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  • American Institute of Physics (AIP)  (31,708)
  • 1985-1989  (31,708)
  • 1950-1954
  • 1945-1949
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Publisher
Years
Year
  • 101
    Electronic Resource
    Electronic Resource
    Infrared spectra of diatomic polar molecules in rare-gas liquids. I. Spectral theory (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3435-3442 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A theoretical model for the infrared (0–1) band of dilute solutions of diatomic polar molecules in nonpolar rare-gas liquids is presented. The model is based on decomposition of the rotational motion of the diatomic molecule into two limiting cases, according to the Bratos model: quasifree rotation and rotational diffusion. Contribution to the infrared absorption coefficient due to quasifree rotation is analyzed within a non-Markovian formalism using a stochastic directing intermolecular field (DIF) model to describe the diatomic molecule–solvent interaction. The P and R branches appear as a consequence of the quasifree contribution, which is also important in the Q-branch region. The contribution due to rotational diffusion is calculated making use of the Debye model and is mainly significant in the Q-branch region.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456918
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  • 102
    Electronic Resource
    Electronic Resource
    Infrared spectra of diatomic polar molecules in rare-gas liquids. II. Application to the HCl in Ar, Kr, and Xe solutions (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3443-3449 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In paper I (the preceding paper), a spectral theory describing the near infrared fundamental band spectra of diatomic polar molecules impurities in nonpolar solvents has been developed in terms of a reduced set of parameters. In this paper, this spectral theory is applied to HCl in Ar, Kr, and Xe liquid solutions. The parameters obtained by fitting the experimental and calculated profiles are in good agreement with those calculated using several microscopic theories.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456872
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  • 103
    Electronic Resource
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    Pressure dependence of the polarized reflectance spectrum of the solid charge-transfer complex, perylene–TCNQ: Estimation of microscopic parameters (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3450-3455 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The polarized reflectance spectrum of the perylene–TCNQ complex has been measured at high pressure up to 27 kbar using a diamond anvil cell incorporated into a microspectrophotometer. The degree of charge transfer (ρ), the transfer integral (t), and the site-energy difference (Δ) are estimated at each pressure from analysis of the charge-transfer band. The pressure dependences of t and Δ are found to be ∂ ln t/∂P=+2.3% kbar−1 and ∂Δ/∂P=−6.4 MeV kbar−1, respectively. It is shown that the observed pressure dependence of the stabilization energy of the charge-transfer excited state is mainly attributable to the change in the electrostatic energy accompanying the lattice contraction.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456873
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  • 104
    Electronic Resource
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    Triplet dipoles in the absorption spectra of dense rare gas fluids. II. Long range interactions (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3456-3462 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A model calculation is presented to evaluate the long range dispersion dipole occurring when three nonoverlapping dissimilar atoms interact. Deviation from pairwise additivity is taken into account by a simplified theory based on electrostatic arguments. By comparing the long range triplet dispersion dipole with the one resulting from exchange overlap effects previously investigated [J. Chem. Phys. 90, 650 (1989)], it is shown that a cancellation between both contributions occurs in the vicinity of the van der Waals separation. The far infrared spectra of dense rare gas fluids are next generated by molecular dynamics simulation including long range dispersion and short range overlap dipoles. An estimate of the expected absorption in liquid krypton is given, αmax =3.6×10−9 cm−1 am−2, which might give some hope to experimentalists. As far as rare gas mixtures are concerned, the model calculation reproduces surprisingly well the experimental absolute spectral density of a Kr–Ar liquid mixture (15% Kr, 135 K) over a large domain of frequency but fails to reproduce the low frequency dip. Nevertheless, irreducible three-body dipoles contribute significantly to the spectrum. The difficulty of obtaining accurate results from an approximate model is emphasized and a guideline for further improvements is sketched.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456874
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  • 105
    Electronic Resource
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    Hole and antihole profiles in nonphotochemical hole-burned spectra (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3463-3469 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The shapes of the real-phonon and pseudophonon sideband holes (PSBH) which occur in hole-burned spectra in amorphous solids are both wavelength and burn time dependent. The theory previously proposed to simulate hole shapes in the short burn time limit is extended to examine the PSBH shapes at arbitrary times and at different burn wavelengths. The simulated spectra are compared with experimental data for tetraphenyl porphin in polystyrene. It is also shown how the simulated spectra may be used to deconvolve the antihole spectrum from the hole spectrum.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456875
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  • 106
    Electronic Resource
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    Quantum chaos of Ar3: Statistics of eigenvalues (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3470-3476 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The successive diagonalization–truncation method is applied to the calculation of the vibrational eigenvalues of the Ar trimer bound by pairwise Lennard-Jones potentials. The statistics of the eigenvalues reveal strongly chaotic behavior of the cluster, consistent with the classical dynamics studies. Moreover, the zero-point energy is higher than the highest energy at which regular dynamics were found classically, indicating that for all energies physically accessible to the cluster, the dynamics are chaotic.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456876
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  • 107
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    Oriented molecule beams: Focusing and orientation of t-butyl bromide with analysis by polarized laser photofragmentation (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3477-3482 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Tert-butyl bromide is photodissociated at 277.3 nm (in the tail of the first absorption continuum). Both fragments, i.e., the t-butyl radical and the Br(2P1/2) atom, are detected by laser MPI-TOF mass spectrometry. Seeded, supersonic beams of this pseudosymmetric top molecule are readily focusable and orientable (via its first-order Stark effect) using the electrostatic hexapole lens. The linearly polarized laser-induced photofragmentation technique reveals unusually high degrees of orientation, significantly greater than that for t-C4H9I, due to the less serious hyperfine disorientation effect for the Br- (vs I-) containing molecule. Thus t-C4H9Br is an excellent candidate reagent for reactive asymmetry scattering studies of the steric effect via the oriented molecule beam method.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456877
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  • 108
    Electronic Resource
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    Branching ratios in the N+CH3 reaction: Formation of the methylene amidogen (H2CN) radical (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3483-3491 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The branching ratios for the reaction N+CH3 →Products, have been determined in a discharge-flow system coupled with mass-spectrometric detection of both reactants and products. The major products are H2 CN+H, with about 10% of the reaction proceeding to give HCN+H2 . Experiments carried out on the reaction of N atoms with the deuterated methyl radical showed that the branching ratio for formation of D2 CN+D is about 0.9 and for DCN+D2 formation about 0.1 independent of T from 200 to 363 K. The results are consistent with the energetics and orbital symmetry properties of the reactant and product molecules. Implications for the atmosphere of Titan are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456878
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  • 109
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    Block Lanczos approach combined with matrix continued fraction for the S-matrix Kohn variational principle in quantum scattering (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3504-3508 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An iterative method is proposed for calculating the S matrix in the Kohn variational approach. Instead of solving the system of linear equations directly, the method consists of a block Lanczos algorithm extended to complex symmetric matrices and a matrix continued fraction procedure. Applications to inelastic and reactive scattering calculations indicate a reasonable rate of convergence.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456880
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  • 110
    Electronic Resource
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    Reaction rates of H(H2), D(H2), and H(D2) van der Waals molecules and the threshold behavior of the bimolecular gas-phase rate coefficient (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3492-3503 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We study low-energy quantal phenomena in the rearrangement of three-atom systems composed of H and D. All calculations are carried out on the double many-body expansion potential energy surface for the hydrogen trimer. The unimolecular rearrangements of van der Waals molecules, such as D⋅⋅⋅H2→HD⋅⋅⋅H, are studied as a model for the exchange transfer reaction in condensed phases, and the gas-phase bimolecular reactions, such as D+H2→HD+H, are studied to probe the limiting low-temperature threshold behavior, which is compared to that predicted by quantum mechanical threshold laws. The reaction rates are studied down to temperatures of 10−3 K. We also predict the spectroscopic tunneling shift on the lowest energy levels of the H⋅⋅⋅H2 complex.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456879
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  • 111
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    Vibrationally inelastic collision cross sections of He+SO2: Distorted wave approach (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3509-3515 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Vibrational excitation and relaxation cross sections of He colliding with SO2 molecule are evaluated. The results obtained rely on several approximate methods. The infinite-order-sudden approximation provides the framework for treating the rotational motion, while the vibrational processes are described using both exact close-coupling and approximate distorted wave techniques. The latter scheme permits the computations to be extended to much higher collisional energies than so far reported in the literature. Vibrational excitation and relaxation cross sections are presented over a range of collision energies.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456881
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  • 112
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    The rate coefficient for quenching N(2D) by O(3P) (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3516-3524 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have studied the quenching of N(2D) by atomic oxygen in a discharge-flow reactor. Vacuum ultraviolet resonance fluorescence is used to detect the metastables. Three different discharge-based sources provide the atomic oxygen: discharged O2/Ar, discharged N2O/Ar, and the reaction of N with NO. Results from the three different approaches are congruent and indicate the lack of significant systematic errors in the study. The rate coefficient at 300 K is (1.06±0.26)×10−12 cm3 molecule−1 s−1.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456882
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  • 113
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    Ultrafast vibrational dynamics of a photoexcited metalloporphyrin (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3525-3531 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The ultrafast photodynamics of four-coordinate nickel(II) porphyrins in noncoordinating solvents has been studied using femtosecond time resolved optical spectroscopy. Unambiguous evidence has been found for the formation of a metastable metal (d,d) excited state in less than 350 fs following excitation of the macrocycle. However, the transient absorption spectrum of this ligand-field electronic excited state continues to evolve and reaches the steady-state form only after about 20 ps. This spectral behavior and the attendant complex kinetics can be modeled phenomenologically in terms of a broad distribution of the (d,d) excited states evolving to a narrower distribution. The dynamics are associated with vibrational relaxation. Intramolecular and intermolecular contributions to this process are considered.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456883
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  • 114
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    The branching of nonradiative processes in isoquinoline (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3532-3538 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have measured the absolute rates of internal conversion and of intersystem crossing in isoquinoline excited to its S2 vibronic states (3131–3062 A(ring)) in supersonic beams. The combination of direct detection of the triplets, absorption and excitation spectra allowed us to extract these rates. The intersystem crossing rate barely depends on the excess vibrational energy in the S2 manifold, being about 2×109 s−1. The internal conversion rates grow by more than two orders of magnitude over the range of 0–800 cm−1 of excess vibrational energy in the S2 state, up to 5×1010 s−1. At the S2 electronic origin intersystem crossing is the dominant nonradiative channel, while at vibrational energy exceeding 400 cm−1 internal conversion becomes dominant.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456884
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  • 115
