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  • American Institute of Physics (AIP)  (31,708)
  • Oxford University Press  (23,294)
  • American Association for the Advancement of Science (AAAS)  (10,976)
  • 1985-1989  (65,978)
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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 5118-5119 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The vertical ionization potentials of the D2d and C2v structures of B2H4 are reported at the G1 level of ab initio molecular orbital theory. The results support the interpretation by Ruscic, Schwarz, and Berkowitz of their new photoionization experiment in terms of a C2v bridged structure of B2H4. In addition, the bridged C2v B2H+4 cation is found to have a very small barrier to inversion (1 kcal/mol).
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  • 2
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 5123-5124 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The radii and electrostatic potential of singly−negative ions of the 3d,4d and 5d transition series have bee calculated. This gives a quantitative estimate of the size of these ions.(AIP)
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  • 3
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 4061-4072 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Various aspects of the intramolecular proton transfer in malonaldehyde have been investigated theoretically within the reaction surface Hamiltonian framework, which was recently applied with a two-dimensional surface to this molecule by Carrington and Miller. The present calculation, which involves a three-dimensional reaction surface and a high level of ab initio accuracy, gives a tunneling splitting which is ∼50% smaller than experiment and a hydrogen/deuterium isotope effect that is within 40% of experiment with no adjustable parameter. The vibrational wave function has been analyzed to extract an effective curvilinear tunneling path on the hypersurface. The path calculations, and other analysis, clearly demonstrate the limitations of one-dimensional models for polyatomic tunneling systems like malonaldehyde. In addition, tunneling splittings have been calculated for excited vibrational states of malonaldehyde, leading to new insight into the multidimensional character of proton transfer.
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  • 4
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 4098-4105 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The solvent viscosity and polarity dependence of the magnetic field effect in polymethylene-linked radical ion pairs, which were generated by photoinduced intramolecular electron transfer in compounds of the type pyrene–(CH2)n–N,N-dimethylaniline, has been studied. A stochastic Liouville equation is used, in which the dynamics of the polymethylene chain, the spin Hamiltonian as a function of the varying radical distance (exchange interaction), and a distance-dependent back electron transfer rate are incorporated. The results are compared with predictions made on the basis of the (static) subensemble approximation.
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  • 5
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 4106-4110 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A direct method for determining time delays for scattering processes is developed using the R-matrix propagation method. The procedure involves the simultaneous generation of the global R matrix and its energy derivative. The necessary expressions to obtain the energy derivative of the S matrix are relatively simple and involve many of the same matrix elements required for the R-matrix propagation method. This method is applied to a simple model for a chemical reaction that displays sharp resonance features. The test results of the direct method are shown to be in excellent agreement with the traditional numerical differentiation method for scattering energies near the resonance energy. However, for sharp resonances the numerical differentiation method requires calculation of the S-matrix elements at many closely spaced energies. Since the direct method presented here involves calculations at only a single energy, one is able to generate accurate energy derivatives and time delays much more efficiently and reliably.
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  • 6
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 4111-4117 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The Hamiltonian-based model developed by the authors [N. J. Cotes and M. G. Sceats, J. Chem. Phys. 89, 2816 (1988)] for bimolecular reactions which exhibit severe configurational restrictions, such as orientational requirements for reaction at surface sites, provides an analytical expression for the reaction rate which involves the evaluation of the potential of mean force along the radial coordinate that describes the binding. The expression for the reaction rate is evaluated for the case of a charged molecule interacting with a binding site on a dipolar particle. The results of the model are compared with the multidimensional Brownian-dynamics simulations of Northrup et al. [J. Chem. Phys. 84, 5536 (1986)] and excellent agreement is obtained in the diffusion limit.
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  • 7
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 4128-4133 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A red wing in the absorption of H2O in the first continuum which extends beyond 200 nm is induced by the matrix. A threshold energy of 6.95 eV is found for the quantum efficiency of permanent dissociation of H2O into OH and H. This corresponds to an additional barrier of 1.8 eV due to the matrix cage. The barrier is attributed to the sum of repulsive H–Ar pair potentials in an interstitial site of D3 symmetry, which has to be surmounted by the H atom on the way from the initial substitutional H2O site to the final stable Oh site of the abstracted H atom.
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  • 8
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 4213-4222 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new algorithm for the calculation of molecular integrals involving STOs is reported. The algorithm enables us to obtain every two-center one-electron integral and the long-range many-center one- and two-electron integrals. The efficient implementation of the algorithm is discussed and its performance is thoroughly tested. The analysis on the stability of the relations employed in the calculation of multipolar moments is included. Futhermore, the computer time required to carry out each step (construction of basic matrices, calculation of multipolar moments, and calculation of two-electron integrals) has also been analyzed. The range of validity of this approach is shown in several molecular integrals.
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  • 9
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 4223-4228 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: By applying projection techniques a unified description of weakly and strongly correlated electronic systems can be given. For that purpose the theory is formulated in Liouville instead of Hilbert space. By introducing an appropriate metric in terms of cumulants it is ensured that all energies are size consistent. An energy expression is derived which is a generalization of Goldstone's linked cluster theorem for arbitrary Hamiltonians. It is suitable for applying the projection method of Zwanzig and Mori. The equations for the correlation energy within the Local Ansatz and within coupled electron pair approximation CEPA-2 follow straightforwardly from the present theory.
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  • 10
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    The Journal of Chemical Physics 91 (1989), S. 4242-4248 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The present paper clarifies a number of issues concerning the general problem of constructing improved short time quantum mechanical propagators. Cumulant methods are shown to be a particularly convenient tool for this task. Numerical results comparing methods based on partial averaging and on gradient approaches are presented for simple model problems and for many particle quantum fluids.
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  • 11
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    The Journal of Chemical Physics 91 (1989), S. 4229-4241 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Neutral and cationic MgYk clusters with Y=Na, Li (k=2–8) have been investigated using all-electron ab initio configuration interaction (CI) procedures. The alkaline earth metal (or group IIa) atom takes the central position in the most stable geometries of all clusters studied except for MgNa3, MgLi3, and MgNa+3 . The importance of the chemical nature of the atoms involved in determining the stability and other properties of the clusters, is clearly demonstrated by comparing the neutral and cationic series of MgNak , MgLik , and BeLik . The properties obtained can be fully explained by considering the s–p promotion, the role of Jahn–Teller distortions, and the stereochemical aspects. The appropriate treatment of electronic correlation effects is of crucial importance for correctly predicting the stability of the clusters. The low stability of small clusters, especially MgNak (k〈5), is particularly noticeable. The atomization energies per atom for MgY6, MgY7, and MgY8 with Y=Na or Li, are all found to be comparable. The general pattern for atomization energies per atom and for other stability measures as well as for fragmentation channels and ionization potentials as functions of the cluster size is explained in the paper.
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  • 12
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 4249-4253 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The connected moments expansion with use of variational Monte Carlo technique (CMX-VMC) is applied to the calculation of the ground state energies of H, H−, Be, and Li2. Exponential-type wave functions for H, Hylleraas-type wave functions for H−, and a Hartree–Fock single determinant constructed with a single-zeta Slater-type orbital multiplied by a pair correlation factor of Jastrow-type for Be and Li2 are employed as approximate trial wave function. The results of the present computation are found to agree with the corresponding exact values quite well. The overlap between the approximate and exact wave function is also estimated simultaneously by the new technique.
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  • 13
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 5139-5153 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The problem of classical vs nonclassical structure of protonated acetylene (vinyl cation) C2H+3 has been studied using high resolution infrared spectroscopy. The spectrum has been observed in the 3.2 μm region in air-cooled and water-cooled plasmas using C2H2:H2:He mixtures and in liquid nitrogen-cooled plasmas using CH4:H2:He mixtures. The difference frequency spectrometer with the velocity modulation method has been used to conduct the Doppler-limited, high sensitivity spectroscopy.The observed vibration–rotation pattern with the band origin at 3142.2 cm−1 has been identified as due to the antisymmetric CH stretching ν6 band of the C2H+3 ion with the nonclassical (bridged) structure. The observed spectral pattern was anomalous, but definitive assignments could be made for a part of the spectrum using the ground state combination differences which fit to the usual asymmetric rotor pattern. The discrimination between the classical and nonclassical structures is based on the observed spectral intensity pattern due to spin statistical weights. Agreement of vibrational band patterns and the rotational constants with ab initio values gives supporting evidence. The anomaly of the spectrum is at least partly ascribed to the small energy difference between the classical and nonclassical structures and possible rearrangement between them, the idea used by organic chemists over the years in wet chemistry. Systematic splittings with the intensity ratio of 2:1 have been noticed in some parts of the spectrum indicating that the protons tunnel between the apex and the two end equilibrium positions of the bridged structure. Using a simplified internal rotation model proposed by Hougen, the barrier height of the tunneling has been estimated. Chemical kinetics in plasmas related to C2H+3 is also discussed.We conclude that (1) the nonclassical structure is lower in energy than the classical structure, and (2) the apex proton and the two end protons exchange their positions with a measurable time scale.
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  • 14
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 5154-5159 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Using a purely ab initio minimum energy path for the trans-tunneling motion in the HF dimer, the energy levels for the K-type rotation and trans-tunneling motion for (HF)2 and (DF)2 are calculated with a one-dimensional semirigid bender Hamiltonian and no adjustable parameters. The transition moments for rotation-tunneling transitions are calculated, using our ab initio value for the dipole moment of an isolated HF molecule, and we also calculate B¯ values. The energy levels we obtain are in close agreement with experiment; for example, the K=0 tunneling splitting in (HF)2 is calculated as 0.65 cm−1 compared to the experimental value of 0.658 69 cm−1. As well as showing that our ab initio minimum energy path is very good, the calculation demonstrates that the semirigid bender formalism is able to account quantitatively for the unusual K dependence of the rotational energies resulting from the quasilinear behavior, and that the trans-tunneling motion is separable from the other degrees of freedom. We use the results to predict the locations, and transition moments, of the ΔK=0 and ±1 subbands in the tunneling spectra of (HF)2 and (DF)2, many of which have not yet been observed.
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  • 15
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 5164-5169 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The inelastic neutron scattering (INS) spectra of some ammonium halides and ammonium nitrate are analyzed in terms of the internal vibrations of the NH+4 ion convoluted with multiphonon lattice modes up to the eighth term. Due to the low mass of this ion, most of the intensity of the internal-mode region of the INS spectra is in the "phonon wings,'' differences between the salts in this spectral region being mainly due to differences in their lattice-mode spectra. Refinement of the lattice Debye–Waller factor is included in the profile refinement of interatomic force constants.