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    The ClHCl− anion: Its chemical bond, vibrations, and free energy (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3539-3548 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The geometry of ClHCl− has D∞h symmetry, [Cl–H–Cl]−, as opposed to C∞v symmetry [Cl–H...Cl−], which is predicted to be favored at the Hartree–Fock (HF) level of calculation with a basis set of [11s9p2d1f(Cl)/5s2p1d(H)]. The chemical bond of the D∞h structure is a nonclassical four-electron three-center bond, while the C∞v geometry corresponds to an ion–dipole complex. The energy difference between the two is small. Calculations with inclusion of configuration revealed that both have predominantly a HF configuration. D∞h is realized by excitations from σ molecular orbitals (MO) to their antibonding σ* MO, whereas excitations to π MO are ineffective. Normal vibrational frequencies were calculated as follows (units are cm−1): G82(MP2): (ν1,σg): 346; (ν3,σu ): 547; (ν2,πu): 854; 1D Analysis(MP4): (ν1,σg ): 338; (ν3,σu): 830; (ν2, πu ):...; 2D Analysis(MP4): (ν1,σg)@B: 310; (ν3,σu) : 709; (ν2,πu) : 877. The potential for the asymmetric vibration is shown to be quartic. The standard free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) changes (i.e., at 1 atm and 298.16 K) were evaluated from the reaction, HCl+Cl−→ClHCl−, to be as follows: ΔG°=−16.5 kcal/mol, ΔH°=−23.5 kcal/mol, and ΔS°=−23.5 cal/K/mol.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456885
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  • 116
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    Improved dynamic hyperpolarizabilities and field-gradient polarizabilities for helium (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3549-3551 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Accurate and explicitly electron-correlated wave functions have been used to determine the dynamic hyperpolarizability tensor (γ) and the dynamic field-gradient polarizabilities (B and C) for helium. The γ-tensor is related to nonlinear optical processes such as the Kerr effect, second- and third-harmonic generation and degenerate-four-wave mixing. In the static limit our results agree completely with a previous highly accurate variational-perturbation treatment. For second-harmonic generation at λ=4880 A(ring) the ratio γzzzz/γxxzz is 2.950, which may be compared with the experimental value of 2.949.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456886
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  • 117
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    Geometries and energy separations of 14 electronic states of Au4 (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3561-3570 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Multiconfiguration SCF (MCSCF) followed by multireference configuration interaction calculations are carried out on 14 electronic states of Au4 . The low-lying electronic states are studied further using multireference singles + doubles CI (MRSDCI) calculations including excitations from the d shells. The ground state of Au4 is found to be a closed shell 1 A1 state with an equilibrium geometry of rhombus. Two excited states of 3 B2 symmetry with rhombus and square geometries were found to lie 0.98 and 1.12 eV above the ground state. The spin–orbit effects were included using the relativistic configuration interaction method (RCI). Accurate MRSDCI calculations which included up to 1.5 million configurations on the ground states of Au2 and Au3 are also performed with the objective of comparing the binding energies of Au2 –Au4 . The gold tetramer is found to be much more stable in comparison to the trimer and dimer.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457625
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  • 118
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    Calculation, display, and analysis of the nature of nonsymmetric nuclear magnetic resonance shielding tensors: Application to three-membered rings (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3552-3560 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The components of the full nuclear shielding tensor are analyzed with particular regard to the origins of the antisymmetric component of the shielding of nuclei at low-symmetry molecular sites. As an aid we propose the study and display of the shielding response vector, i.e., the nuclear shielding field per unit applied magnetic field. The analysis is based on ab initio calculations in the localized orbital-local origin method for cyclopropane, cyclopropene, ethylene oxide, ethylene imine, and diazirine, and also includes a discussion of the mechanism for the large antisymmetric component predicted for the unsaturated nuclei in cyclopropene and diazirine, both in terms of bond and lone pair contributions and in terms of a decomposition of the dominant paramagnetic contributions into molecular point group species. Display of the component of the shielding vector parallel to the applied field is shown to be a valuable alternative to the common ellipsoid representations. Display of the components perpendicular to the applied field provides a picture of the relation of the antisymmetry to molecular structure and suggests its direct observation through monitoring the perpendicular response. Finally, we show that, contrary to a recent claim, the eigensolutions of the full tensor in the presence of antisymmetry play no role in the observed anisotropies.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456887
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  • 119
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    Doubly ionized states of ethylene: Auger spectrum, potential energy surfaces and nuclear dynamics (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 1734-1753 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Theoretical investigations of the outer valence doubly ionized states of ethylene are presented using ab initio Green's function and configuration interaction methods. The vertical double ionization potentials computed by the Green's function method using the ADC(2) scheme are discussed in connection with the Auger spectrum of ethylene and found to reproduce the experimental spectrum quite accurately. Another main purpose of the present work is the investigation of the potential energy surfaces of selected states via ADC(2) and the study of the nuclear dynamics on them. It is shown that only the ground state of the dication is nonplanar with a torsional angle of 90°. The investigation of the topology of the surfaces reveals a low energy conical intersection of the dicationic ground and first excited states. The associated vibronic coupling problem turns out to include, additionally, the second excited state and thus, we encounter here a three-state problem. In the approximation of linear vibrational and vibronic coupling the static and dynamic aspects of the vibronic interaction are investigated by a model Hamiltonian comprising three nonseparable nondegenerate vibrational modes. The input data for the model are extracted from the ADC(2) results. With the aid of this Hamiltonian we have computed a hypothetical "sudden'' double ionization spectrum of ethylene whose complex structure is analyzed stepwise and by cuts through the pertinent adiabatic and diabatic surfaces. The analysis exhibits that the conical intersection of the two lower lying states dominates the vibronic dynamics and leads to strong vibronic mixing between them. This mixing is indirectly reinforced by the interaction between the intermediate and the upper states. As a consequence, strong nonadiabatic effects occur in the ethylene dication. Furthermore, two of the three surfaces involved in the conical intersection are also involved in a multidimensional avoided crossing. A more detailed analysis presented earlier has exhibited that the states belong to an interesting class where two potential energy surfaces are likely not to coincide although plenty of nuclear degrees of freedom exist, in principle, to allow for an intersection to occur.
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    URL: http://dx.doi.org/10.1063/1.457080
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  • 120
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    Rigorous bounds to molecular electron repulsion and electrostatic potential integrals (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3596-3602 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Novel rigorous upper and lower bounds, at primitive level, to general electron-repulsion integrals (ERIs) involving Gaussian basis sets have been derived and interconnections with the earlier works in the literature are brought out. New optimal strategies for a preemptive elimination of insignificant ERIs at atom and contraction levels are discussed and tested, resulting in a significant reduction in CPU time. Similar analysis is carried out for the computation of the molecular electrostatic potential for the first time in the literature, leading to a marked savings in computer time.
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    URL: http://dx.doi.org/10.1063/1.456892
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  • 121
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    Multireference and relativistic effects in NiH (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3589-3595 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Large multireference CI calculations have been performed for the ground state of NiH. The effects of relativity were investigated using both a variational (no pair) theory and perturbation theory. The largest CI calculations included up to 29 reference states and were performed at the contracted CI level. The calculated and experimental results (within parentheses) are for Re 2.76 a0 (2.76 a0), for ωe 1997 cm−1 (2003 cm−1) and for the dipole moment μ 2.32 D (2.4±0.1 D). The effects of relativity are −0.03 a0, +60 cm−1 and −0.3 D, respectively. The effects of going from a reference selection threshold of 0.05 to a selection threshold of 0.02 and a different set of CASSCF orbitals was before applying Davidson's correction +0.07 a0 for Re and about +30 cm−1 for ωe, and after applying this correction +0.06 a0 and −80 cm−1, respectively. The most accurate results for the dipole moment was obtained using the multireference ACPF method.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456891
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  • 122
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    The contribution of hydrogen bonds to bulk and surface thermodynamic properties of dimethylsulfoxide–water mixtures (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3603-3613 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A simple model for a hydrogen-bonded mixture treated in mean field approximation is used to study the excess bulk and interfacial thermodynamic properties of DMSO/water solutions at various compositions. According to the model, the system is treated as an ensemble of hydrogen bonding pairs involving water and DMSO molecules. In the interfacial layer of thickness equal to the length of the hydrogen bond, the number of hydrogen bonding pairs is reduced. Considering the difference in the hydrogen bonding strength for the water–water and water–DMSO hydrogen pairs, as well as orientational restrictions at the liquid/vapor interface, the number densities of water–water and water–DMSO hydrogen bonds are calculated as functions of the distance from the interface. The concentration profile is determined by minimizing the free energy function of the system. The model describes reasonably well the experimental data for excess enthalpies and free energies of mixing, as well as the excess surface free energy over the entire composition range. The resulting excess surface free energy is considered as a contribution of the hydrogen bond interaction among the two species to the total surface tension.
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    URL: http://dx.doi.org/10.1063/1.457641
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  • 123
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    Erratum: Predissociation of O2 in the B state [J. Chem. Phys. 90, 2836 (1989)] (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 1947-1947 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457683
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    The equilibrium structure and harmonic vibrational frequencies of ozone: Coupled cluster results including triple excitations (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 1945-1947 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The highly correlated CCSDT-2 model of electron correlation is applied to computation of the equilibrium geometry and harmonic vibrational frequencies of O3. Comparison of the present results with those of a recent study [J. F. Stanton, W. N. Lipscomb, D. H. Magers, and R. J. Bartlett, J. Chem. Phys. 90, 1077 (1989)] reveal the importance of certain simultaneous three-particle correlation effects (T3) which enter perturbation theory in fifth and higher orders, particularly for asymmetric structures of ozone and, consequently, the B2 stretching frequency. The CCSDT-2 value for ω3 is 1182 cm−1, in reasonable agreement with the experimental value of 1089 cm−1, and considerably better than values obtained with other methods which make more approximations in treating T3. To treat this difficult problem, it appears to be necessary to include terms involving T3 which are nonlinear in the cluster amplitudes into the CC equations.
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    Semiempirical determination of intermolecular anharmonicity in molecular crystals: The case of 1,2,4,5-tetrabromobenzene (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3624-3630 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The molecular crystal 1,2,4,5-tetrabromobenzene (TBB) has been studied using Brillouin scattering and lattice dynamical calculations. A method for mapping acoustic mode anharmonicity is developed that exhibits substantial directional behavior and correlates well with the directions of the molecular movements associated with the phase transition. It is shown that by comparing the calculated sound velocities with the experimental values, insight can be gained into the displacive phase transition and the lattice dynamics of TBB. The sound velocities are plotted for three crystallographic planes containing the crystal axes. The relationships between sound velocities and lattice dynamics are discussed. The phonon dispersion curves for three directions are also presented.
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    Restricted rotational diffusion of rigid bodies (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 1232-1241 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Analytical solution to the restricted-diffusion-in-a-cone model for a symmetric top is obtained using a set of orthogonal functions of nonintegral order. It is shown that the case of the restricted-diffusion-in-a-cone model for an asymmetric top cannot be solved in analytical closed form: Series solution is, however, possible. The correlation functions for arbitrary functions of the orientation of a rigid body are also obtained. These functions are required in obtaining the correlation functions for the restricted rotational diffusion of articulated bodies and are useful in interpreting the data from relaxation experiments for determining the diffusion coefficients and conal restriction angles.
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    Thermal diffusion in helium–hydrocarbon and helium–fluorocarbon systems at 300 K (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 1258-1260 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Thermal diffusion factors are reported for a series of helium–hydrocarbon and helium–fluorocarbon systems. The data are used to test the first-order corresponding states theory: An amusing empirical correlation is also reported.
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    Enhanced energy transfer between donor and acceptor molecules near a long wire or fiber (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 1279-1286 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A theory is developed for the energy transfer between donor and acceptor molecules near a long metallic wire or dielectric fiber. The theory is valid for arbitrary wave lengths. Interference effects are noted and discussed.
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    Optical excitation spectra of adsorbates on dielectric surfaces (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 1294-1302 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Optical absorption spectra of adsorbates on dielectric surfaces have been considered at low coverage. Radiative and nonradiative processes have been taken into account. Expressions for the spectral functions have been derived describing the spectra of the symmetric and antisymmetric modes, respectively. At low coverage and when the damping of the surface electron–hole pairs is much greater than the effective radiative damping, the relative intensities of the symmetric and antisymmetric modes take positive and negative values describing the physical processes of absorption (attenuation) and stimulated emission (amplification), respectively. Thus, peaks of the symmetric and antisymmetric modes arising from two nearby excited states of the adsorbed atom may cancel each other out, provided that the frequency profiles of the peaks in question overlap. Numerical calculations have been made for the spectra of rare-gas atoms He, Ne, Ar, Kr, and Xe which are physisorbed on graphite. The resulting excited geometrical configurations arising from the cancellation process between the spectra of the symmetric and antisymmetric modes have been computed as a function of the distance R from the atom to the dielectric surface, and they are graphically presented. The disappearance or the persistence of the excited geometrical configurations may help to determine the distance R at which the atom is physisorbed on the dielectric surface under investigation.
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    Prediction and design of first super-strong liquid-crystalline polymers (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 1326-1338 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: This paper presents the details of the theoretical prediction and design (atom by atom, bond by bond) of the molecule chemical structures of the first candidate super-strong liquid-crystalline polymers (SS LCPs). These LCPs are the first LCPs designed to have good compressive strengths, as well as to have tensile strengths and tensile moduli significantly larger than those of existing strong LCPs (such as Kevlar). The key feature of this new class of LCPs is that the exceptional strength is three dimensional on a microscopic, molecular level (thus, on a macroscopic level), in contrast to present LCPs (such as Kevlar) with their one-dimensional exceptional strength. These SS LCPs also have some solubility and processing advantages over existing strong LCPs. These SS LCPs are specially-designed combined LCPs such that the side chains of a molecule interdigitate with the side chains of other molecules. This paper also presents other essential general and specific features required for SS LCPs. Considerations in the design of SS LCPs include the spacing distance between side chains along the backbone, the need for rigid sections in the backbone and side chains, the degree of polymerization, the length of the side chains, the regularity of spacing of the side chains along the backbone, the interdigitation of side chains in submolecular strips, the packing of the side chains on one or two sides of the backbone, the symmetry of the side chains, the points of attachment of the side chains to the backbone, the flexibility and size of the chemical group connecting each side chain to the backbone, the effect of semiflexible sections in the backbone and side chains, and the choice of types of dipolar and/or hydrogen bonding forces in the backbones and side chains for easy alignment. The chemical syntheses of several of the theoretically designed SS LCPs presented in this paper are nearing completion at Los Alamos. This paper also makes some predictions about chemical crosslinking of some of the structures in SS LCPs.
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    Ab initio molecular orbital study on the methylcation affinity of N2 (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 1376-1377 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457162
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    A comparison between integral equation theory and molecular dynamics simulations of dense, flexible polymer liquids (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 1357-1364 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Recently we (J.G.C. and K.S.S.) formulated a tractable "reference interaction site model'' (RISM) integral equation theory of flexible polymer liquids. The purpose of this paper is to compare the results of the theory with recent molecular dynamics simulations (G.S.G. and K.K.) on dense chain liquids of degree of polymerization N=50 and 200. Specific comparisons were made between theory and simulation for the intramolecular structure factor ωˆ(k) and the intermolecular radial distribution function g(r) in the liquid. In particular it was possible to independently test the assumptions inherent in the RISM theory and the additional ideality approximation that was made in the initial application of the theory. This comparison was accomplished by calculating the intermolecular g(r) using the simulated intramolecular structure factor, as well as, ωˆ(k) derived from a freely jointed chain model.The RISM theory results, using the simulated ωˆ(k), were found to be in excellent agreement, over all length scales, with the g(r) from molecular dynamics simulations. The theoretical predictions using the "ideal'' intramolecular structure factor tended to underestimate g(r) near contact, indicating local intramolecular expansion of the chains. This local expansion can be incorporated into the theory self consistently by including the effects of the "medium induced'' potential on the intramolecular structure.
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    Vibrational state dependence of ionic rotational branching ratios in resonance enhanced multiphoton ionization of CH (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 1374-1376 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We show that rapid evolution of a Rydberg orbital with internuclear distance in a resonance enhanced multiphoton ionization (REMPI) process can have a profound influence on the production of molecular ions in alternative rotational states. This is illustrated by calculations of ionic rotational branching ratios for (2+1') REMPI via the O11 (20.5) branch of the E' 2Σ+(3pσ) Rydberg state of CH. The rotational propensity rule for ionization changes from ΔN=odd (ΔN=N+−Ni) at lower vibrational excitation, as expected from the ΔN+l=odd selection rule, to ΔN=even at higher vibrational levels. This effect is expected to be quite general and should be most readily observable in diatomic hydrides.
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    Comment on: "Mean field theory of nematic order for grafted rods'' (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 1381-1383 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The calculation of Halperin et al. [J. Chem. Phys. 86, 6550 (1987)] has been repeated, but with the orientational distribution function expressed as a sum of Legendre polynomials. The result is that the inflections in the curves of nematic order parameter and surface pressure vs surface concentration are no longer present.
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    URL: http://dx.doi.org/10.1063/1.457165
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    Reply to the Comment on: Mean field theory of nematic order for grafted rods (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 1383-1383 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
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    URL: http://dx.doi.org/10.1063/1.457166
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    Optimization of kinetic and thermodynamic properties in irreversible open chemical and thermal engines (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3677-3684 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Periodic variations of an external parameter or constraint of open chemical systems have been shown to induce changes in time averaged kinetic and thermodynamic quantities. We examine the effects of the analytic form of the periodic variation on the time averaged quantities and find the maximum changes obtainable through periodic variations. A variational procedure is proposed, based on a Fourier expansion of the form of the periodic perturbation, the laws of thermodynamics, conservation of matter, and the kinetics. The efficiency of power production in a combustion system is examined with this method in a numerical example. A unique maximum in the efficiency is found, with the gains achievable for more complex functions exceeding those for a sinusoidal perturbation. We interpret the changes in efficiency in terms of the magnitude of the response of the system (resonance) and phase shifts between the periodic perturbations and the response of the system. We illustrate the mechanisms of efficiency changes in this system with two examples; one in which the periodic perturbation affects the phase relations and one in which the periodic perturbation affects the magnitude of the response. Finally, we note that multiple attractors may coexist in this system for certain forms of the periodic perturbation, each with a distinct efficiency.
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    Experimental measures of the electron distribution and bonding in bis(η5-cyclopentadienyl) osmium from He(I) and He(II) valence photoelectron spectroscopy (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 663-673 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The He(I) and He(II) high resolution photoelectron spectra of osmocene are reported. Vibrational fine structure is observed in all the valence metal-based ionizations and in the cyclopentadienyl π ionizations which derive from the e1g and e1u symmetry combinations. Analysis of the vibrational progressions found in the metal-based ionizations provides a measure of the force constants and vibrational frequencies for the metal–ring stretch in the positive ions. The vibrational analysis for the 2E2(5/2) and 2E2(3/2) states of the osmocene cation [derived from the spin–orbit split ionization of the metal e2g(dx2−y2, dxy) set] indicates an 0.12 A(ring) greater metal–ring bond length in the cation compared to the neutral molecule. The sharp 2A1(1/2) ionization [correlating with removal of an electron from the metal a1g(dz 2) orbital] is observed to have a much shorter vibrational progression. The adiabatic ionization is the most intense (vertical) band of the series, indicating that there is no appreciable change in metal–ring bond distance upon ionization from the nonbonding a1g(dz 2) orbital to produce the 2A1(1/2) cationic state. A large spin–orbit coupling is observed in the metal ionization region and a moderate amount in the cyclopentadienyl ring π ionization region. From evaluation of the spin–orbit coupling, the ionization band which correlates primarily with the e1g combination of the cyclopentadienyl π orbitals is shown to contain about 27% metal character. The spin–orbit coupling effects are negligible in the ionization which corresponds to the e1u combination of the ring orbitals, as expected from the lack of symmetry interaction with the metal d orbitals. The ionizations of this complex also provide experimental measures of relative metal-based and carbon-based ionization cross sections with He(I) and He(II) energy sources which are compared with theoretical calculations. All of the observations indicate that covalent bonding is more prevalent in osmocenethan in ferrocene.
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    Polarized resonant Raman scattering of cis polyacetylene (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 732-737 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Polarized resonance Raman measurements of highly oriented films of cis polyacetylene have been carried out. The intensities of the three Raman fundamentals in the four different scattering configurations have been measured and corrected for the strong anisotropy of the optical properties. The experimental data are indicative of an almost perfect alignment of the cis polyenic chains along the stretching direction. Preliminary measurements of the depolarization ratios carried out on isotropic samples in the resonance region appear to be inconsistent with the view that the visible band is dominated by a single transition polarized along the chain axis.
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    Laser-induced fluorescence spectroscopy of NCS in a free jet expansion (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 762-774 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Laser-induced fluorescence excitation spectra have been recorded for many vibronic transitions of the A(2Π)–X(2Π) and B(2Σ+)–X(2Π) systems of NCS under supersonic free jet expansion conditions. New assignments have been made for many of these bands and several of the assignments from previous work have been revised. Vibronic energies have been determined for levels involving excitation of all three vibrational modes in the ground electronic state and both excited electronic states. A detailed rotational and vibrational analysis has been carried out for levels involving excitations of the two stretching modes and the many rotational and potential function constants have been determined for the A and X states. The variation of the spin–orbit constant with vibrational level has also been investigated. Results of this paper provide the ground work for a detailed analysis of the Renner–Teller effect in the bending vibrational mode of this radical for both the X2(Π) and A(2Π) states to appear in a forthcoming paper. They also provide a spectroscopic database for future dynamical and kinetics studies of processes involving this radical.
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    Investigations on the convergence rate of the thermodynamic and structural parameters from Monte Carlo simulations of aqueous solutions of methanol and methylamine (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3707-3715 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Monte Carlo simulations using statistical perturbation theory (SPT) were applied to calculate the free energy, solvation enthalpy, and partial molar volume changes for transforming the solute methylamine to methanol in an aqueous solution of 264 water molecules. Using preferential, as well as random, sampling the calculated thermodynamic parameters were compared in the averaging phase after considering 1500 and 3000 K configurations. The relative free energy shows little variance after taking 1500 K configurations. The solvation enthalpy and the partial molar volume agree well with the experimental values after taking 3000 K configurations for methanol and 4500 K configurations for methylamine. There are 3.0 to 3.3 water molecules in the first hydration shells of the polar groups of the solutes with ∼2 of the waters involved in hydrogen bonds with methylamine and ∼2.4 hydrogen bonded to methanol. The calculated numbers of hydrogen bonds do not change after taking 1500 K configurations, and coordination numbers of the polar sites with water were stable after 3000 K. The similarity of the solution structures with the two different solutes suggests little difference in the solvation entropy in accordance with experiment.