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  • 16
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 5160-5163 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new upper bound for the dissociation energy of acetylene, D00(HCC–H) =529.89(±0.01) kJ/mol, has been determined by Stark anticrossing spectroscopy. The zero-pressure extrapolated (unimolecular) decay rates of levels of S1 (A˜ 1Au) v'3=2 and 3 (quanta of the trans-bending vibration) increase upon application of an electric field of 113 kV/cm. We attribute this increase in molecular decay rate to predissociation rather than any other Stark-induced nonradiative or radiative phenomenon. The lowest level (v'3 =2, J'=2, K'=1) from which we have observed an increase in decay rate (i.e., predissociation) has an internal energy of 44 295.65 cm−1 relative to v=0, J=0 of S0 (X˜ 1∑+g). This corresponds to a value about 24 kJ/mol lower then the consensus upper bound result from four different recent experimental determinations (including one of ours) as well as current ab initio results. The new value agrees, however, with earlier work and with recent modeling studies of acetylene pyrolysis kinetics.
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  • 17
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    The Journal of Chemical Physics 91 (1989), S. 5285-5290 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The technique of Brillouin spectroscopy has been used to determine the adiabatic elastic constants of single crystals of the orientationally disordered phase (phase I) of CBr4 .The values of the elastic constants at 333 K are C11 =42.2, C12 =33.6, and C44 =16.1 (in units of kbar) and suggest that there is strong rotation–translation coupling in the k=0 acoustic wave vector regime. Because x-ray Laue diffraction was not possible in solid CBr4 , the necessary orientation of the single crystals was determined by also using the Brillouin scattering spectra.
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  • 18
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    The Journal of Chemical Physics 91 (1989), S. 5313-5315 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The ν3 (O–Si stretch) fundamental bands of the HOSi+ and DOSi+ molecular ions in the 9 μm region have been detected for the first time, using a tunable infrared diode laser spectrometer and a hollow cathode discharge cell. Analysis of the results yielded accurate values for the molecular rotational and centrifugal distortion parameters, as well as for the band origins, which are 1127.009 cm−1 for HOSi+ and 1103.112 cm−1 for DOSi+ . The ground vibrational state parameters are in excellent agreement with those determined from the ν1 bands of the two isotopes.
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  • 19
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    The Journal of Chemical Physics 91 (1989), S. 5324-5328 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Fragment ions from N2 in the states of (K)−1(nl)1 have been measured at 54.7° with respect to the polarization direction of the incident photon by using a time-of-flight (TOF) mass spectrometer. Simulation fitting calculation to the measured TOF spectra gave distributions of the kinetic energy for these ions. Ion yield spectra observed showed that no singly charged molecular ion was produced from the states of (K)−1(nl)1 in contrast to that of (K)−1(1πg)1. These results are discussed in connection to the dissociation pathway of N2 in these states.
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  • 20
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    The Journal of Chemical Physics 91 (1989), S. 5329-5334 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Dissociation of O2 stimulated by soft x-ray absorption has been studied by using a monochromatized synchrotron radiation and a time-of-flight mass spectrometer. The parent molecular ion O+2 was formed only at 531 eV (excitation of 1s→1πg), and a fragment ion O+ had the highest intensity in all the energies giving core–hole states. The measured time-of-flight spectra were reproduced by a simulation calculation, which provided kinetic energy distributions of O+ and O++. The dissociation pathways from the core–hole states of O2 were discussed using the obtained kinetic energy distribution and ion intensity ratios as well as Auger electron spectra in the literature.
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  • 21
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    The Journal of Chemical Physics 91 (1989), S. 5316-5323 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The general theory for inelastic scattering of molecules 1Δ electronic states is outlined and applied to the specific case of 1Δ states arising from a π2 electron occupancy, e.g., NH(a 1Δ). Integral cross sections for rotational transitions out of the lowest rotational level (J=2) of NH(a 1Δ) v=0 are reported for several targets. A pulsed beam of rotationally cold NH(a 1Δ) was produced by 193 nm photolysis of a dilute mixture of hydrazoic acid in nitrogen seed gas at the tip of a nozzle. The target beam was also prepared as a pulsed supersonic beam. The final rotational state distribution was measured in the collision zone by laser fluorescence excitation. The state-to-state cross sections were found to decrease significantly with increasing final rotational quantum number J'. The magnitude of the J=2→J'=3 cross sections were compared for the different targets. Isotopic scrambling in NH(a 1Δ)–D2 collisions was also searched for but not observed.
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  • 22
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    The Journal of Chemical Physics 91 (1989), S. 5335-5342 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The yield of metastable oxygen atoms through dissociative recombination of O+2 ions with electrons has been studied using a plasma flow tube experiment. For O+2 with high vibrational excitation (around v=9) it was found that half of the oxygen atoms are formed on the O(1D) state and that the branching ratio toward O(1S) is large (∼0.4). Using Xe+ instead of Ar+ as precursor ions, it was shown that the O(1S) yield is much less for ions with low vibrational excitation. However, the present experimental results are not compatible with the extremely low theoretical value of this yield which was reported recently for O+2 (v=0).
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  • 23
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    The Journal of Chemical Physics 91 (1989), S. 5343-5351 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Quenching and vibrational energy transfer in the B2Π state of the NS free radical have been studied using temporally and spectrally resolved laser-induced fluorescence in a low-pressure discharge flow reactor. The collision partners were He, Ar, H2, N2, O2, SF6, CO2, and N2O. Total removal cross sections show an oscillatory behavior with v' in the range of unperturbed levels, v'=4–7, for all colliders studied save O2 where a nearly monotonic increase is seen. Vibrational transfer occurs for H2 and the polyatomics; the rates vary little with v'. Δv=2 transfer occurs with the polyatomic colliders for v'=5. Fluorescence decay traces from the perturbed v'=3 and 8 levels differ from the unperturbed levels and from each other. v'=3 is perturbed by quartets and shows "gateway'' level behavior whereas v'=8 is perturbed by a doublet and shows efficient interelectronic transfer for all rotational levels.
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  • 24
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    The Journal of Chemical Physics 91 (1989), S. 5359-5364 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: State-to-state charge–transfer cross sections have been computed for N+2(X;v=0,1,2) +Ar at 12 collision energies between 1.2 and 320 eV. A classical path method is used, whereby the vibronic degrees of freedom are treated quantum mechanically as the system moves along a classical trajectory. The calculations use the potential energy surfaces computed by Archirel and Levy. Comparison is made with experimental results for this system, including the recent work from Ng's laboratory. In most cases the agreement is quite good. There is, however, a significant difference in the charge–transfer branching ratios to produce Ar+(2P3/2) or Ar+(2P1/2) products. Possible explanations of the discrepancy are discussed. As expected, the cross sections obey the Franck–Condon principle at energies above 200 eV.
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    The Journal of Chemical Physics 91 (1989), S. 5365-5373 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The reaction between ground state atomic oxygen and acetylene was studied using the crossed molecular beam method with an average collision energy of 6 kcal/mol. The two major primary reaction channels are (a) formation of CH2 and CO and (b) formation of HCCO and H. Product angular distributions and time-of-flight spectra were measured and the translational energy release was determined for each channel. The reaction proceeds primarily on the triplet surface through a long-lived intermediate. For both channels the translational energy distributions were found to peak at about 30% of the total available energy, indicating the existence of an exit channel barrier in each case. The branching ratio between channel (a) and (b) was found to be 1.4±0.5.
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  • 26
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    The Journal of Chemical Physics 91 (1989), S. 5387-5391 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A pump-and-probe technique is utilized to yield a population distribution over the rotational quantum states of the nascent product MgH in the reaction of Mg(1P1) and H2. The resulting normalized profile of the MgH bimodal distribution at 693 K coincides with that at 733 K, as well as with the results obtained at 380 K by Breckenridge and co-workers. This temperature dependence demonstrates that the bimodality actually results from the insertive reaction alone. This conclusion is consistent with the isotopic effect.
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    The Journal of Chemical Physics 91 (1989), S. 5425-5439 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Using recent ab initio interaction potential energy surfaces for the CN (X 2Σ+, A 2Π)+He system [H.-J. Werner, B. Follmeg, and M. H. Alexander, J. Chem. Phys. 89, 3139 (1988)], we have calculated fully quantum cross sections for inelastic transitions between individual rovibrational levels of the A 2Π and the X 2Σ+ states of CN. We have concentrated on the transitions studied experimentally by Dagdigian and co-workers for CN+Ar, namely transitions between the rotational levels of the A, v=8 and X, v'=12, the A, v=7 and X, v'=11, and the A, v=3 and X, v'=7 vibrational manifolds. In the case of the 8→12 and 7→11 transitions the cross sections are large (0.1–1 A(ring)2), and the dependence on initial Λ doublet level and on final rotational quantum number displays the same subtle alternations as seen experimentally. In the case of the 3→7 transitions, for which the vibrational levels are energetically much more separated, the calculated cross sections for CN+He are extremely small (10−5 A(ring)2), far smaller than observed experimentally for CN+Ar. In order to resolve this discrepancy, we have carried out some additional ab initio calculations for the CN+Ar system, but the change in the interelectronic coupling potential appears not to be large enough to explain the magnitude of the experimental cross sections.
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    The Journal of Chemical Physics 91 (1989), S. 5451-5454 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The multiconfigurational spin tensor electron propagator approximation to the electron propagator is applied for the first time to the calculation of electron affinities. The electron affinities of Li, Na, and K are calculated as the ionization potentials of the negative ions. On average these electron affinities differ from experiment by 0.003 eV, giving the best theoretical values reported to data.
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    The Journal of Chemical Physics 91 (1989), S. 5633-5641 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Starting with the Green–Kubo formulas for the phenomenological coefficients of irreversible thermodynamics, the molecular expression for the distinct diffusion coefficient Dd[R],αβ in any internal reference frame R is derived. It follows that Dd[R],αβ is given by the time integral of a time correlation function (tcf) Λd[R],αβ(t) whose dynamical variables are the velocities of two different particles of species α and β, each relative to a microscopic reference velocity wˆR . These relative dynamic variables have the welcome feature of being orthogonal to the total momentum of the system so that Λd[R],αβ(t) vanishes at long times. Hence the divergence of the barycentric distinct diffusion coefficients, suggested by their usual interpretation in terms of a time correlation function that lacks an explicit dependence on the reference frame, is resolved. The influence of the reference frame on the behavior of the barycentric distinct tcf 's is analyzed. In great measure this influence is exposed by considering initial values of the tcf 's; in turn the initial values are decisive in establishing the sign of the distinct diffusion coefficients in ideal or nearly ideal mixtures. Finally, by applying new linear relations connecting the self and distinct tcf 's, a complete description of some of the barycentric distinct tcf 's may be given in the limiting cases of a pure fluid and a tracer in a host liquid.
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  • 30
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    The Journal of Chemical Physics 91 (1989), S. 5655-5664 
    ISSN: 1089-7690
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The revised capillarity approximation (RCPA) of McGraw and Reiss [J. Stat. Phys. 20, 385 (1979)] is reexamined using established liquid drop models to evaluate the effects of physical cluster excluded volume on the thermodynamic barrier to nucleation near a critical temperature Tc. A significant heightening of the nucleation barrier is predicted, and the anomalous behavior observed experimentally near Tc is examined within the framework of classical nucleation theory using the RCPA. Definitive methods for experimentally separating this pure nucleation effect from those dependent on slowing down of the post-nucleation completion times for phase separation are described.
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  • 31
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    The Journal of Chemical Physics 91 (1989), S. 5672-5678 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The homogeneous nucleation of three supersaturated alcohol vapors carried in argon is investigated experimentally. A gasdynamic shock tube method serves to subject the vapors to short supersaturation pulses. The growing nuclei are detected in number and size by Mie-light scattering. The results provide the supersaturation over a wide range of temperature for the nucleation rate of 108 cm−3 s−1. Comparison with the classical nucleation theory (CNT) exhibits increasing divergence with decreasing temperature.
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  • 32
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    Topics: Physics , Chemistry and Pharmacology
    Notes: A permutation group direct configuration interaction program, using graphical techniques, has been extended to second order properties. Using this program, absolute and relative two-photon cross sections have been evaluated for the rotational lines of three vibronic bands 0–0, 1–0, 1–1 of the A 2Σ+←X 2Π transition of OH. The computed absorption coefficients are compared with the available experimental data.
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  • 33
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    The Journal of Chemical Physics 91 (1989), S. 5648-5654 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The structure of liquid carbon tetrachloride (CCl4) has been measured at −20, 20, 60, 110, and 160 °C along the liquid–vapor coexisting curve by means of pulsed neutron total scattering technique. Characteristic changes of the structure factor Sm(Q) have been observed with increase of temperature. Experimental results show that orientational correlation still exists even at 160 °C, the highest temperature studied in this work. The observed Sm(Q)'s are discussed in terms of a packing or a number density of effectively uncorrelated molecules and preferred orientations introduced between the nearest neighbors. Experimental facts of broadening of the first peak of Sm(Q) and the shift toward the lower Q side of this peak with an increase of temperature are attributed simply to a decrease of molecular number density, while broadening both of a small second peak located around 2.2 A−1 and a valley following this peak is attributed to a decrease in number and broadening in distribution of correlated molecules.
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  • 34
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    The Journal of Chemical Physics 91 (1989), S. 5665-5671 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The adiabatic dynamics of an electron in a polar solvent has been studied by means of a molecular dynamics technique for the coupled electron–solvent system. The method hinges on a separation of time scales for the solvent motion and certain classical dynamical degrees of freedom which represent the ground-state wave function of the electron. For the solvated electron these variables are taken to be the positions and widths of Gaussians that comprise a distributed basis set. The method is applied to an electron in liquid ammonia, and the conditions for the validity of the classical separation of time scales are investigated. Time correlation functions describing the electron diffusion coefficient, and the fluctuation and dissipation of electron energy, are compared with analogous quantities for a classical solute (a chloride ion) in the same solvent. The electron diffusion coefficient in ammonia is calculated to be about twice that of a chloride ion. Experimental mobility data on the electron–ammonia system suggest that the real diffusion coefficient is a factor of 3 larger than calculated for the model used here. Possible reasons for this discrepancy are noted.
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  • 35
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    The Journal of Chemical Physics 91 (1989), S. 5687-5693 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: We present results of numerical simulations of a new model for the clustering process in self-associating polymer systems. We focus on two experimentally interesting systems; telechelic ionomers and reverse micelles. The clustering dynamics introduced by us is an extension of the Eden method, previously used in the context of single particle aggregation. The structure of the aggregates seen in the simulations is qualitatively similar to that observed in experimental systems. The quantitative predictions of the model are found to be similar to those found in simulations of a model based on a diffusion-limited-aggregation-type process.
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  • 36
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    The Journal of Chemical Physics 91 (1989), S. 5701-5706 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The adsorption of NO on Ag(111) at 80 K has been studied by high resolution electron energy loss spectroscopy (HREELS) and thermal desorption spectroscopy (TDS). At low exposures (≤0.05 L) NO is adsorbed in part dissociatively and in part molecularly in two different threefold bridge states (in upright and bent or tilted orientation with respect to the surface normal). The NO molecules in the threefold bridge position are desorbed at 100 K. With increasing exposure the desorption temperature shifts gradually to 110 K. At medium exposures (∼0.13 L) additional NO is adsorbed molecularly in an atop position with an upright orientation in admixture with atomic N and O and molecular NO adsorbed in threefold bridge states. The NO molecules adsorbed in atop position are weakly bonded on the surface and are desorbed at about 90 K. Simultaneously, N2O is formed and adsorbed on the surface. At saturation a new bent or tilted NO species in atop position appears on the surface and the amount of N2O significantly increases. The N2O molecules are desorbed at 90, 110, and 190 K. At saturation all the NO and NO-derived species (the threefold bridge and atop states, N2O and atomic oxygen and nitrogen) coexist on Ag(111).
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  • 37
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    The Journal of Chemical Physics 91 (1989), S. 5707-5721 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Static and dynamic properties related to the internal configurational motions have been calculated for the alkyl chains of phospholipid molecules in a membrane environment in the liquid crystal phase. The calculations have been performed for the chain 1 of 1,2-dipalmitoyl 3-sn-phosphatidylcholine (DPPC), a typical constituent of phospholipid membranes. Under the assumption of fixed bond lengths and bond angles, the internal dynamics of the chain is described in terms of 15 dihedral angles. The time evolution of the angular variables is assumed to be diffusional in character, and a master equation for transitions among the stable conformers is constructed from the energetics and hydrodynamics of the chain. This method is an extension to the time domain of the rotational isomeric state (RIS) approximation, which has been widely used to compute static properties of the chains. After calculation of the suitable correlation functions, effective rate constants relevant for spectroscopic and kinetic observables have been computed, and the results have been compared with those obtained by recent Brownian dynamics (BD) calculations. The position dependence of the rate constants along the chain has been examined with special reference to understanding the effects resulting from cooperativity in the conformational transitions. The overall spinning and tumbling of the chain has also been described by a diffusive model. The calculated spectral densities for the composite motional process have been used to rationalize the behavior of the relaxation times T1, T2, and T1ρ measured in deuterium nuclear magnetic resonance (NMR) experiments.
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  • 38
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    The Journal of Chemical Physics 91 (1989), S. 5743-5755 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: A simple theory is developed to relate the optical absorption spectrum of a two-dimensional dye aggregate, either dispersed in a transparent matrix or deposited on a transparent substrate, to the spectra of a crystal that consists of a stack of such aggregate layers. Spectral data can then provide evidence on whether the structure of such an aggregate is similar to that of a layer in the crystal. A thiapyrylium dye aggregate dispersed in a polymer film is treated as an example.
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  • 39
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    The Journal of Chemical Physics 91 (1989), S. 5860-5860 
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  • 40
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    The Journal of Chemical Physics 91 (1989), S. 5861-5861 
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  • 41
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    The Journal of Chemical Physics 91 (1989), S. 4827-4837 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The structure of mixtures of hard chains (modeled as a pearl necklace of freely jointed hard spheres) and hard spheres in slitlike pores is studied using a canonical ensemble Monte Carlo method. Simulation results for the density profiles in pores with wall separations varying from two to ten hard sphere diameters are presented at overall volume fractions of 0.12 and 0.34, and the effect of pore size, chain concentration, and chain length on the structure of the chains is investigated. It is found that the chains are depleted at the wall at the lower density, but enhanced at the wall (relative to the center of the pore) at the higher density; this depletion increases as the chain length is increased. It is found that the enhancement of monomers and depletion/enhancement of chains at the wall becomes more marked as the pore size is increased. As all the pores we study are integer multiples of the bead diameter, we do not observe the oscillatory variation of the wall density with wall separation which is expected at high densities. The fluid is uniform in the middle of the larger pores, but is not uniform anywhere in the smaller pores. At the same total packing fraction, increasing the mole fraction of the monomers tends to remove the chains from the wall and also make the oscillations in the chain profile more pronounced. The density of end sites of the chain at the wall is higher than it is for middle sites. In the smaller pores, the chains are found to be flattened in the direction perpendicular to the walls; in large pores the fluid in the middle of the pore is essentially uniform.
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  • 42
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    Notes: Heat capacity measurements of four lanthanide sesquisulfides La2S3, Ce2S3, Nd2S3, and Gd2S3, prepared in the γ phase, have been obtained between 6 and 350 K by adiabatic calorimetry. The total heat capacity has been resolved into lattice, electronic, magnetic, and Schottky components. The Schottky contributions agree well with the calculated values based on the observed splitting of the ground-state manifold of the rare earth ions occupying sites of S4 symmetry in the Th3P4 structure. The observed splitting is obtained from an analysis of the hot bands in the absorption spectrum and from direct observation of the Stark levels in the far infrared. The Stark levels (all doublets) for Ce2S3 (2F5/2) are 0, 185, and 358 cm−1; for Nd2S3(4I9/2), they are 0, 76, 150, 180, and 385 cm−1. For La2S3, which has no Schottky or magnetic contributions to the heat capacity, the thermal data can be extrapolated to 0 K. The entropy for La2S3 at 298.15 K (as S0/R) is 19.51. Schottky and magnetic ordering at lower temperatures in Ce2S3, Nd2S3, and Gd2S3 preclude such extrapolation techniques. Therefore the entropy at 298.15 K for these compounds {S0–S0(7 K)}/R, is 21.34, 22.38, and 20.05, respectively.
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  • 43
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    The Journal of Chemical Physics 91 (1989), S. 4861-4868 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The observation that the intramolecular direct-correlation-function matrix is very closely approximated by its large-r form for rigid diatomics suggests an approximation such that the site–site Ornstein–Zernike equation becomes analytically solvable for several molecular models of interest. Alternatively, approximation of the particle–particle direct correlation function by its asymptotic form directly yields the same approximation, in particle–particle (in contrast to site–site) language. Applications of this approximation, which can be regarded as an extended mean spherical approximation when viewed in particle–particle terms, are made to rigid hard diatomics, dipolar diatomics, and a tetrahedral molecular model. The close but subtle connections between our approximation procedure and earlier approximations are discussed.
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  • 44
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    The Journal of Chemical Physics 91 (1989), S. 4869-4878 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Analytical equations are obtained for the pair correlation functions in the mean spherical approximation (MSA) for two charged hard-sphere ions in a model molecular solvent. Model solvents of dipolar dumbbells (which may be partially dissociative) or charged hard-spheres fused in a tetrahedral configuration are treated in an extended MSA. The resulting expressions for the Born solvation free energy of an ion are very similar to that obtained earlier by Chan et al. for a charged hard-sphere ion in a dipolar hard-sphere solvent. The solvent averaged ion–ion potentials are obtained in the hypernetted chain approximation. Comparisons among results of various solvents are also made. In order of increasing oscillation amplitudes of the solvent averaged ion–ion potential for given molecular density and dipole moment we have continuum (very-much-less-than)dumbbell≤tetrahedral〈dipolar hard sphere.
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  • 45
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    The Journal of Chemical Physics 91 (1989), S. 4885-4890 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The solvation dynamics of a pair of ions rigidly held a fixed distance from each other in either a dipolar hard-sphere solvent or a dipolar dumbbell solvent is investigated under the mean spherical approximation (MSA) applied to an interaction-site model in which the pair of ions is treated as a rigid extended dipolar dumbbell. An analytical expression for the reorganization free energy for a charge transfer reaction is obtained and compared with numerical results obtained via a thermodynamic cycle. Solvent effects on the rate of a charge transfer reaction are also investigated using the dielectric Green-function theory developed by Newton and Friedman.