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    Brownian dynamics study of the end-to-end distribution function of star and linear polymers in different regimes (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3721-3723 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Brownian dynamics computer simulations are used to investigate the shape of the mean-squared end-to-end distance distribution function in the three regimes: (1) excluded volume; (2) θ; (3) collapsed. It is found that Mazur's function fits regime (1), that the Gaussian function fits regime (2), and that neither of these appear to describe the collapsed state. The implications of these results to theories of ring formation during polymerization is pointed out.
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    A stability condition for the Hartree–Fock solution of the infinite one-dimensional system (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3724-3728 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A simplified stability condition for the Hartree–Fock (HF) solution giving the self-consistent field crystal orbitals (SCF-CO) of the infinite one-dimensional system is derived. Since the present formulation, particularly for the systems having nearly or entirely degenerated highest occupied and the lowest unoccupied COs, contains only two physical parameters, that is, the density of states and the Coulomb repulsion integrals both at the Fermi level, it is tractable to examine the stability of the HF solutions of such polymers as well as the ordinary molecular systems. An example of its application to metallic trans-type polyacetylene is also shown.
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    The adsorption of hydrogen on a ruthenium (101¯0) surface (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3729-3743 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Video-LEED, HREELS, TDS, and Δcursive-phi measurements were used to investigate the adsorptive, structural, and vibrational properties of the Ru(101¯0)/H system between 100 and 500 K. At all temperatures investigated hydrogen adsorbs dissociatively with very high initial sticking probability (s0(approximate)1.0) with apparent precursor mechanism. The saturation coverage at 100 K is extraordinarily high (aitch-thetamax =2(approximately-equal-to)1.728×1015 H atoms cm−2), up to this coverage four H binding states α, β1, β2, and β3 can be distinguished having desorption energies between 56 KJ/mol (α) and 80 KJ/mol (β3). The H binding states are intimately correlated with the four observable ordered H phases: At aitch-theta=1 a c(8×2) or "1×2'' structure with weak split spots appears which transforms at higher coverages into a clear 1×2 phase with likewise weak spots and with maximum intensity at aitch-theta=1.2. It follows a c(2×2)-3H phase (I maximum at aitch-theta=1.5) with rather more intense ‘extra' spots which fade away with increasing coverage until at aitch-theta=2.0 a (1×1)-2H pattern is reached. The (positive) H-induced work function change Δcursive-phi runs through two maxima and saturates at ∼250 mV. The vibrational loss spectra which were measured in two perpendicular azimuths exhibit a variety of bands which can be correlated with the ordered H phases and point to H species bound in two different kinds of threefold coordinated sites. Our data suggest several structural similarities with the neighboring system in the periodic table, Rh(110)/H, but also interesting differences.
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    The liquid–gas transition and the polymer–magnet analogy (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3762-3773 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We propose a new unified field-theoretic path integral treatment of liquid–gas transition occurring in simple neutral fluids and neutral monodisperse polymer solutions. Obtained theoretical results indicate that both systems belong to the same Ising-type universality class which is strongly supported by the most recent experimental results on the liquid–gas transitions.
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    Distributed electronic relaxation and nonexponential fluorescence in polymers: Reversibility in donor–excimer pairs—A perturbation theory treatment (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3744-3761 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An ensemble of distributed donor molecules that undergoes rotational transitions into a discrete excimer state has been analyzed, in an attempt to model distributed electronic relaxation and nonexponential fluorescence of aromatic polymers in presence of rotational sampling processes. In case of irreversible trapping, the donor survival function has been formulated in terms of the one-sided Laplace transform and specified for a modified Gaussian distribution to yield a closed-form expression for the donor decay. The formalism permits a time-dependent rate function to be derived that makes possible the construction of the excimer excitation function by means of the convolution theorem. In case of reversible constraints, a generalized treatment based upon time-dependent transition and transform matrices has been given which applies a perturbation technique for approximately solving the system of nonautonomous differential equations in the time domain. In the limit of weak coupling, the method develops approximate Mth order expressions (M=2,4,6, and 8) to the fluorescence response functions of donor and excimer. The perturbational solutions are well behaved up to relatively long time scales and they prove useful for providing the typical nonexponential time behavior of such a system affected by dispersion and dissociation. The physical restriction of this mathematical analysis (weak reversibility) has been addressed and the implications of distributed event times in future analyses of polymer fluorescence have been discussed.
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    The Thouless–Anderson–Palmer approach to random copolymer glasses (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3774-3787 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The experimental evidence that the critical properties of homopolymer solutions are the same as Ising-type ferromagnets suggests that the critical properties of the random copolymer solutions might be analogous to that of Ising spin glasses. Using newly developed path integral methods, the analogy between the polymer and the magnets is carefully analyzed in order to be extended to the case of random copolymers. Established similarities permit one to use the Thouless–Anderson–Palmer (TAP) method in order to investigate the possibility of the formation of the new phase for random copolymers analogous to the spin glass phase for the magnets.
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    Implications for multidimensional effects on isomerization dynamics: Photoisomerization study of 4,4'-dimethylstilbene in n-alkane solvents (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 943-952 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Studies of the photoisomerization of 4,4'-dimethylstilbene in n-alkane solvents are reported. As with similar studies it is possible to identify an activation barrier to isomerization and the viscosity dependence of the isomerization is not adequately explained by a one-dimensional hydrodynamic Kramers equation. Comparison of the isomerization rates with those of other stilbenes suggests that the use of a frequency dependent friction for the solvent does not consistently explain the data. These results and those of other stilbenes can be explained by a two-dimensional isomerization coordinate where the reaction trajectory is linked to the solvent viscosity.
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    The decay of triplet pyrazine in supersonic jets (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 7331-7339 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The decay rates of optically excited triplet states of pyrazine in supersonic expansion were measured by using three different methods. The excess energy dependence of the radiationless rate constants in the energy range between the T1 and the S1 electronic origins of the isolated molecule was explored. Decay rates between 7×102 –2.5×104 s−1 were found in the 1500 cm−1 range of excess vibrational energy from the origin of the T1 state. The decay rates are free of mode specificity and rotational effects. The pure radiative lifetime in the measured range is rovibronic independent. The results support a model which suggests that certain vibrational modes, those which undergo large frequency changes in the excited state, control the strong vibrational energy dependence of the T1 →S0 intersystem crossing of pyrazine.
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    Water dimer Coriolis resonances and Stark effects (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 7348-7354 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: E±2 symmetry states for K=0, 1 of (H2O)2 are found to be perturbed and a Coriolis resonance model is used to give a satisfactory treatment of the observations. The effective b-type Coriolis coupling constant ζ is 1409.4 MHz, implying appreciable vibrational angular momentum since ζ/2C≈1/8. The spacing Δ between the upper K=0 levels and the lower K=1 levels is 10 719 MHz. The rotational constants from the deperturbation analysis are now found to be in substantially better agreement with those from tunneling states of other symmetries and with the structure of the dimer. The Stark effects of the perturbed states have been analyzed. The a component of the electric dipole moment is well determined; the c component appears to be small, but the results are not completely consistent.
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    A study of some Rydberg states of CO2 by (3+1) multiphoton ionization spectroscopy (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 7399-7407 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The MPI spectra of CO2 were recorded in the range 275 to 338 nm in both a conventional MPI cell and a molecular beam apparatus. The polarization ratio of each MPI peak was measured in order to assign the symmetry of excited states. The 3pσu 1Πu and 3pσu 1Πu110 Rydberg states were observed, while the 3pπu 1Σ+u Rydberg state was missing, and a new Rydberg state, 3pπu 1Δu was observed at 333.03 and 333.70 nm. In the nf (n=4,5) Rydberg series, evidence was found of interaction with other nearby Rydberg states. A mass selected pure CO+2 ion signal was measured for the 3pπu 1Δu state, but both CO+ and CO+2 ion signals were detected for the other Rydberg states. Predissociation and fragmentation in MPI processes are discussed. Lower limit lifetimes for the observed Rydberg states of CO2 are given.
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    Conformational stability, barriers to internal rotation, vibrational assignment and ab initio calculations of 2-fluoropropenoyl fluoride (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 7361-7373 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The far infrared spectrum of gaseous 2-fluoropropenoyl fluoride CH2CFCFO, has been recorded at a resolution of 0.10 cm−1 in the 350–35 cm−1 region. The asymmetric torsional fundamental of the more stable s-trans (two double bonds oriented trans to one another) and the high energy s-cis conformations have been observed at 84.1 and 68.5 cm−1, respectively, each with excited states falling to lower frequencies. From these data, the asymmetric torsional potential function governing internal rotation about the C–C bond has been determined. The potential coefficients are V1=−201±4, V2=2106±16, V3=382±6, V4=−41±5, and V5=−74±3 cm−1. The s-trans to s-cis and s-cis to s-trans barriers have been determined to be 2232 and 2124 cm−1, respectively, with an enthalpy difference between the conformations of 108±26 cm−1 (309±74 cal/mol). From variable temperature studies of the Raman spectrum, the conformational enthalpy difference has been determined to be 187±39 cm−1 (535±112 cal/mol) and 370±82 cm−1 (1058±234 cal/mol) for the gas and liquid, respectively. A complete assignment of the vibrational fundamentals observed from the infrared (3500–50 cm−1) spectra of the gas and solid and the Raman (3200–10 cm−1) spectra of all three physical states is proposed. All of these data are compared to the corresponding quantities obtained from ab initio Hartree–Fock gradient calculations employing both the 3-21G and 6-31G* basis sets. Additionally, complete equilibrium geometries have been determined for both rotamers. The results are discussed and compared with the corresponding quantities obtained for some similar molecules.
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    Absorption spectra from high vibrational levels of He2 (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 7408-7414 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Absorption spectra of dense helium gas at cryogenic temperatures has been acquired while the sample was irradiated using a 6.5 MeV proton beam. By chopping the proton beam, rather than the source lamp, we were able to achieve one part in 104 spectral sensitivity. The spectra showed six new bands in 4He2 and three in 3He2. These have been identified as transitions between high-lying vibrational levels, with the strongest originating on the highest bound level of the a 3Σ+u potential. The temperature and pressure dependence of these features, as compared to low-lying molecular and atomic features, offers some insight into the reaction dynamics of this fundamental system.