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  • 46
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    The Journal of Chemical Physics 91 (1989), S. 4879-4884 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The Born solvation free energy (BSFE) of two ions at a fixed distance from one another in a model polar solvent is obtained via two approaches. In the interaction-site approach, the two ions are modeled as a rigid extended dipolar dumbbell. Analytical expressions for the BSFE for such a dumbbell model in a dipolar dumbbell and a dipolar hard-sphere solvent are obtained under a mean spherical approximation (MSA). In the second approach, a thermodynamic cycle is established such that the BSFE for two ions a fixed distance apart can be expressed in terms of the solvent-averaged potential between the two ions and other known quantities. The results obtained via these two approaches are reasonably consistent, with the thermodynamic-cycle BSFE as a function of distance exhibiting more of the structure one expects to find in a molecular solvent. Both BSFE functions are substantially different from the corresponding continuum-solvent result. When the distance between two ions goes to infinity, our results reduce to earlier results for the single-ion BSFE obtained by us for a dipolar dumbbell solvent and by Chan et al. for a dipolar hard-sphere solvent. The BSFE of two ions in an ionic solution with a dielectric-continuum solvent is also obtained; it is found that the contribution of other ionic particles to the total BSFE is usually negligible if the dielectric constant of the solution is assumed to be unchanged.
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  • 47
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    The Journal of Chemical Physics 91 (1989), S. 4891-4901 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The nonlinear response of a two-state chemical transition to an oscillating electric field is examined. A reaction for which this analysis is particularly relevant is a conformational transition of a membrane protein exposed to an ac electric field. Even a modest externally applied field leads to a very large local field within the membrane. This gives rise to nonlinear behavior. The applied ac field causes harmonics in the polarization and can cause a dc shift in the state occupancy, both of which can be observed and used to determine kinetic parameters. Fourier coefficients are calculated for the enzyme state probability in the ac field, exactly for infinite frequency, and in powers of the field for finite frequency. Kramers–Kronig relations are proved and response functions are given for the leading terms of the harmonics. The results are extended to the spherical symmetry relevant to suspensions of spherical cells, vesicles, or colloidal particles. If the protein catalyzes a reaction, free energy is transduced from the electric field to the output reaction, even if that reaction is electrically silent. Many transport enzymes are ideal examples. The ac field can cause the enzyme to pump ions or molecules through the membrane against an (electro)chemical potential. The efficiency of this energy transduction can be as high as 25%.
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  • 48
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    The Journal of Chemical Physics 91 (1989), S. 4909-4911 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The dissociation equilibrium AlBr=Al+Br was studied by effusion beam mass spectrometry over the range 1970 to 2260 K and the dissociation energy D00(AlBr) was derived as 4.41±0.06 eV. This value is in general agreement with other fragmentary thermochemical results, but it is lower than a value derived from a short extrapolation of vibrational levels in the excited 1π state, doubtless because of a potential maximum of about 0.22 eV in that state. A Birge–Sponer extrapolation of the ground state vibrational levels, when corrected for degree of ionicity, yields a D00 value in close accord with the experimental result, but an electrostatic model calculation falls short by 0.45 eV.
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    The Journal of Chemical Physics 91 (1989), S. 4902-4908 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The phase diagrams and the spinodal lines of different types of charged hard sphere fluid mixtures (CHSM) are determined through the use of the mean spherical approximation (MSA). The attention in the paper is particularly addressed to the low density regime of these systems, where the physical parameters are chosen in such a way so to mimic polyelectrolyte solutions with an added salt, or dilute electrolyte solution mixtures of simple salts. The phase diagrams at densities corresponding to the molten salt regime, obtained in a previous paper, are also reported, in order to get a complete picture of the phase stability behavior of CHSM. In all cases investigated the MSA predicts the existence of a closed solubility loop in the low density domain of the phase diagram. The location and the extension of this feature turns out to be crucially affected by the pressure, by the diameter ratios of the ionic components, and by the asymmetry of the ionic charges. A comparison of the MSA predictions in the low density regime with more refined theoretical approaches, or with computer simulation data, has not been possible for reasons which are explained in the text. However, the theory is expected to work at the same level of accuracy as in other physical contexts, where its predictions for the phase stability conditions of various systems have been found to be qualitatively reliable; moreover, for the case in which the model mimics a polyelectrolyte solution with an added salt, the form of the theoretical spinodal closely resembles the cloud points curve of an ionic micellar solution experimentally investigated by other authors.
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    The Journal of Chemical Physics 91 (1989), S. 4912-4919 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: A Smoluchowski equation is derived that depicts the relative diffusion of a sphere with respect to a fixed nonspherical convex body. The transport equation is expressed in terms of the surface-to-surface separation s, measured along the surface normal k. In this coordinate system, the effects of particle shape anisotropy can be subsumed by an appropriate mapping of the radius of the convex excluded volume surface onto that of an equivalent spherical system. Steric effects in diffusion-controlled reaction rates and shape anisotropy corrections to the recollision kinetic theory of rotation are analyzed in light of the chosen coordinates and the explicit Smoluchowski equation.
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    The Journal of Chemical Physics 91 (1989), S. 4920-4925 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: A new scheme of experimental criterion is proposed for estimating the carrier mobility and main-chain conformation of an electroactive polymer chain by the technique of transient electric birefringence (TEB). The rise response Δnr (t) and the decay response Δnd (t) in the TEB satisfy in the Kerr regime (i) Δnr (t)=Δnst -Δnd (t) for a polymer molecule of arbitrary conformation with an induced dipole moment alone arising from carriers highly mobile along the main chain, (ii) Δnr (t)=Δnst -(3/2)Δnd (t/3)+(1/2)Δnd (t) for a rodlike polymer molecule with a permanent dipole moment alone originating from carriers with low mobility, and (iii) Δnr(t)=Δnst-2Δnd (t/2)+Δnd (t) for a random-coiled polymer with a permanent dipole moment alone due to low-mobility carriers, where Δnst =limt→∞ Δnr(t) . Then, comparison of the TEB rise and decay responses gives us information on the carrier mobility and main-chain conformation. This criterion is valid also for polydispersed polymer samples. By applying the criterion to the exemplifying data of the TEB responses for soluble urethane-substituted polydiacetylenes, it is found that the polydiacetylene molecules take a random-coil conformation with a permanent dipole moment in a good solvent, while in a poor solvent the carrier mobility depends crucially on the solvent condition.
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    The Journal of Chemical Physics 91 (1989), S. 4932-4941 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Polystyrene spheres with attached functional groups that ionize in solution repel one another; at sufficiently high sphere concentrations the spheres self-assemble into a crystalline lattice with lattice constants large enough to diffract visible light. We have experimentally and theoretically examined diffraction phenomena from colloidal crystals of polystyrene spheres of diameters between 69 and 127 nm in water. We relate the diffraction bandwidths to the sphere scattering powers in the context of the dynamical diffraction theory and demonstrate the importance of the dynamical theory for predicting the observed diffraction angles, intensities, and bandwidths. We also discuss the mechanism contributing to the diffuse scattering and show the significance of the coherent scattering by lattice phonons.
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    The Journal of Chemical Physics 91 (1989), S. 4926-4931 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Fourier transform infrared multiple specular reflectance (FTIR-MSR) spectroscopy has been applied to study the chemisorption of n-hexadecanoic or palmitic acid [CH3(CH2)14COOH] on aluminium as a function of surface coverage. The intensity of the symmetric carboxylate stretching vibration has been used to estimate the surface coverage. The intensities and band positions of the CH stretching modes are presented as a function of surface coverage. A sudden transition at about 80% of monolayer coverage is observed for the intensity as well as the position of the CH2 stretching modes. The intensity changes correspond to a change from a more or less planar orientation of the polymethylene chains at low surface coverage to an almost perpendicular orientation at high surface coverage. The asymmetric CH3 stretching vibration exhibits a shift opposite to and smaller than what is observed for the CH2 vibrations. No significant changes are observed for the position of the symmetric CH3 stretching vibration. However, for the symmetric carboxylate stretching vibration a sudden shift of the band position is observed of the same magnitude, but opposite to the shifts of the CH2 vibrations.
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    The Journal of Chemical Physics 91 (1989), S. 4942-4948 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The rate of catalytic carbon monoxide oxidation on a Pt(100) single crystal surface under isothermal, low-pressure conditions exhibits for certain ranges of parameters (O2 and CO partial pressures, temperature) sustained temporal oscillations whose mechanism had been explored in previous work. Coupling between reaction and diffusion leads to spatial pattern formation as manifested by patches with different work function on the intrinsically homogeneous surface. Imaging is performed by means of the novel technique of scanning photoemission microscopy. Typically, nuclei with dimensions of a few microns, as determined by the instrumental resolution, are formed spontaneously and expand with sharp fronts and velocities of about 0.5 mm/min (at 480 K) up to sizes ≥1 mm. Waves with even more extended fronts propagating with somewhat higher velocities across the sample surface are responsible for the occurrence of large amplitude temporal oscillations of the integral reaction rate.
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    The Journal of Chemical Physics 91 (1989), S. 4974-4984 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Sticking of light gas atoms (He,D,H) on the (0001) surface of graphite is studied theoretically in the framework of the quantum mechanical perturbation theory. The formulas include a detailed microscopic description of the solid structure and vibrations. The calculations are employed in conjunction with the published experimental data to investigate the properties of the inelastic gas–surface coupling in the He/graphite system. The inelastic coupling is suggested to have a significant attractive component, and to be confined to highly localized spots on the surface. The low energy sticking probability is shown to be very sensitive to the weak coupling between the graphite layers. The high energy sticking via excitation of the optical out-of-plane phonons is found to be very inefficient. The isotopic effect is investigated in the H,D/graphite system. While the heavy isotope sticks about two times more efficiently in the low energy limit, the decay of the sticking probability with the energy is slower for the lighter isotope. Therefore the difference in the sticking probability of the two isotopes decreases strongly towards higher energies.
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    The Journal of Chemical Physics 91 (1989), S. 4961-4973 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The dc electrical conductivity above the Peierls-transition temperature Tc of quasi-one-dimensional (1D) organic metals is calculated by an ab initio single-particle theory based on the Kubo–Mori formalism. The respective inverse relaxation time in this approach is calculated numerically. The theory allows for a reliable reproduction of experimentally derived normalized conductivity curves of highly anisotropic organic metals. The applicability of the model is reduced with decreasing anisotropy. Phenomenologically it can be shown that a T−2 law of decay of the dc electrical conductivity above Tc is conventionally connected with large anisotropies. Decreasing anisotropy leads to T−n curves, where n is sizeably smaller than 2. The theoretical approach reproduces a T−n, n≈2, law of decay in the framework of one-phonon electron scattering processes. Therefore it is suggested that the deviations from the T−1 behavior of conventional three-dimensional metals is caused by the strong Kohn anomaly in 1D systems. The influence of the electron–phonon coupling and the Debye temperature on the analytic structure of the normalized dc electrical conductivity curves has been studied. A general theoretical description is suggested to account for the influence of electronic correlations on the magnitude of the electron–phonon coupling as well as on the Peierls-transition temperature. It is shown qualitatively that the organic metals belong to a class of valence fluctuating systems where charge fluctuations are conserved also in the limit of strong electronic correlations.
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    The Journal of Chemical Physics 91 (1989), S. 4949-4960 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The rate of reaction for oxidation of CO over (210) and (111) single-crystal surfaces of platinum has been studied as a function of reactant pressures (PO2,PCO) and sample temperature (T), both experimentally and by computer simulation. Experimental results on both surfaces show regions with a steady high rate of reaction followed by a nonsteady transition region and, at high CO pressures, a region with low reactivity caused by CO poisoning of the surface. At constant sample temperature, the transition region can be narrow and depends critically on the ratio of the gas phase concentration of reactants (PCO/PO2). The temperature dependences of the experimental data indicate that the critical ratio and the details for the occurrence of CO poisoning are strongly affected by surface processes such as adsorption, desorption, and diffusion ordering and reconstruction phenomena. A computer simulation model of the Langmuir–Hinshelwood surface reaction as developed by Ziff et al. was used for the simulation of the reaction under flow conditions. The initial fair agreement between this model and the experiment can be significantly improved if processes such as adsorption, desorption, and diffusion are taken into account in an extended simulation model which in turn provides an insight into the kinetics of adsorbate poisoning and the effect of adsorbate-induced processes on the reaction.
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  • 58
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    The Journal of Chemical Physics 91 (1989), S. 4985-4993 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The rotationally inelastic diffraction probabilities for H2, HD, and D2 from Cu(100) were computed as a function of surface temperature. The surface is treated in a quantum mechanical fashion using a recently developed formalism. The center of mass molecular translational motion is treated semiclassically, using Gaussian wave packets (GWPs), and the rotations are described quantum mechanically. Strong attenuation of the phonon elastic diffraction peaks with temperature is observed. This Debye–Waller-like attenuation increases with increasing molecular mass and kinetic energy, and decreases as the peaks become more off-specular. The phonon summed rotation–diffraction probabilities show a moderate temperature dependence for the most part. The 0→2 rotational excitation of D2 appears to be strongly phonon assisted above 300 K. At low temperatures our method reproduces the selection rules predicted by previous studies. As the temperature is increased these selection rules become less restrictive. The probability distribution for a scattering molecule exchanging an amount of energy ΔE with the surface was also computed. Rayleigh phonons were found to dominate the energy transfer, with bulk vibrations becoming more important for larger molecular masses, beam energies, and surface temperatures.
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  • 59
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    The Journal of Chemical Physics 91 (1989), S. 4994-5001 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Molecular dynamics simulations have been used to study the structure and dynamics of monolayers of long-chain molecules on a metallic substrate. The system consisted of 90 molecules held at a fixed surface density and periodically replicated in the plane of the surface. Two models have been studied as alternative representations of the admolecule–surface interactions in layers formed by the self-assembly of alkyl thiol [SH(CH2)15CH3] molecules onto a gold substrate. The principal difference between the two models is that in one, the S–C bond is required to lie nearly parallel to the substrate surface. After lengthy equilibration at room temperature, both models yield monolayers in which the chains are aligned and tilted with respect to the surface normal. Although the tilt angle is different for the two models, the thickness of the monolayers is the same and the influence of the "modified-headgroup'' on the detailed structure of the film is confined to the region closest to the metal surface. There are striking differences in the chain rotational dynamics of the two models which are likely amenable to experimental verification.
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  • 60
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    The Journal of Chemical Physics 91 (1989), S. 5030-5036 
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    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The dielectric dispersions of the β process are analyzed with respect to the configurational contributions for the densified glasses of poly(vinyl chloride). The vitrification pressure intensifies the intermolecular potential energy and affects the energy disparity induced by the electric field, but it only slightly affects the energy barrier for the β process. The relaxation magnitude varies with volume in accordance with the configurational change in the glasses. About 40% of the relaxation magnitude is the configurational contribution.
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    The Journal of Chemical Physics 91 (1989), S. 5002-5010 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: If a Pt(100) sample is kept at steady-state conditions of O2 and CO partial pressures and temperature which are similar to those for the occurrence of autonomous temporal oscillations in the rate of CO2 formation, then the surface will be largely covered by adsorbed CO which inhibits oxygen adsorption and keeps the catalytic rate low. Irradiation of a small spot with a high power laser pulse causes momentarily local thermal desorption of CO and creation of a reaction front which propagates as a chemical wave across the surface area, as was monitored by the excursion of the integral reaction rate and locally by means of a work function (=oxygen coverage) probe placed at several mm distance from the spot of irradiation. The velocity for wave propagation rises from about 2 mm/min at 480 K to 4 mm/min at 507 K and is not noticeably dependent on the partial pressures. The mechanism is closely related to that for self-sustained kinetic oscillations of this system and exhibits the typical features of trigger waves: Coupling between autocatalytic reaction and diffusion, as well as the occurrence of a refractory period during which the system is "dead,'' and of a threshold for the intensity of the excitation.
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    The Journal of Chemical Physics 91 (1989), S. 5011-5020 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A Pt(111) surface saturated with molecular oxygen was irradiated by UV light from a mercury arc lamp with various cutoff filters. High-resolution electron energy-loss spectroscopy (HREELS) and temperature programmed desorption mass spectroscopy (TPD) were used to characterize the products retained on the surface. Upon UV irradiation at 95 K, chemisorbed O2 (peroxo) undergoes dissociation, desorption and rearrangement. Different wavelength dependences were observed for the three processes: dissociation was not observed at wavelengths longer than 295 nm, in agreement with gas phase photodissociation of the O–O bond in hydrogen peroxide; desorption and rearrangement became negligible at wavelengths longer than 420 nm, in agreement with the photolysis of an organometallic peroxoplatinum complex. For wavelengths between 230 and 315 nm, the average cross sections of dissociation and desorption were estimated to be 5.7×10−20 and 1.2×10−19 cm2 , respectively. The possible origins of the three processes are discussed.
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  • 63
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    The Journal of Chemical Physics 91 (1989), S. 5021-5029 
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    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The dynamics of chemisorption and decomposition of SiH2 on Si(111)–(1×1) and recontructed Si(111)–(7×7) surfaces have been investigated using classical trajectories on a previously described [Surf. Sci. 195, 283 (1988)] potential-energy surface modified to yield the experimental bending frequencies for chemisorbed hydrogen atoms and to incorporate the results of ab initio calculations of the repulsive interaction between SiH2 and closed-shell lattice atoms. The Binnig et al. model is employed for the (7×7) reconstruction. Sticking probabilities are found to be unity on the (1×1) surface and near unity on Si(111)–(7×7). The major mode of surface decomposition on the (7×7) surface is by direct molecular elimination of H2 into the gas phase. Hydrogen atom dissociation to adjacent lattice sites is a much slower process and the chemisorbed hydrogen atoms thus formed exhibit very short lifetimes on the order of (1.13–10.6)×10−13 s. The calculated rate coefficients for these two decomposition modes are 3.4×1010 and 0.79×1010 s−1 , respectively. The rate coefficients for the corresponding reactions on the (1×1) surface are 6.6×1010 and 5.3×1010 s−1 , respectively. The rates on the (1×1) surface are faster due to the increased exothermicity released by the formation of two tetrahedral Si–Si bonds upon chemisorption compared to a single Si–Si bond on the (7×7) surface. Molecular beam deposition/decomposition experiments of SiH4 on Si(111)– (7×7) surfaces reported by Farnaam and Olander [Surf. Sci. 145, 390 (1984)] indicate that chemisorbed hydrogen atoms are not formed in the SiH4 decomposition process whereas the present calculations suggest that such a reaction, although slow, does occur subsequent to SiH2 chemisorption. It is suggested that energetic differences between SiH4 and SiH2 chemisorption are responsible for these differences.
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    The Journal of Chemical Physics 91 (1989), S. 4615-4624 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: We report three-dimensional L2 basis-set calculations of eigenvalues and eigenfunctions of CIHCI and IHI for zero total angular momentum. Comparisons are made to previous calculations of resonance energies and the bound state in IHI. These eigenfunctions are used in simulations of the photodetachment spectra of ClHCl−→CIHCI+e− and IHI−→IHI+e−. The spectra are convoluted with Gaussian weight functions as was done in very recent simulations of Schatz, based on coupled-channel scattering calculations, and in the experiments of Neumark and co-workers.
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    The Journal of Chemical Physics 91 (1989), S. 4636-4642 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Rotationally inelastic scattering of carbon dioxide by translationally hot H, D, and Cl atoms was studied by time-resolved diode laser absorption. The high J rotational distribution falls off quite rapidly between J=60 and J=80. D atom collisions have roughly twice the excitation cross section versus H atom collisions, with the H*/D* ratio decreasing with increasing J. These results are consistent with a constraint on the total reagent orbital angular momentum available for rotational excitation. Transient Doppler profiles measured immediately after hot atom/CO2 collisions indicate that CO2 molecules excited to high J levels have a larger recoil velocity than molecules excited to lower J levels. This result is consistent with predictions based on a simple model which treats the CO2 potential as a hard shell ellipsoid.
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    The Journal of Chemical Physics 91 (1989), S. 4625-4635 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The stochastic classical trajectory method is used to calculate the energy relaxation of a highly excited diatomic rotor trapped in rare gas crystal at T=20 K. The friction kernels, which appear in the generalized Langevin equations characterizing the motions of the molecule and of nearest neighbor crystal atoms, are expressed in terms of the interaction potentials. The influence of the surrounding crystal on the relaxation mechanism and the efficiency of the various dissipation channels are analyzed by changing the rare gas species and by artificially switching off some channels. Within the limits of the model (classical two-dimensional rotation of the diatomic molecule, coupled on the one hand to a restricted number of first shell atoms themselves coupled to the bulk crystal and on the other hand to the other first shell atoms considered as pertaining to the bath), the results of the calculations show that, in the present case, rotational relaxation is a rapid process, over the picosecond scale, and that the local mode connected to the motions of the molecular center of mass plays a major role in the mechanism. This local mode is responsible, at short times t≤0.5 ps, for the relaxation of 95%, 75%, and 60% of the rotational energy excess in Ar, Kr, and Xe crystals, respectively.The remaining energy is then dissipated over longer times via the local mode or directly towards the crystal modes. A striking energy saturation phenomenon of the local mode is exhibited in xenon crystal.
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    The Journal of Chemical Physics 91 (1989), S. 4651-4657 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: This work describes a new approach for the study of atom–diatom reactive collision employing the time-dependent wave packet Schrödinger equation. The method makes use of the projection operator formalism to form a coupled system of time dependent Schrödinger equations and of optical potentials to circumvent the necessary use of products coordinates. Two versions are presented. As an example we applied the method to the collinear reactive H+H2 system for which are calculated both transition probabilities and rate constants.