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    Intramolecular dynamics of van der Waals molecules: An extended infrared study of ArHF (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 2790-2807 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The near-infrared spectrum of ArHF prepared in a slit supersonic expansion is recorded with a difference frequency infrared laser spectrometer. By virtue of the high sensitivity of the technique, and the lack of appreciable spectral congestion at the 10 K jet temperature, we observe 9 of the 11 vibrational states with energies below the Ar+HF(v=1, j=0) dissociation limit. These include (1000), the lowest bound HF (v=1) state, the singly, doubly, and quadruply van der Waals stretch excited states (1001) (1002), and (1004), both the Σ bend (1200) and Π bend (111e,f 0), and the multiply excited, Π bend plus van der Waals stretch (111e,f 1). Two Ar+HF(v=0) states, (0000) and (0001), are also characterized. This spectroscopic information is quite sensitive to the Ar+HF potential energy surface away from the equilibrium configuration, and thus provides a rigorous test of trial potential energy surfaces. Excellent agreement is obtained between experiment and the predictions of a recently reported Ar+HF(v=1) potential.
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    Vacuum ultraviolet absorption spectra of thiirane and thietane (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 2808-2813 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Photoabsorption cross sections of thiirane and thietane vapors have been studied in the 110–240 nm region using synchrotron radiation. A number of peaks from thiirane and thietane are arranged into four and five Rydberg series, respectively, converging to the first ionization potential. Many Rydberg peaks from both molecules are observed to possess vibronic structures. The vibrational progressions from thiirane are assigned to the CH2 wagging (ν4) and CS symmetry stretching (ν5) modes. For thietane, the CH2 wagging (ν5) and CS symmetry stretching (ν7) modes of the excited states are found to be active. Molecular geometries and vibrational frequencies for the excited states are discussed on the basis of ab initio calculations.
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    High-pressure Raman study of a one-dimensional hydrogen-bonded crystal of cyanoacetylene (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 2814-2817 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: At room temperature, cyanoacetylene undergoes a liquid–solid transition at 70±30 MPa and further a polymerization reaction at 1.5 GPa. Raman spectra indicates that the crystal structure of the high-pressure phase is isomorphic with the monoclinic low-temperature structure (P21/m, Z=2), in which linear cyanoacetylene molecules are joined with CH NC hydrogen bonds to form infinite one-dimensional chains. The internal stretching frequency of the C–H proton donor decreases wtih increasing pressure, while that of the C 3/4 N proton acceptor increases. The librational modes associated with the rotational motions of the rigid molecules show large shifts to higher frequencies with pressure. The observed frequency shifts are attributed to the strengthening of the CH⋅⋅⋅NC hydrogen bond in the one-dimensional molecular chain.
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    Solvent dynamical effects in electron transfer: Predicted influences of electronic coupling upon the rate-dielectric friction dependence (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 906-916 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The predicted dependence of the bimolecular rate constant for outer-sphere electron exchange kex upon the longitudinal relaxation time τL for Debye solvents is examined numerically on the basis of a suitably combined rate formulation in order to examine the manner and extent to which the rate-solvent friction dependence, of particular experimental significance, should be sensitive to the degree of donor–acceptor electronic coupling and related factors. The treatment accounts for the contributions to kex from a spatial distribution of reactant pairs as well as for the effects of donor–acceptor interactions upon the unimolecular rate constant, ket (s−1), for each encounter geometry. The latter include the influence of electronic interactions as prescribed by the matrix coupling element H12 upon the effective frequency for adiabatic barrier crossing νn, as well as upon the electronic transmission coefficient κel (i.e., the degree of reaction nonadiabaticity). The anticipated dependence of the free-energy barrier, as well as νn and κel, upon the donor–acceptor separation is accounted for in the kex calculations. The form of the resulting log kex −log τ−1L plots exhibit, as expected, a marked dependence on the value of H12 for reactant contact, H(open circle)12. Over the τ−1L range appropriate for common solvents at ambient temperatures, ∼1×1011 to 5×1012 s−1, the log kex −log τ−1L slopes, x, are significantly (or substantially) below unity for H(open circle)12(approximately-less-than)0.5 kcal mol−1; x decreases markedly with increasing τ−1L and/or for decreasing H(open circle)12. The approach to the solvent inertial limit (for large τ−1L) also tends to decrease x further. Numerical comparisons are made with corresponding log kex −log τ−1L plots obtained using the simple "encounter preequilibrium'' treatment which presumes that only a fixed narrow range of encounter-pair geometries contribute to kex. While the form of theplots are not greatly different, former more sophisticated treatment generally yields smaller slopes. Some corresponding numerical calculations for electrochemical-exchange reactions are also included.
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    Relaxation behavior in a bistable chemical system. Plateau behavior (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 938-942 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The relaxation behavior of the bistable iodate–arsenous acid reaction near a hysteresis limit has been investigated. Outside the bistability region, relaxations to the unique stable steady state may exhibit a plateau in concentration as a function of time, corresponding to a local minimum in rate of reaction. Very close to the saddle-node bifurcation defining the bistability limit, the relaxing system remains in the plateau for long periods of time. Experimental measurements of plateau behavior are compared to behavior predicted by a one-variable model of the iodate–arsenous acid system.
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    Development of the flowing afterglow/Langmuir probe technique for studying the neutral products of dissociative recombination using spectroscopic techniques: OH production in the HCO+2+e reaction (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 963-973 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The flowing afterglow/Langmuir probe (FALP) technique has been extended to enable the neutral products of electron–ion dissociative recombination in thermalized afterglows to be identified by spectroscopic methods. Absolute number densities of H atoms in the afterglow have been determined using vacuum ultraviolet (VUV) absorption at the Lα wavelength. By exploiting the reaction H+NO2→OH+NO, all of the H atoms can be incorporated into OH molecules and thus observation of the intensity of laser induced fluorescence (LIF) If, obtained by exciting the (1,0) band of OH(A 2Σ←X 2Π), allows a calibration to be obtained of If against the known number density of OH X 2Π(ν‘=0) in the afterglow. Following this procedure, a recombining HCO+2 /electron afterglow was probed for production of ground state OH X 2Π(ν‘=0) using LIF and it was established that OH(ν‘=0) resulted from 17% of the recombining ground state HCO+2 ions. It was also established that a further 17% of the recombinations resulted in OH(ν‘〉0), i.e., that, in total, (34±6)% of the HCO+2 ions recombine to produce OH X 2Π radicals, either directly or via the electronically excited A 2Σ state. Details of the calibration procedure for H and OH number densities, of the ion chemistry involved in the production of the HCO+2 afterglow plasmas and of the checks carried out to establish that the fluorescence observed was from OH produced in the recombination reaction are presented. During these experiments, the rate coefficient at 300 K for the H+NO2 reaction was determined to be 1.3×10−10 cm3 s−1 from observations of the H-atom decay as a function of NO2 number density in the afterglow (in good agreement with previous determinations). Also the rate coefficient for the quenching reaction of OH(ν‘〉0) with NO to produce OH(ν‘=0) was determined to be 6×10−11 cm3 s−1.
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    A simple model for correcting the zero point energy problem in classical trajectory simulations of polyatomic molecules (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 2863-2868 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A simple model is proposed for correcting problems with zero point energy in classical trajectory simulations of dynamical processes in polyatomic molecules. The "problems'' referred to are that classical mechanics allows the vibrational energy in a mode to decrease below its quantum zero point value, and since the total energy is conserved classically this can allow too much energy to pool in other modes. The proposed model introduces hard sphere-like terms in action–angle variables that prevent the vibrational energy in any mode from falling below its zero point value. The algorithm which results is quite simple in terms of the cartesian normal modes of the system: if the energy in a mode k, say, decreases below its zero point value at time t, then at this time the momentum Pk for that mode has its sign changed, and the trajectory continues. This is essentially a time reversal for mode k (only!), and it conserves the total energy of the system. One can think of the model as supplying impulsive "quantum kicks'' to a mode whose energy attempts to fall below its zero point value, a kind of "Planck demon'' analogous to a Brownian-like random force. The model is illustrated by application to a model of CH overtone relaxation.
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    Outer-sphere electron transfer reactions in non-Debye solvents. Theory and numerical results (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 2869-2880 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In this paper we present a theory that predicts, for a wide range of reaction adiabaticity, the time evolution of the reactants' population, σR(t), for outer-sphere electron transfer reactions in polar solvents exhibiting a non-Debye relaxation behavior. A number of stochastic trajectory studies have been performed to test the validity of the theory in the limit where solvent relaxation constitutes the rate-limiting step. The agreement between the numerical and theoretical results is excellent. The theory is then used to study a number of different reactive regimes ranging from nonadiabatic to solvent-controlled adiabatic limits. The studies performed show that the character of the decay of σR(t) for low activation barrier reactions constitutes a useful qualitative way of identifying a priori the dielectric relaxation properties of the solvent: For a non-Debye solvent the decay of σR(t) is strongly nonexponential in contrast to the results for Debye solvents where σR(t) decays mainly exponentially. The solvent relaxation time scales that are important for the reaction kinetics are identified. Our results are also expressed in terms of rate kernels. For high energy barriers the Stable States Picture results for the forward and reverse rate constants are recovered.
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    Examination of a generalized model for radiationless energy transfer in dyes. Comparisons of theory and experiments (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 2892-2897 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A modified general model for radiationless energy transfer is examined. The model does not rely on selection criteria and is intended to apply over the range of diffusion and resonance energy transfer parameters normally encountered in quenching experiments. Experimental data is compared with theoretical predictions and found to confirm the model's validity for a wide range of physical conditions.
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    Photodissociation of Cr(CO)6 in a molecular beam at 248 nm: Observation of primary and secondary photoproducts by mass spectrometry (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 2881-2891 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Photodissociation of Cr(CO)6 induced by a KrF* excimer laser (248 nm) was examined under molecular beam conditions, using a quadrupole mass spectrometer with electron impact ionization (11–19 eV) to detect the products as their respective ions. The ions of all Cr(CO)x products (x=0–5) were produced upon photodissociation of Cr(CO)6 at laser fluences from 1 to 10 mJ cm−2. The quantitative behavior of the Cr(CO)+x product ion yields vs laser fluence as well as statistical rate calculations indicate that Cr(CO)4 is the major product formed upon single-photon dissociation of Cr(CO)6 under collisionless conditions at 248 nm. The additional Cr(CO)+x (x〈4) product ions observed at low fluences are formed primarily by electron-impact fragmentation of Cr(CO)4 in the mass spectrometer ionizer. At laser fluences (approximately-greater-than)5 mJ cm−2, secondary photodissociation of Cr(CO)4 becomes important. Cr(CO)2 is assigned as the major photoproduct formed via secondary photodissociation of Cr(CO)4.
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    A state-selected study of the unimolecular decomposition of C2H+2(A˜,B˜) using the photoion–photoelectron coincidence method (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 2898-2900 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The photoion–photoelectron coincidence spectra for C2H+ and C2H+2 have been measured in the wavelength range of 645–765 A(ring). The C2H+2(A˜ 2Ag,B˜ 2∑+u) ions prepared with internal energies above 17.39 eV are found to dissociate completely into C2H++H in the temporal range 〈12 μs. An upper bound of 17.33±0.05 eV is determined for the appearance energy of the process C2H2+hν→C2H++H+e− at 0 K.