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    The Journal of Chemical Physics 91 (1989), S. 4643-4650 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The molecular dynamics simulation method is extended to study a model of diffusion-controlled reactions. This allows a molecular description of solvent at an equal footing of reactants. Nondiffusional dynamic behavior of reactive molecules is found at short times. It enhances the rate of reactive encounter in comparison to the prediction of Smoluchowski theory. The model studied in this work can be regarded as a theoretical prototype of fluorescence quenching. In this context it is shown that the nondiffusional dynamics is mainly responsible for the discrepancy between Stern–Volmer plots measured in a continuous excitation experiment or obtained by integrating the time resolved fluorescence intensity. The other aspects such as the long-time behavior of survival probability, solvent effect as well as competing effect from finite concentration of one reactive species are also studied in some detail.
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    The Journal of Chemical Physics 91 (1989), S. 4658-4663 
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    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A theoretical model was proposed for the orientational effects in diffusion-controlled enzyme reactions. To this purpose, we attained the exact solution of the rotational–translational diffusion equation (RTDE) with mixed boundary conditions (MBC). In steady-state conditions, the assumed MBC were: (i) the reactant molecules are chemically modified at the enzyme active site, provided that their relative orientation lies within a given angular range; (ii) the reactant molecules outside the active site or for uncorrect orientations are reflected; (iii) the concentration of reactants is constant at infinite distance from the active site. The developed exact analytical procedure led to a system of linear algebraic equations, which was numerically solved for the simple case of only one angular variable (plane–rotor approximation). The procedure was found to be well suited to calculate the kinetic constant of the enzymatic as a function of different physical parameters, such as the translational and rotational diffusion coefficients, the range of allowed orientations of the reactants, and the size of the active site of the enzyme.
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    The Journal of Chemical Physics 91 (1989), S. 4679-4699 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: An ergodic theoretic basis for the statistical theory of isomerization is provided. A strong mixing assumption is used to derive the absorbing boundary method of computing isomerization dynamics. In addition, the absorbing boundary method is shown to fail in systems exhibiting certain long-time correlations. In order to account for these correlations, we construct a new statistical theory termed the "flux renewal model.'' The new model is based on the consistent application of strong mixing, with the incorporation of nonzero relaxation time. It utilizes statistical calculations to eliminate the explicit computation of long-time trajectories exhibiting characteristics of chaos. The flux renewal model is tested and compared with the absorbing boundary method via numerical computations of the isomerization dynamics of the chaotic siamese stadium billiard. The flux renewal model is shown to give the best approximation to the isomerization flux–flux correlation. It does this by simultaneously handling nonstatistical correlations and long-time scale exponential decay. In addition, with less computational effort, the flux renewal model generally provides more accurate rate constant estimates than the absorbing boundary method.
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    The Journal of Chemical Physics 91 (1989), S. 4700-4713 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: We present an approach to quantum dynamics, based entirely on Cartesian coordinates, which covers vibrational as well as rotational motion. The initial state is represented in terms of multidimensional Gaussian wave packets. Rotational adaptation to angular momentum eigenstates is done by using angular momentum projection operators. This gives an initial state represented as a weighted superposition of Gaussians with different average orientation in space. It is shown that the subsequent dynamics can be determined from the dynamics of Gaussians corresponding to just one of these orientations. An application to the 3D photodissociation dynamics of ICN is presented. All six degrees of freedom which describe the internal motion of the triatomic are included, the only approximation introduced in the present calculation being the thawed Gaussian wave packet approximation for the dynamics. The total absorption spectrum out of vibrational–rotational eigenstates of ICN as well as fully resolved final product distributions are calculated.
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    The Journal of Chemical Physics 91 (1989), S. 4664-4678 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The results of a microcanonical variational transition state theory study of the HO2 and HeH+2 systems are reported. The calculations were carried out using a modification of the Wardlaw–Marcus flexible transition state theory method in which the sum of states N(E,R) is calculated directly. For the HO2 system, the results obtained using the Melius–Blintpotential surface are in excellent agreement with previously reported variational transition state theory calculations of Rai and Truhlar on this system. In particular, two transition states were found for the HO2→OH+O half-reaction. However, calculations using the Lemon–Hase surface produced only one transition state. From calculations carried out on surfaces constructed by combining the two original surfaces and calculations carried out using an orbiting transition state variational model (i.e., ignoring the angular part of the potential), it is shown conclusively that the two transition states found for the Melius–Blint surface arise from the peculiar shape of the minimum reaction path for this surface. Also, the orbiting transition state calculationssuggest that phase space theory can describe this half-reaction on the Lemon–Hase surface over a large energy range provided that the potential along the minimum reaction path is used in the phase space theory calculations. Other calculations, in which the parameters of the Lemon–Hase surface were adjusted, were also carried out in order to determine the conditions necessary to support multiple transition states. The results of these calculations suggest that multiple transition states are more likely to occur when a strong bottleneck is present in the angular part of the potential, especially when the intermediate complex exists in a shallow well. These predictions were confirmed by theresults obtained for the HeH+2 system using the McLaughlin–Thompson–Joseph–Sathyamurthy surface. Two transition states were found for the HeH+2→HeH++H half-reaction and calculations using the orbiting model showed that the origin of the multiple transitionstates is not the minimum reaction path as in the case of the Melius–Blint surface. Furthermore, the energy at which "transition state switching'' occurs is in reasonable accord with the energy at which phase space theory calculations begin to diverge from the experimental data on the cross section for this system. Lifetime calculations for the pseudocomplex lying between the two transition states suggest that the observation of multiple transition states may be possible using femtosecond transition state spectroscopy. Finally, calculations on the HeH+2→He+H+2 half-reaction showed that one transition state exists for this half-reaction at low energy, while at higher energy there are no transition states.
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    The Journal of Chemical Physics 91 (1989), S. 4714-4727 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The reactions of neutral iron clusters Fe7–27 with water are studied in a laser-vaporization cluster source coupled to a continuous-flow reactor. Reaction products are detected via laser ionization and time-of-flight mass spectrometry. The reactions of room-temperature clusters with H2O show adsorbate decomposition and hydrogen desorption, as do the reactions with D2O at elevated temperatures. The room-temperature reaction with D2O appears not to involve any decomposition, and is at equilibrium under the conditions of these experiments. The dependence of reaction extent on D2O pressure yields equilibrium constants for the addition of the first and second D2O molecules. The analysis is complicated by the presence of two-photon ionization processes that are treated quantitatively with a rate-equation model. This treatment also yields estimates for cluster photoabsorption cross sections, which are found to be approximately linear in cluster size, having a magnitude of 2.3×10−17 cm2 per iron atom. From the derived equilibrium constants and estimated adsorption entropies, approximate D2O–cluster binding energies are determined. They range from 0.42 to 0.59 eV, and their dependence on cluster size shows a remarkable similarity to the dependence of the rate constants for reaction of iron clusters with H2. The implications of this similarity are discussed.
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    The Journal of Chemical Physics 91 (1989), S. 4733-4737 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Theoretical calculations have been performed to determine the spectroscopic constants for the ground and selected low-lying electronic states of the transition-metal noble-gas ions VAr+, FeAr+, CoAr+, CuHe+, CuAr+, and CuKr+. Analogous calculations have been performed for the ground states of the alkali noble-gas ions LiAr+, LiKr+, NaAr+, and KAr+ and the alkaline-earth noble-gas ion MgAr+ to contrast the difference in binding energies between the simple and transition-metal noble-gas ions. The binding energies increase with increasing polarizability of the noble-gas ions, as expected for a charge-induced dipole bonding mechanism. We find that the spectroscopic constants of the X 1Σ+ states of the alkali noble-gas ions are well described at the self-consistent field level. In contrast, the binding energies of the transition-metal noble-gas ions are substantially increased by electron correlation. The difference arises from the contribution of metal-neutral noble-gas-ion character in the wave functions. This correlation effect increases as the ionization potential of the noble gas decreases from He to Kr and as the ionization potential of the metal atom increases.
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    The Journal of Chemical Physics 91 (1989), S. 4728-4732 
    ISSN: 1089-7690
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    Topics: Physics , Chemistry and Pharmacology
    Notes: We present an ab initio calculation of polarizability and second hyperpolarizability for the benzene molecule including electron–electron correlation. The finite field method is used. For each selected strength of the applied electric field the energy of the benzene molecule is calculated using the self-consistent field method (SCF) as well as with its Møller–Plesset correction in the second order (MP-2). Then the microscopic optical nonlinear responses are calculated by fitting both the SCF energy and the MP-2 energy to a polynomial in the field strength. We find that electron correlation significantly enhances the second hyperpolarizability. For the polarizability, our computed value shows an excellent agreement with the experimentally measured value. For the second hyperpolarizability, the computed value using MP-2 energy shows a reasonable agreement with that reported by the electrical field induced second harmonic (EFISH) generation but a poor agreement with the result of degenerate four wave mixing (DFWM). Finally, we also compare our ab initio results with those previously reported using semiempirical methods.
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    The Journal of Chemical Physics 91 (1989), S. 4738-4744 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Relativistic effect Erel upon the total electronic energy of an atom is discussed with particular reference to obtaining the nonrelativistic total energy Eexact from the experimental total energy. Numerical values of this effect obtained by various authors by different nonempirical methods are compared for neutral atoms of rare-gas elements. It is shown that methods either of a Hartree–Fock-type or of a Dirac–Hartree–Fock-type give much the same Erel value for He through Ar. It is pointed out that Erel calculated with Hartree–Fock wave functions is not adequate for use in obtaining Eexact from the experimental total energy and that the Erel value calculated with wave functions including electron correlation should work well, although an actual demonstration can be done only for two-electron systems for lack of data. A semiempirical formula is therefore proposed, which is useful for least-squares fit of experimental total energies of isoelectronic series of atoms to extract nonrelativistic total energies along with the relativistic effect. From nonrelativistic energies thus derived, semiempirical values of correlation energies of atoms are obtained. The results thus obtained are in reasonable agreement with correlation energies derived by Clementi along somewhat different lines. The power series expansion in Z of the fitted formula for the He series shows that numerical values of expansion coefficients agree reasonably well with the corresponding values obtained by accurate relativistic and nonrelativistic Z expansion-type calculations.
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    The Journal of Chemical Physics 91 (1989), S. 4758-4762 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The recently proposed method of density mapping between momentum and position spaces is applied to the ground state of the helium atom. The Kellner densities and wave functions are used as reference, and the Eckart densities γ(p) and ρ(r) are employed as test densities keeping their parent wave functions in blind. From the momentum density γ(p), the corresponding position density ργ(r) and the position moments 〈rn〉γ (−2≤n≤4) are generated within a few percent error. The inverse procedure generates the momentum density γρ(p) and the momentum moments 〈pn〉ρ (−2≤n≤4) from the position density ρ(r) with a better accuracy. Density-functional calculations are performed in a simplified manner and the energies close to the Hartree–Fock limit value are obtained.