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    Calculation of resonant states using an optimal path of continuous extension of the bound states: The cubic and quartic energy distortion of the harmonic oscillator (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 7780-7786 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Resonant eigenvectors are calculated within the framework of the complex rotation theory using a continuous deformation and extension of the localized bound states through the potential barrier. The vector is obtained by gradually increasing the basis size and defining for each new basis a lesser perturbation path which best verifies the complex extension of the virial theorem. The modifications resulting from the increase of the basis set N and from the variations of the optimal rotation angle θ(N) are well handled using a Bloch wave operator formulation.
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    Analysis of the potential energy surface of Ar–NH3 (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 7809-7817 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The combination of supermolecular Møller–Plesset treatment with the perturbation theory of intermolecular forces is applied in the analysis of the potential energy surface of Ar–NH3. Anisotropy of the self-consistent field (SCF) potential is determined by the first-order exchange repulsion. Second-order dispersion energy, the dominating attractive contribution, is anisotropic in the reciprocal sense to the first-order exchange, i.e., minima in one nearly coincide with maxima in the other. The estimated second-order correlation correction to the exchange effect is nearly as large as a half ΔESCF in the minimum and has a "smoothing'' effect on the anisotropy of ε(20)disp. The model which combines ΔESCF with dispersion energy (SCF+D) is not accurate enough to quantitatively describe both radial and angular dependence of interaction energy. Comparison is also made between Ar–NH3 and Ar–PH3, as well as with the Ar dimer.
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    The adsorption of hydrogen on Si(111)-7×7 as studied by multiple internal reflection spectroscopy (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 7978-7987 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Multiple internal reflection (MIR) infrared spectroscopy has been used to study the adsorption of hydrogen on the Si(111)-7×7 surface over the temperature range of 100–400 K. The high resolution of this technique has enabled us to observe three different silicon monohydride stretch modes at 2076, 2086, and 2095 cm−1. Using the polarization selection rules for infrared spectroscopy we have assigned these modes to a silicon monohydride whose bond axis is parallel to the surface normal and two monohydrides whose bond axes are tilted with respect to the surface normal. We have also observed the temperature dependent formation of surface dihydride and trihydride species. The formation of both the dihydride, observed at 2119 cm−1, and the trihydride, seen at 2143 cm−1, is favored by low adsorption temperatures. Finally, we also report a temperature dependent competitive formation between the two higher hydrides with the formation of the dihydride species maximizing near 200 K and lower temperatures favoring the surface trihydride.
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    Cooper minima and rotationally resolved resonance enhanced multiphoton ionization spectroscopy (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 7995-7997 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We demonstrate that a Cooper minimum, close to threshold, in photoionization via an excited molecular Rydberg state can have a dramatic influence on the ionic rotational branching ratios. It is also shown that this behavior can be exploited to produce ions selectively in a specific rotational level. To illustrate this effect we present the results of ab initio calculations for (2+1') resonance enhanced multiphoton ionization via the O11 (23.5) branch of the H 2Σ+(3d,4s) state of NO, where a Cooper minimum is found in l=3 of the kσ and kπ continua at photoelectron kinetic energies of 2.6 eV and 2.9 eV, respectively.
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    URL: http://dx.doi.org/10.1063/1.457219
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    Observation of direct and collision-induced double resonance of a molecular ion (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 7997-7999 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The double resonance technique has been applied to a molecular ion. We have observed microwave-infrared double resonance signals of HN+2 produced in a hollow cathode discharge at low pressures (20–200 mTorr). A color center laser with 2–20 mWatts of power provided the infrared signal radiation, and a millimeter wave klystron with ∼400 mWatts of power provided the microwave pump radiation. The frequency of the microwave radiation was swept and the double resonance signal was detected through variation of the infrared power. Using the R(1) and P(1)infrared transitions we have observed the J=1←0 rotational transition in both the ground state and the ν1 state. We have observed not only the direct three-level double resonance signals, but also collision-induced four-level double resonance signals indicating the existence of some selection rules for the Langevin potential dominated ion–neutral interaction. The major results of this paper are: (1) The double resonance method is now applicable to molecular ions and (2) ion–molecule collisions have selection rules.
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    URL: http://dx.doi.org/10.1063/1.457220
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    Analysis of CH2 a˜ 1A1 (1,0,0) and (0,0,1) Coriolis-coupled states, a˜ 1A1–X˜ 3B1 spin–orbit coupling, and the equilibrium structure of CH2 a˜ 1A1 state (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 6566-6578 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The symmetric and antisymmetric stretch spectra of a˜ 1A1 CH2 are measured with Doppler-limited resolution by infrared flash-kinetic spectroscopy between 2600 and 3050 cm−1. The spectra are significantly perturbed by spin–orbit interactions between near-resonant levels of X˜ 3B1 in both lower and upper levels of the transitions, and by Coriolis interaction between symmetric and antisymmetric stretch states. The triplet character of the perturbed levels is detected by observing broadening of MJ structure in a magnetic field of about 1 kG. The levels that are likely to be perturbed by triplet methylene are identified using the experimentally observed and theoretically calculated 3B1 energy level structure. The term values of the remaining symmetric and antisymmetric stretch levels are simultaneously fit to Watson's Hamiltonian including Coriolis coupling. This analysis provides the remaining information needed to determine the equilibrium rotational constants [Ae =19.8054(206), Be=11.2489(45), and Ce =7.2386(36) cm−1 ], and, therefore, the equilibrium structure (re=1.107 A(ring), and θe =102.4°) of this fundamentally important carbene. The experimental and theoretical structures are in good agreement.
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    The intramolecular vibrations of the ammonia water complex. A matrix isolation study (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 6604-6612 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The infrared spectrum of the ammonia water complex has been recorded from 10 to 4000 cm−1 for complexes trapped in neon and krypton matrices. Spectra have been observed for NH3 or ND3 complexed with H2O, HDO, or D2O. The observed intramolecular fundamentals are compared with frequencies calculated from a rigid-molecule harmonic oscillator model.
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    An adapted form of the collocation method for calculating energy levels of rotating atom–diatom complexes (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 6598-6603 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A method is presented for calculating energy levels of atom–rigid-diatom systems for various values of the total angular momentum (J) of the complex. The technique is based upon the collocation method for the vibrational motions of the system and the Galerkin approach for the total rotation. Unlike the Rayleigh–Ritz variational principle, the method does not require the evaluation of integrals over the Hamiltonian and so is very simple to implement. An important feature of the method is that the wave function is obtained in an analytic form and so it is a simple matter to calculate many quantities of spectroscopic interest such as rotational constants and spectral intensities. It is also shown that contracted basis sets can be used in conjunction with the collocation method to enhance the efficiency of the calculation. The method is demonstrated by calculating rovibrational levels of the van der Waals complex Ar–HCl for J up to 10.
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    URL: http://dx.doi.org/10.1063/1.457378
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    Time-resolved electron energy loss spectroscopy study of water desorption from Ag(011) (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 7964-7971 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present time-resolved electron energy loss spectroscopy (EELS) results for the desorption of submonolayer coverages of H2O and D2O from Ag(011). The measurements were performed isothermally and effectively at constant coverage. At coverages above a few percent of a monolayer, we observe that the desorption is characterized by zero-order kinetics. That is, the desorption rate is independent of coverage. We hypothesize that this results from the formation of two-dimensional water islands on the surface at all coverages and temperatures measured. The desorption process is governed in part by the dissociation of water from the edges of these islands. Rapid diffusion on the surface maintains a quasiequilibrium between the molecules bound to islands and isolated adsorbed water molecules, thereby leading to pseudo-zero-order kinetics. This result is interpreted upon general thermodynamic grounds, and also using a simple kinetic model which leads to pseudo-zero-order desorption kinetics. An unusual hydrogen kinetic isotope effect is reported which lends support to our interpretation.
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    URL: http://dx.doi.org/10.1063/1.457214
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    Photoionization thresholds of rare gas clusters (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 7972-7977 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Photoionization efficiency curves of rare gas clusters generated by supersonic expansion of pure argon, krypton, and xenon gas have been measured as a function of photon energy using light from the Berlin Electron Synchrotron Facility (BESSY). Assuming that the relative shapes of the efficiency curves are independent of cluster size, ionization potentials for each cluster can be extracted from the data by using the known absolute ionization thresholds of the dimer or trimer ions. The data agree with the model that ionic clusters are composed of an ionic substructure surrounded by shells of atoms. In particular, it has been confirmed that Ar+3 is the central substructure in Ar+n for n〈15±1. In addition we have estimated the ratio of the covalent binding energy to the charge-induced dipole binding energy for the ions. The larger dipole-induced binding energy in the case of xenon may explain the differences between the mass spectra of Xe and Ar clusters.
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    URL: http://dx.doi.org/10.1063/1.457215
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    Raman-ultraviolet optical double resonance spectroscopy in gas-phase acetylene (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 7994-7995 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457218
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    The Stark effect in the B 3Π–X 3Δ band system of TiO (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 8000-8002 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457222
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    Time dependence of hole spectrum due to dispersive burning kinetics in dye-doped polymers (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 6579-6587 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Time evolution of a persistent hole spectrum has been investigated for dyes doped in polyvinyl alcohol films at 2 K. The zero-phonon hole profile measured for resorufin using a single-mode dye laser has been found to be nearly Lorentzian even in the case of relatively deep holes. The observed fluence-dependent depth and width of the zero-phonon hole are reproduced well by the numerical simulation based on a dispersive burning-kinetics model. It is pointed out that a large distribution of the burning rate is responsible for the fluence-dependent hole broadening. Time evolution of the phonon-sideband hole simulated using the same model is also compared with the experimental data. It is shown that the low-energy sideband hole is enhanced when the burning kinetics is dispersive.
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    The rotational spectrum and structure of chlorine peroxide (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 6588-6597 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The products of the ClO self-reaction have been studied in a flowing chemical reactor using submillimeter wave spectroscopy. The complete spectrum between 415 to 435 GHz has been measured as well as selected transitions in the range 285 to 415 GHz. The major products have been identified as the ClO dimer (Cl2O2) and chlorine dioxide (OClO). The observed rotational b-type spectra of the most abundant isotopic species35 ClOO35Cl and 37ClOO35Cl have been analyzed. The observed nuclear spin statistics for the main species, the relative abundance of the lesser species, and the structure determination demonstrate unambigiously that the ClO dimer must possess identical chlorine atoms in a peroxide structure. The rotational constants as well as a complete set of quartic centrifugal distortion constants have been determined. Structural parameters for the vibronic ground state have been calculated: rOO=142.59(21) pm, rClO=170.44(4) pm, (angle)ClOO=110.07(1)° and dihedral angle=81.03(1)°. Rotational transitions in the first excited torsional state have been measured and analyzed for the isotopic species 35,35 and 35,37. An effective molecular structure for the first torsional excited state has been calculated. A torsional frequency has been derived from relative intensity measurements and the cis and the trans barrier heights estimated.
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    High-resolution near-infrared spectroscopy of water dimer (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 6613-6631 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: High-resolution near-infrared spectra are reported for all of the O–H stretch vibrational bands of the water dimer. The four O–H vibrations are characterized as essentially independent proton-donor or proton-acceptor motions. In addition to the rotational and vibrational information contained in these spectra, details are obtained concerning the internal tunneling dynamics in both the ground and excited vibrational states. These results show that for tunneling motions which involve the interchange of the proton donor and acceptor molecules, the associated frequencies decrease substantially due to vibrational excitation. The predissociation lifetimes for the various states of the dimer are determined from linewidth measurements. These results clearly show that the predissociation dynamics is strongly dependent on the tunneling states, as well as the Ka quantum number, indicating that the internal tunneling dynamics plays an important role in determining the dissociation rate in this complex.
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    URL: http://dx.doi.org/10.1063/1.457380
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    Dynamics of resonant third-order optical nonlinearity in perylene tetracarboxylic dianhydride studied by monitoring first- and second-order diffractions in subpicosecond degenerate four-wave mixing (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 6643-6649 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present a novel approach using simultaneous monitoring of temporal behavior of the usual phase conjugate signal and the second-order diffraction produced in a degenerate four-wave mixing experiment to obtain information about dynamics of resonant third-order nonlinear optical processes. The second-order diffraction is interpreted as arising from the presence of the appropriate Fourier component of the excited state grating. The higher Fourier components are expected to be generated in the presence of such excited state processes as bimolecular decay, two-photon absorption, saturation of absorption and diffusion of excitation. Third-order nonlinear optical properties of an organic dye: perylene tetracarboxylic dianhydride were studied by this approach using subpicosecond degenerate four-wave mixing at a wavelength of 602 nm. The dye is found to exhibit a strong resonant nonlinear effect with an effective χ(3) of ≈2×10−10 esu. The decay of the phase conjugate signal is power dependent and can be theoretically simulated by using a combination of monomolecular and bimolecular decay laws. The temporal behavior of the phase conjugate signal and the second-order diffraction are investigated at different intensities. The observed characteristics are well simulated by using a dominant bimolecular decay mechanism at higher excitation density.
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    Observation of an excimer emission band of the ArOD van der Waals molecule (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 6657-6663 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An excimer emission band attributed to a transition from the first excited state to the ground state of the van der Waals Ar(OD) molecule has been observed in experiments where D2O was dissociated in presence of rare gases by two photons of a quadrupled YAG laser (266 nm). KrOD has also been observed in the same experiments. The ArOD excimer band comprises three main peaks at 313.9, 315.1, and 316.0 nm as well as several minor ones nearer to the main OH (A2 Σ+ ,v=0–X2 Π,v=0) emission band. The emission profile is realistically simulated by a one dimension model using recently published potentials for the excited state and modeling the ground state by that of the ArF X 2 Σ+ state. This result tends to demonstrate that the ground state geometry of Ar(OD) involved in the transition is probably L shaped although the high intensity of the observed excimer band is not satisfactorily explained.
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    Magnetic circular dichroism spectroscopy and magnetization studies of iron atoms isolated in xenon matrices (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 6677-6683 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Absorption and magnetic circular dichroism (MCD) spectra of iron atoms isolated in xenon matrices have been measured and their magnetization properties studied. The high signal intensity and optical quality of the matrices has allowed the assignment of many more transitions than hitherto. The magnetization measurements reveal considerable guest host interaction, though there is little evidence of this in the spectra. Detailed analysis of the magnetization data shows that they can be interpreted quite well in terms of an essentially axial crystal field splitting of the 5 D4 ground state which places the two ||MJ||=4 levels about 100 cm−1 below the ||MJ||=3. By extending the theoretical treatment to include a small mixing of the two ||MJ||=4 levels it is possible to improve the fit of the higher temperature data. These results demonstrate the value of MCD measurements, particularly where a system is electron paramagnetic resonance (EPR) silent.
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    Observation of "magic numbers'' in the population distributions of the (NH3)n−1NH+2 and (NH3)nH+2 cluster ions: Implications for cluster ion structures (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 6684-6690 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have measured the relative yields of the cluster ions (NH3)n−1NH+2 and (NH3)nH+2 produced by electron impact ionization of a supersonic molecular beam of neutral ammonia clusters, as functions of cluster ion size, neutral beam stagnation temperature, and electron impact energy. Our observation of a magic number at n=7 in the distribution of (NH3)n−1NH+2 cluster ions is interpreted in terms of an intracluster bimolecular association reaction between a nascent NH+2 daughter ion and an adjacent NH3 solvent molecule, giving rise to a protonated hydrazine product ion. Our observation of magic numbers at n=5 and n=8 in the (NH3)nH+2 cluster ion distribution is consistent with the production of the N2H+8 Rydberg radical cation via an intracluster ion–molecule reaction. Our experimental results are consistent with recent theoretical predictions concerning the structure and stability of N2H+8 ions.
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    Surface chemistry of metastable oxygen. I. Production and loss of the 4–5 eV states (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 7936-7946 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In an attempt to isolate the important parameters governing the production of the 4–5 eV states of O2 generated from O-atom recombination on metallic surfaces, we have investigated the effects of substrate composition and temperature on the intensity of the UV glows that are generated. Of the 22 metals investigated, glow-producing activity is localized in the VIIIb group on the Periodic Table, led by Ni and including Pt, Co, Pd, Fe, and also Au. One parameter that appears to be directly or indirectly linked to this effect is the electronic heat capacity. The six active metals exhibit unique temperature signatures, with Fe, Ni, and Co being distinctly different from Pd, Pt, and Au, measured over the 220–470 K range. The spectra of the UV glows, consisting of the A–X, A'–a, and c–X transitions of O2, differ only in intensity between the metals, not in state or vibrational level distributions. This suggests that the observed spectra are mediated by gas-phase interactions. The data indicate that at high temperatures, surface quenching of the A, A', and c states is rapid, and that under low-pressure conditions, this effect can be observed even for excited molecules generated in the gas phase. Studies with nickel alloys indicate that the percentage of Ni in a sample is the overriding determinant of the glow intensity, regardless of the other components. Although existing data on energy accommodation coefficients lead to the conclusion that most of the energy in O2 molecules leaving surfaces should be in vibrational modes of the ground state, we have not been able to substantiate this idea.
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    A hole burning study of excitonic states of chain molecules in glasses (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 7988-7993 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Thermal broadening of spectral holes burnt into excitonic states of long chain molecular aggregates of pseudoisocyanine iodide is investigated over a temperature range from 350 mK to 80 K. The results differ very much from the usual behavior of small molecules in glasses. We found that extended states are almost completely decoupled from the amorphous host lattice and spectral diffusion effects play a minor role. The homogeneous linewidth is independent of temperature below 10 K. Above, thermal broadening occurs in two steps: there is a weak onset around 15 K and a strong onset around 65 K. The data can be fitted by a superposition of two exponentials.
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    Equilibrium distributions of alkanes at a fixed partial pressure of molecular hydrogen (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 7999-8000 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In calculating chemical equilibrium distributions in gaseous systems containing hydrocarbons and molecular hydrogen, the degrees of freedom are usually taken to be T, P, and H/C, but there may be reasons for taking T, P, PC2H4 or T, P, PH2. The standard Gibbs energies of formation in homologous series are closely represented by A+Bn, where n is the carbon number and A and B depend only on the temperature. At a specified ethylene partial pressure, the equilibrium distribution within a homologous series depends only on B, and this note shows that at a specified hydrogen partial pressure, the equilibrium distribution depends only on A.
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    Comment on "Relation between stretched-exponential relaxation and Vogel–Fulcher behavior above the glass transition'' (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 8002-8003 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
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    URL: http://dx.doi.org/10.1063/1.457223
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    An electron spin-echo envelope modulation study of the lowest triplet state of pyridine-d5: Spin-density distribution and structure (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 6549-6565 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Recently we have shown that the lowest triplet state (T0) of pyridine, incorporated in a single crystal of benzene, may be studied by electron spin-echo (ESE) spectroscopy. From the nitrogen hyperfine structure in the ESE detected electron paramagnetic resonance (EPR) spectra, we were able to conclude that pyridine, a planar molecule in the ground state, becomes nonplanar upon excitation into T0. Here we report the results of a detailed investigation of this distortion and of the electronic nature of the lowest triplet state of pyridine-d5. We have performed electron spin–echo envelope modulation (ESEEM) spectroscopy. From the modulation spectra, the electron-nuclear double resonance (ENDOR) frequencies corresponding to the various deuterium nuclei are obtained. Analysis of the dependence of these frequencies on the orientation of the magnetic field with respect to the triplet fine-structure axes system allows for a determination of the deuterium hyperfine and quadrupole tensors. From these tensors and the known nitrogen hyperfine tensor, the structure and spin-density distribution of pyridine in its lowest triplet state are deduced. Pyridine adopts upon excitation into T0 a boatlike structure, in which the nitrogen atom is tilted by about 40° with respect to the plane through the ortho- and meta-carbon atoms and the para-carbon/para-deuterium fragment by about 10° with respect to this plane. Thereby the hybridization of the ortho-carbon atomic orbitals strongly deviates from that for aromatic hydrocarbons and becomes almost sp3; the hybridization of the atomic orbitals on the other carbon atoms changes much less, while the nitrogen atomic orbitals remain sp2 hybridized. Approximately half of the spin density is found to be localized on the nitrogen atom with a remarkable distribution over the atomic orbitals: the π orbital carries 40%, the n orbital 10%. The rest of the spin density is distributed over the para- (30%) and ortho-carbon atoms (10% each). The lowest triplet state of pyridine is neither an nπ* state nor a ππ* state, but a state of mixed character as the result of a strong vibronic coupling between the 3B1 (nπ*) and 3A1 (ππ*) states.
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    Photoionization and photoelectron spectroscopy of O2 with coherent vacuum ultraviolet radiation (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 6632-6642 
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    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The photoionization of O2 near its ionization limit has been studied with coherent vacuum ultraviolet radiation produced by third harmonic generation in free jet expansions of the rare gases. High resolution (∼2 cm−1) photoionization spectra were obtained in the ionization threshold region from 103 to 98 nm which includes three vibrational levels of the H 3Πu (3sσ) Rydberg state. The H, v=0 photoionization spectrum was assigned by simulating the H 3Πu←X 3∑+g Rydberg excitation, yielding spectroscopic constants as well as the overall autoionization lifetime. The v=1 and 2 levels have distinctly different rotational band contours which reflect perturbations with bound and dissociative levels of nearby "dark'' states. The photoionization dynamics were probed further through measurements of photoelectron angular distributions for the v+=0 and 1 vibrational levels of O+2. In addition to strong variations in the asymmetry parameter (β) across the H state autoionization resonances, spectrally narrow variations in β were found in the surrounding continuum. These latter results suggest the presence of weak resonance features imbedded in the background continuum which nonetheless strongly influence the photoelectron ejection dynamics.