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    The Journal of Chemical Physics 91 (1989), S. 4763-4773 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Propargylene was identified in a matrix as a product of photolysis of cyclopropenylidene and diazopropyne. The molecule is a triplet. The optimum geometry predicted by ab initio calculations corresponds to a structure HC≡C–C¨H. The transition structure in the interconversion HC≡C–C¨H(arrow-right-and-left)HC(overdot)=C=C(overdot)H(arrow-right-and-left)HC¨–C≡CH is very low in energy and close to the energy of the vibrational ground state. Owing to this nonrigidity, computed infrared (IR) frequencies based on a harmonic treatment do not match the experimental spectrum. When this nonrigidity is taken into account by using a nonharmonic approximation calculated UMP2/6-31G** IR spectra are in good agreement with the observed spectra of HCCCH and DCCCD.
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    The Journal of Chemical Physics 91 (1989), S. 4774-4778 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Linearly averaged position moment (LAPM) [Rk11Rk22⋅⋅⋅ Rk3N3N] is defined as the linear (not quadratic)average of the position moment operator Rk11Rk22⋅⋅⋅ Rk3N3N over the N-electron wave function Ψ({Rj}), where {Rj} are 3N Cartesian coordinates of electrons and {kj} are non-negative integers. When all the LAPM's are well defined, it is shown that the Schrödinger equation is equivalent to a set of an infinite number of equations between LAPM's involving the potential-energy operator. The kinetic energy operator does not appear. The spherical polar representation of the LAPM equation is also presented. Illustrations are given for simple one- and two-electron atoms, where the LAPM equation is applied to the determination of approximate wave functions and associated energies.
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  • 80
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    Notes: In this work the fine structure splitting of the X 3Σ− state of NH together with the spin-forbidden dipole-allowed radiative transitions(b 1Σ+,a 1Δ)→X 3Σ− in that system are considered. In addition the spin-allowed A 3Π→X 3Σ− and c 1Π→(b 1Σ+,a 1Δ) transitions which provide valuable optical probes of the NH radical are studied. Symbolic matrix methods permit the use of large configuration state function (CSF) spaces (170–280×103 CSFs) in characterizing these effects. The fine structure splitting and spin-forbidden decay are described within the context of the Breit–Pauli approximation. In the determination of the fine structure splitting both HˆSO, the full microscopic spin–orbit and spin-other-orbit operator and HˆSS, the dipolar spin–spin operator, are considered through second order in pertubation theory. Thecompletely ab initio determination of λ0≡[E(X 3∑−1, v=0)−E(X 3∑−0+ , v=0)]/2, presented here gives λ0=0.903 (0.9198) cm−1 in good accord with the experimental value given parenthetically. The predicted radiative lifetimes for the v=n level of the A 3Π state τn (A 3Π)(corresponding to A 3Π, v=n→X 3Σ−) are τ0(A 3Π)=392[418±8, 453±10]ns and τ1(A 3Π)=438[420±35, 488±10] ns in good agreement with the experimental values given parenthetically. The predicted radiative rate for the v=0 level of the c 1Π state is somewhat slower than the total decay rate measured experimentally suggesting predissociation of even the lowest rotational levels. The radiative lifetime for the v=0 level of the a 1Δ state, τ0(a 1Δ) corresponding to the spin-forbidden dipole-allowed transition a 1Δ2→X 3∑−1 was found to be τ0(a 1Δ)=2.18(〉1.9)s which compares favorably with the lower bound determined from matrix isolation experiments given parenthetically.For the b 1Σ+→X 3Σ−(0,0) transition the ratio of the parallel to the perpendicular transition moment was found to be μ(parallel)/μ⊥=−0.30 (−0.35±0.05) which again compares favorably with the experimental result given parenthetically. This result is qualitatively different from that in the isovalent systems NF, NCl, and NBr for which −μ(parallel)||μ⊥ 〉1. A partial explanation for this result in terms of molecular dipole moments is offered. The predicted radiative lifetime for the b 1Σ+, v=0→X 3Σ− transition τ0(b1 Σ+)=100 ms, which is in excellent agreement with the independent theoretical determination τ0(b 1Σ+)=97 ms of Marian and Klotz, is significantly longer than the most recent gas phase measurement τ0(b 1Σ+)=53(+17−13)ms.
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  • 81
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    The Journal of Chemical Physics 91 (1989), S. 4779-4784 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The first-order interaction energy between two He atoms is calculated for a range of interatomic distances with a large explicitly correlated basis set reproducing over 99.998% of the correlation energy of the He atom. The interaction energy obtained in basis sets of increasing size converges to within 0.1 μhartree. This accuracy is comparable to accuracy of calculations which use orbital basis sets of spdfgh quality. Our results agree well with values extracted from experiments. We also show that our largest He wave functions behave properly in the outer region of the He atom.
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  • 82
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    The Journal of Chemical Physics 91 (1989), S. 4793-4799 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Quadrature rules for the evaluation of matrix elements relevant for one-dimensional quantum mechanical problems are developed and tested. Nodes and weights are determined from a related problem and the general transformation between localized and delocalized basis representations is discussed.
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  • 83
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    The Journal of Chemical Physics 91 (1989), S. 4785-4792 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The ground and low-lying states of CuF2 and CuCl2 have been studied using the single and doubles configuration interaction (SDCI) and coupled pair functional (CPF) methods in a large Gaussian basis set including up to g-type functions. The results include a determination of the bond distances for all the ligand field states (the three states with one hole in the 3d shell) and charge transfer (CT) states, and force constants for the ligand field states. Overall the SDCI (with or without the Davidson correction for higher excitations) and CPF results are in good agreement. The CPF results, which we believe to be the most accurate, can be summarized as follows. The computed value for the asymmetric stretching frequency in the 2Σ+g ground state of CuF2 is 740 cm−1, compared to the experimental value of 765 cm−1. The d–d transition energies for CuF2 are 2500 and 10 800 cm−1. The two lowest charge transfer states are at 31 200 and 33 900 cm−1, respectively. CuCl2 has been found to have a ground state which is an almost equal mixture of 2Σ+g and 2Πg when an estimate of the spin–orbit interaction is included. Three d–d transitions are found: 1600 cm−1 ( J=1/2), 7500 cm−1 (J=3/2), and 9700 cm−1 (J=5/2). The lowest charge transfer states have been computed to lie at 16 700 and 19 600 cm−1. Two bands have been found in the gas-phase spectrum of CuCl2 at 9000 and 19 000 cm−1, in good agreement with the present results. The computed f value for the CT band is 0.017, to be compared to the experimental value of 0.014.
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  • 84
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    The Journal of Chemical Physics 91 (1989), S. 3296-3305 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: In this paper, we discuss the nonlinear optical effects that arise when stochastic light waves, with different correlation times, interfere in an absorbing medium. It is shown that four-wave mixing signals are generated in several directions that spectrally track the incoming light fields. This effect is particularly relevant to transient hole-burning experiments, where one of these signals could easily be misinterpreted as a genuine hole-burning feature.
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  • 85
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    The Journal of Chemical Physics 91 (1989), S. 4425-4426 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The dissociation energy of boron hydride has been calculated ab initio using a very accurate theoretical model, with an expected accuracy of ±0.2 kcal mol−1. The values of 85.7 and 82.4 kcal mol−1 for De and D0, respectively, confirm the conclusion of Curtiss and Pople [J. Chem. Phys. 90, 2522 (1989)], that the predissociation limit of 82.6±0.4 kcal mol−1 might be close to the actual dissociation energy, and that an earlier proposed De of 78.9 kcal mol−1 is in error.
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  • 86
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    The Journal of Chemical Physics 91 (1989), S. 4426-4427 
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    Topics: Physics , Chemistry and Pharmacology
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  • 87
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    The Journal of Chemical Physics 91 (1989), S. 3287-3295 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The reduced potential curve (RPC) method has so far been successfully employed for the discussion of the ground states of diatomic molecules, however, its efficacy in the complicated world of the excited states has been doubted by some spectroscopists. The present paper should prove that such doubts are in general unjustified. It is shown that, for an excited state of definite symmetry and order (e.g., 1 1Πu, 2 1Πu, etc.) in a group of affiliated molecules, e.g., the group of homo- or heteronuclear alkali diatomic molecules, the same rules hold in the reduced potential curve (RPC) scheme as have been shown before to hold for the ground state. Small deviations and also anomalies with respect to this rule exist for some excited states as must, of course, be expected. The RPC method just seems most suited to visualize such anomalies. Rydberg–Klein–Rees (RKR) and theoretical ab initio calculated potentials are studied in reduced form. The RPC scheme makes possible a systematic comparative study of excited states of diatomic molecules. The RPC method may be also used for detection of errors (inaccuracies) in the analysis of the spectrum or of deficiencies in the theoretical calculation, and for estimation of the potentials of excited states.
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  • 88
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    The Journal of Chemical Physics 91 (1989), S. 3327-3334 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The torsional Raman spectra of C2H6 and C2D6 have been experimentally investigated with an improved conventional Raman spectrometer. Wave numbers and absolute cross sections of all observed bands have been satisfactorily interpreted within the experimental accuracy by means of a simple, pure-torsional Hamiltonian and a model for the torsional dependence of the molecular polarizability. The effective threefold potential barrier parameters for C2H6 have been taken from the recent literature, whilst for C2D6 the values V3eff (C2D6)=990±2 cm−1 and V6eff (C2D6)=7.9±1 cm−1 were obtained.
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  • 89
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    The Journal of Chemical Physics 91 (1989), S. 3335-3339 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The high resolution spectrum of the ν2 (C≡O) stretching vibration in the hydrogen bonded dimer, OC---HF, has been recorded in a continuous wave (cw) supersonic molecular jet using a diode laser spectrometer. Spectroscopic analysis gives the following rovibrational parameters (in cm−1): ν0=2167.69 9 04(11); B0=0.102 200 647(13); D0J =3.244(18)×10−7; B2=0.101 552 5(15); D2J =3.449(36)×10−7. Investigation of observed line profiles allows a lower limit of 0.68 ns to be made for the excited state vibrational predissociative lifetime.
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  • 90
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    The Journal of Chemical Physics 91 (1989), S. 3306-3318 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The technique of optical–optical double resonance is applied to study the ion-pair states of Br2 correlating to Br−(1S)+Br+(3P1). The Br2 molecules in the ground state are pumped to the A3Π(1u) state as an intermediate, and subsequently excited into the ion-pair states by a coherent two-photon transition. The double resonance transition is detected through the dispersed fluorescence terminating on the lower valence states. The 1u(3P1) ion-pair state is observed for the first time and the energy level analyses are made on the v=0 to v=10 levels in detail. The 0−u (3P1) state is also identified through the heterogeneous interaction with the 1u(3P1) state, which is interpreted in terms of the pure precession approximation. The molecular parameters thus obtained in the Dunham expansion are Y00=53 257.605(10), Y10=164.1571(69), Y20=−0.7588(15), Y30=5.650(98)×10−3, Y01=0.040 011 6(40), Y11=−1.7488(42)×10−4, and Y02=−8.40(53)×10−9 for the 1u(3P1) state of 79Br2 isotope species (all in cm−1 with 3σ in parentheses). The main parameters of the 0−u(3P1) state are Y00=53 479.72(25), Y10=148.620(54), and Y01=0.038 021 5(79).
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  • 91
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    The Journal of Chemical Physics 91 (1989), S. 3319-3326 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Microwave and infrared spectra of Ar–HCN have been obtained using an electric-resonance optothermal spectrometer. The microwave measurements extend to higher J the previous results of Leopold et al. and Klots et al., allowing the determination of higher-order centrifugal distortion constants for this quasilinear, highly nonrigid complex. A Padé approximant fit to the microwave data indicates a significant rotation-induced asymptotic increase in the zero-point center-of-mass separation between the Ar and the HCN, above that expected from pure radial distortion. This results from the large coupling between the angular and radial degrees in the intermolecular potential forcing the centrifugal alignment of the HCN. Infrared spectra are reported for the C–H streching fundamental ν1 and the combination band ν1+ν15, where ν5 is the van der Waals bending vibration. The band-origin difference between these two bands gives ν5=7.8 cm−1, in rough agreement with the 10 cm−1 harmonic value predicted from the microwave-determined nuclear quadrupole coupling constant. The complexation-induced red shift of the C–H stretching vibration is 2.69 cm−1 and the vibrational predissociation linewidths Γ are 〈10 MHz (FWHM). The vibrationally excited complex predissociates before striking the bolometer detector, implying that the predissociation lifetime τ〈1 ms.
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  • 92
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    The Journal of Chemical Physics 91 (1989), S. 3360-3367 
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    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have performed scattered light intensity autocorrelation measurements on a Winsor type microemulsion system composed of brine, cyclohexane, SDS and a mixture of 1-butanol and 1-pentanol. At high cosurfactant concentration, where the microemulsion phase was considered to consist of individual, spherical water-in-oil droplets of relatively low droplet volume fraction, the autocorrelation functions were observed to be essentially single exponential, as expected. Above a certain droplet volume fraction, however, additional decay modes were observed to enter the correlation data. These modes were interpreted to be due to rotation and/or internal motion of droplet aggregates.
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  • 93
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    The Journal of Chemical Physics 91 (1989), S. 3340-3359 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The electronic spectra of silyl radicals, SiH3 and SiD3, were observed between 310 and 430 nm (46 000–64 000 cm−1) by resonance enhanced multiphoton ionization (REMPI) mass spectroscopy. The spectra were generated through a 2+1 REMPI mechanism. Two Rydberg series originating from planar, D3h point group states were observed. One series, of quantum defect δ=1.45(2), is comprised of the E˜ 2A‘2(4p), J˜ 2A‘2(5p), and M˜ 2A‘2(6p) Rydberg states which have origins at ν0–0 =48 438, 56 929, and 60 341 cm−1 in SiH3 and at ν0–0 =48 391, 56 874, and 60 267 cm−1 in SiD3. In SiD3 theP˜ 2A‘2(7p) Rydberg origin was observed at ν0–0 =62 002 cm−1. The H˜, K˜, and N˜ states observed in the SiD3 spectrum comprise the second Rydberg series, δ=2.09, and were tentatively assigned as ns 2A'1 Rydberg states (n=5, 6, 7). The K˜ and N˜ origins were observed at ν0–0 =58 417 and 61 005 cm−1. A fit of the Rydberg formula to the np 2A‘2(n≥5) origins found the adiabatic ionization potential of the SiH3 and SiD3 radicals to be IPa=8.135(+5,−2) eV and IPa=8.128(1) eV, respectively. Detailed vibrational analyses of these Rydberg states are presented. Analysis showed that in the E˜ 2A‘2 (4p) state of the SiH3 radical ω2 (a‘2 symmetric bend)=796(7) and 2ω4 /2(e' degenerate bend)=870(5) cm−1 and that in SiD3 radical ω'1 (a1 SiH3 symmetric stretch)=1576(3), ω'2 =589(3), and 2ω4 /2=635(6) cm−1. The REMPI spectra exhibited ν‘2 hot bands from vibrational levels as high as Ev =2073 cm−1 in the X˜ 2A1state. Modeling calculations, which fit the numerous ν‘2 hot bands, predicted barriers to inversion of Binv=1935 cm−1 and Binv =1925 cm−1 for SiH3 and SiD3 X˜ 2A1 radicals, respectively.
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  • 94
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    The Journal of Chemical Physics 91 (1989), S. 3368-3377 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The 28SiH+2 29SiH+2 cation radicals have been generated under neon matrix deposition conditions at 4 K by the photoionization (16.8 eV) of SiH4(g). A resolution of the 29Si and hydrogen A tensors was possible which enables a complete characterization of the unpaired electron in this highly reactive ionic species for its X 2A1 ground electronic state. The A tensor assignments were facilitated by the occurrence of preferential orientation of the isolated SiH+2 molecule in the neon lattice. Ab initio theoretical calculations of the nuclear hyperfine tensors were conducted which showed good agreement with the experimental results. The experimental magnetic parameters in neon at 4 K are: gx=2.0043(3), gy=1.9970(3), gz =2.0018(3); Ax(H) =110.4(3), Ay (H)=124.1(3), Az (H)=116.6(3); for 29Si, Ax =−762.9(3), Ay =−760.8(3) and Az=−1005.0(3) MHz where X is perpendicular to the molecular plane and Z lies along the C2 axis of the C2v radical. No experimental evidence for SiH+4 or the complex, SiH+2 ⋅⋅⋅H2 was obtained. Theoretical calculations indicate that hyperfine structure from the "complexed'' H2 should be resolvable if this species was present. However, rotational averaging of H2 above the SiH+2 plane would reduce the hfi of H2 and might prevent its detection.
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  • 95
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    The Journal of Chemical Physics 91 (1989), S. 3378-3383 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: N(2D) can be generated by two-photon dissociation of NO above the threshold wavelength of 279 nm. A comparison of the three-photon NO+ ionization signal with the N(2D) excitation spectrum resulting from its detection by 2+1 resonance-enhanced multiphoton ionization (REMPI) shows both similarities and differences. The NO Rydberg states that are generated in the initial two-photon process either ionize or predissociate. The NO+ and N+ signals are indicative of predissociative lifetimes and channels, since an NO molecule that does not predissociate to N(2D) and O(3P) can separate to two lower atomic limits. In the 268–279 nm region there are three two-photon absorption bands, 5sσ-X 3-0, 4dπ−-X 2-0, and 5dπ−-X1-0, which show very high N(2D) predissociative yields from the upper states. A fourth band, 4dδ-X2-0, behaves in a distinctly different manner from its 4dπ− complex partner. The presence of the valence G2Σ− state, which lies within the experimental energy range, is not in evidence from either the three-photon NO+ spectrum, or from predissociation in the N(2D) channel. It appears that this state predissociates rapidly to the lower N(4S)+O(1D) or N(4S)+O(3P) limits, even above the N(2D)+O(3P) threshold.
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  • 96
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    The Journal of Chemical Physics 91 (1989), S. 3384-3391 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Infrared laser absorption spectra of H2O+ have been recorded in an ac discharge using velocity modulation detection. Eighty-five lines of the 210 fundamental and 20 lines of the 221 hot band were used to derive rotation, spin-rotation, and distortion parameters for the ground state and the first two vibrationally excited states of the bending mode. The results are compared with those from analysis of the A˜–X˜ band system and from rotational and vibration–rotation laser spectra, as well as with recent ab initio results obtained using MRCI wave functions.
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  • 97
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    The Journal of Chemical Physics 91 (1989), S. 3405-3425 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: A high degree of structure and therefore order in chaos is found to exist in the detailed dynamical pathways to conformational isomerization. It is shown that this structure can be used to determine the probabilities associated with the dynamical pathways to reaction, trapping, and back reaction. An earlier publication described the mediation of the dynamics of 3-phospholene by phase space structures we called "reactive islands'' (RIS)21. In this paper we extend the physical and mathematical properties of RIS and develop the corresponding kinetic theory. RIS theory is applied to a model of a hindered rotor and 3-phospholene. It is shown that the RIS kinetic model accurately predicts trajectory simulations of conformer population decay. Comparisons with standard RRKM theory are included. A discussion on the extension of RIS theory to quantum reactive dynamics and its relevance to laboratory experiments is also included.
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  • 98
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    The Journal of Chemical Physics 91 (1989), S. 3392-3404 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: In this paper we focus on the detailed nonlinear classical dynamics of conformational isomerization. In particular we concentrate on systems which admit phase space structures we call "reactive islands.'' Our calculations are on a two degree of freedom model of the molecule 3-phospholene with an experimentally fit potential energy surface by Harthcock and Laane. The reactive islands (RIS) are embedded within and are part of chaotic regions of phase space. We find that the RIS are constructed from a linear stability analysis of the period 1 orbit at the transition state or approximated by a similar analysis on reactive periodic orbits. The two approaches converge as the order of the reactive periodic orbit increases. It is found that the fully constructed RIS have well defined regions of reactivity and thus mediate the process of conformational isomerization.The overlap areas of the RIS give important kinetic information such as probabilities for trapped to reactive motion, reactive to trapped motion, and primary back reaction. The implications of this work lie in precisely identifying the microscopic dynamical pathway for isomerization. The results of this work and its relation to the related work of Gray and Rice as well as its relation to contemporary work in modern dynamics is also discussed.
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  • 99
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    The Journal of Chemical Physics 91 (1989), S. 3426-3430 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The pure rotational spectrum of CCl2 in the ground state was observed for the first time in the millimeter- and submillimeter-wave region. The CCl2 molecule was generated directly in a free space cell by a dc glow discharge in CCl4 . Several transitions showed hyperfine splittings arising from the nuclear quadrupole coupling of two Cl atoms. Analysis of the observed spectrum yielded the rotational, centrifugal distortion, and nuclear quadrupole coupling constants of the C35 Cl2 species. The r0 structure was derived from the rotational constants to be r0 (C–Cl)=1.7157 A(ring) and θ0 (ClCCl)=109.2°. The ionic and π character of the C–Cl bond were estimated from the nuclear quadrupole coupling constants to be 0.26 and 0.32, respectively.
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  • 100
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    The Journal of Chemical Physics 91 (1989), S. 3431-3434 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Molecular orbital calculations have shown that the structures of both trans and cis isomers of N,N'-disubstituted indigo dyes are considerably twisted about the central C=C bond. The twisting angle of the cis isomer is not always larger than that of the corresponding trans isomer, indicating that the large hypsochromic shift on the trans→cis isomerization of N,N'-disubstituted indigo dyes cannot be explained by the twisted structure of the cis isomer. It is shown that the hypsochromic shift can be ascribed to a larger contribution of the zwitterionic form to the resonance hybrid structure in the lowest excited singlet state S1 than in the ground state S0 .This makes the repulsion between the negative charges on the carbonyl oxygen atoms enhanced and the instability of the cis configuration compared to the trans configuration much larger in the S1 state than in the S0 state and leads to a larger vertical S1 ←S0 transition energy for the cis isomer than for the trans isomer.
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