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    URL: http://dx.doi.org/10.1063/1.457381
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    Dipolar relaxations in solid (CF4)1−x(CClF3)x (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3111-3118 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: (CF4)1−x(CClF3)x shows a eutectic phase diagram with a eutectic temperature TE =69 K and a eutectic concentration xE =0.54. In the mixed crystal regime x〈0.015, dielectric audio frequency spectroscopy revealed the dynamics of weakly coupled CClF3 molecules performing thermally activated reorientations. An analysis of the data was performed in terms of a relaxator model including a critical comparison of different distributions of relaxation times. The static susceptibilities of the mixed crystals suggested a two-phase model of CF4, where rigid latice domains coexist with paraelastic regions. Several pure CClF3 samples were investigated using different thermal treatments. The dielectric dispersion of this compound is described by the reverse Havriliak–Negami expression and possible explanations for the origin of the relaxation spectra are discussed. The behavior of the CClF3 rich mixed crystals is considered briefly.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456933
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    A realistic molecular dynamics simulation of the plastic crystalline phase of neopentane. I. The model and its static properties (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3119-3132 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A molecular dynamics simulation of a realistic model of neopentane [C(CH3)4] in its plastic phase has been performed on a sample of 6×6×6 fcc unit cells (i.e., 864 molecules) at 135, 175, and 230 K. The molecules of the simulated sample interact through phenomenological exp-6, atom–atom potentials between all the atoms of nearest neighbor molecules. The orientational probability density function (opdf), the displacement probability density function (dpdf), and its second moment the Debye–Waller factor have been computed. We confirm the very large value and the important thermal variation of the Debye–Waller factor and the strong anisotropy of the opdf deduced from neutron diffraction experiments. The computed opdf is very well reproduced by a mean-field calculation making use only of the microscopic intermolecular potential and of the equilibrium position of the molecular centers of mass, a result in line with the isotropic character of the dpdf, but not valid for other plastic crystals made of molecules with different geometries.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456934
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    High-pressure equations of state of krypton and xenon by a statistical mechanical theory (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3133-3147 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present statistical mechanical calculations for krypton and xenon, employing accurate pair potentials with and without condensed-phase modifications. A unique feature of the present work is that solid- and fluid-phase thermodynamic properties are both computed within a single framework, using our recently developed hard-sphere perturbation theory. Results are applied to analyze experimental fluid, solid, and fluid–solid transition data, ranging up to 2×106 atmospheres at several temperatures. Effective pair potentials for both krypton and xenon, inferred from the analysis, contain short- and long-range modifications to the pair potential of Aziz and Slaman. The long-range correction is repulsive and originates from the well-known Axilrod–Teller three-body potential, while the short-range correction is attractive and is needed for describing high-compression data. Experimental isotherms above 50 GPa for xenon require a further softening of the short-range repulsion from Barker's correction (obtained from experimental data below 50 GPa). Implications of the short-range correction and its possible relation to many-body forces are discussed. Additional tests of the present rare-gas calculations against available computer simulations and Monte Carlo and lattice-dynamics calculations carried out in this work show satisfactory agreement. Computation of solid–fluid transition properties shows that the Axilrod–Teller three-body potential must be included to obtain reliable agreement with experimental melting and freezing data.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456935
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    Investigation of diffusion processes in poly(ethylene oxide)/poly(methyl methacrylate) blends by laser light scattering spectroscopy (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3160-3167 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Mutual diffusion in poly(ethylene oxide)/polymethyl methacrylate blends is investigated by using the photon correlation spectroscopic technique. The mutual diffusion coefficient and the static structural factor are measured as a function of concentration for various temperatures. The Onsager coefficient is obtained by simultaneous measurements of the diffusion coefficient and static structural factor. The result is employed to test theoretical predictions on the concentration dependence of the mutual diffusion coefficient of the polymer blend. The experiment results are found not in agreement with theoretical predictions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457643
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    Isothermal–isobaric molecular dynamics simulation of polymorphic phase transitions in alkali halides (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3148-3159 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: As the first application of the isothermal–isobaric molecular dynamics technique to a system with Coulombic forces, the pressure-induced face-centered-cubic to body-centered-cubic transition of some alkali halides is studied assuming Born–Mayer–Huggins potentials between the ions. The long-range forces are handled by the cubic harmonic expansion of the Ewald summation. The property that any periodic boundary condition of orthorhombic symmetry can be uniformly treated in this expansion allows the independent fluctuation of each box length in the course of the simulation of the isobaric ensemble. This reduces stresses during phase transitions from one crystalline state to another and yields less defects. By the appropriate change in the external pressure, the fcc–bcc transition takes place reversibly only in the presence of a considerable amount of defects. Otherwise, quite large hysteresis is found. It is shown that the [100], [010], and [001] directions of the original fcc lattice become the [ 1/2 1/2 1¯], [1¯01], and [111] directions in the bcc lattice, respectively. This confirms the Watanabe–Tokonami–Morimoto mechanism supported also by most of the experimental observations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456936
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    Contribution of covalent bond force to pressure in polymer melts (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3168-3173 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Molecular dynamics simulations of the freely jointed effective hard sphere model of a polymer melt are performed, with continuous covalent and noncovalent potentials employed for computational convenience. The virial theorem is used to determine the melt pressure from the simulations. Excellent agreement is found over a wide range of density and degree of polymerization with the recently introduced equation of state of Honnell and Hall. The atomic pressure is defined as the contribution made by each atom to the compressibility factor of the system; it is found to be uniform along the interior atoms of the polymer chain, with slightly different values for the end atoms. This uniform value is almost independent of N, the number of bonds in the chain, and explains the small dependence on N of the pressure that is observed. Particular attention is paid to the contribution to the pressure made by the force in the covalent bonds of the system. For packing fractions greater than 0.3, approximately 30% of the pressure in the melt arises from this source.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456937
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    Resonance enhanced electron stimulated desorption (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3174-3181 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A theory is presented which accounts for "giant'' enhancements in electron stimulated desorption (ESD) yields from adsorbate-covered surfaces if the incident electrons become trapped in a shape or Feshbach resonance associated with the adsorbate. The resulting temporary negative ion is displaced inwards towards the surface as a result of the force provided by the image screening charge. Upon reneutralization, the "desorbate'' can be returned high on the dissociative repulsive wall of the neutral-surface potential curve. This process has been modeled within the context of semiclassical Gaussian wave packet dynamics. Recent observations of such giant enhancements in the ESD yields for the system O(a)/Pd(111) are explained in terms of this model, and an atomic physics basis for the resonance in atomic oxygen is proposed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456938
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    Rotational population and alignment distributions for inelastic scattering and trapping/desorption of NO on Pt(111) (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3182-3195 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Rotationally resolved experiments on the NO/Pt(111) system explore the mechanisms of inelastic scattering and trapping/desorption. The rotational dynamics associated with these two regimes are markedly different. A neat supersonic NO beam is scattered at normal incidence from a Pt(111) crystal at 375–475 K. The non-Boltzmann rotational population distribution of the scattered species exhibits considerable rotational excitation beyond the energy available from the incident beam. Thus, a surface vibration to rotational energy transfer mechanism must be operative. The accompanying rotational alignment data reveal that highly excited rotational states exhibit predominantly "cartwheel'' motion. In contrast, rotationally excited molecules that desorb from a 553 K Pt(111) surface show a preference for "helicopter'' motion. The opposite preferences for rotational alignment in the two dynamical regimes provide insight into the anisotropy of molecule–surface interactions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456939
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    Simplified trajectory method for modeling gas–surface scattering: The NO/Pt(111) system (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3196-3207 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A model is presented to describe the dynamical processes of trapping/desorption as well as direct and indirect inelastic scattering on single-crystal surfaces. Newton's equations of motion are integrated for a system consisting of a rigid rotor interacting with a slab of 19 surface atoms. The surface atom which is closest to the center of mass of the molecule is permitted to translate only along the surface normal. In turn, this mobile surface atom is harmonically coupled to a microcanonical heat bath consisting of three subsurface atoms. This method is much less computationally intensive than the typical generalized Langevin equation (GLE) approach. Direct comparison is made between the predictions of this model and experiment for the NO/Pt(111) system. In the case of trapping/desorption, the model accurately describes the observed dependence of rotational alignment on rotational quantum number. For the inelastic scattering regime, the model successfully reproduces the degree of rotational excitation and qualitatively accounts for the observed rotational alignment. In addition, the model predicts correlations between final state velocity and final state rotational angular momentum (both direction and magnitude), as well as the effect of molecular orientation and surface impact parameter on the overall trapping probability.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456940
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    The theory of semipermeable vesicles and membranes: An integral-equation approach. III. Vesicles with internal nonpermeating ions (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3208-3211 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The system of a fluid in the presence of a spherical semipermeable vesicle (SPV) with the freely mobile nonpermeating species inside the vesicle is investigated via an integral-equation approach. This system can be used to model certain feature of a biological cell, permeable to simple ions, in which solute proteins inside the cell are unable to permeate its walls. As an illustrative example of the use of our integral equations, the analytical solution for density profiles in the mean-spherical approximation/Debye–Hückel approximation (MSA/DH) is obtained, where the MSA is used to obtain the density profiles near a membrane and the DH approximation to obtain the bulk pair correlation functions. A method which applies to nonmobile protein fixed inside a cell is also considered.
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    URL: http://dx.doi.org/10.1063/1.456941
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    The effects of attractive and repulsive interaction on two-dimensional reaction-limited aggregation (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3212-3221 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The effects of both attractive and repulsive interactions on reaction-limited cluster–cluster aggregation have been explored using a two-dimensional polydisperse off-lattice model. Interactions with a magnitude on the order of kBT(kB is the Boltzmann constant and T is temperature) have an important effect in both the aggregate structure and the kinetics of aggregation. Attractive interactions lead to more compact structures and repulsive interactions lead to less compact structures with a lower effective dimensionality. The effects of power law particle–particle interactions with interaction energies of the form E=A/r Ε (r is distance) on aggregate structure, aggregation kinetics and the evolution of the cluster size distribution were explored for Ε=2, 4, and 6. A mean field variational theory indicates that fractal dimensionality is unchanged if Ε≥2D0 (where D0 is the fractal dimensionality in the absence of interactions) and that D=D0 (Ε+2)/2(D0+1) for Ε in the range 1≤Ε≤2D0. The simulation results are in accord with this theoretical analysis.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456942
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    Measurement of the viscoelastic properties of silver metal liquid-like films by laser light scattering (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 3222-3227 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The rheological properties of silver metal liquid-like films, trapped between two immiscible liquid phases, were investigated by measuring laser light scattering from the interfacial capillary waves using optical heterodyne mixing spectroscopy. All the films studied were found to have small elasticities in the range 0.27 to 1.0 dyn/cm. These values are consistent with a colloidal model of the structure of the films with diffusional processes possibly short circuiting the interfacial stresses. It also indicates a gaseous state, consistent with the charge of the surfactants and the silver colloidal particles, and the strong effect of perfluorosurfactants.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456943
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