ALBERT

Notes: We have derived theoretical expressions for the incident beam angular dependence of the absorption cross section for ethylene chemisorbed on single crystal surfaces with linearly polarized light. The changes in the π and σ resonance intensities for C 1s near-edge x-ray absorption fine structure (NEXAFS) spectra can be related to the geometry of chemisorption relative to the surface. The derived equations have been used to analyze experimental data in order to determine the structure of chemisorbed ethylene on Ni(100). At low temperatures ethylene bonds with the molecular axis parallel to the surface but with the molecular plane tilted 50°. The geometry of vinyl fragments formed by annealing the crystal to 180 K was also determined. The C–C bond is bent 35° from the surface normal in this case, with the plane of the molecule tilted by 65°.

Notes: It is shown that biphotonic excitation of diarylmethanes leads to the formation of ion pairs with extremely long time scale of charge recombination.

Notes: The internal state distributions of NO desorbed from a Pt(111) surface by visible and near-visible laser radiation (355, 532, and 1064 nm) were measured by laser-induced fluorescence. Non-Boltzmann rotational state distributions and inverted spin–orbit populations were observed and both were found to be relatively insensitive to the desorption-laser wavelength. It is suggested that the internal state distributions arise from the charge exchange processes occuring during desorption via a short-lived negative-ion resonance intermediate.

Notes: Electron impact of dilute Ar/NH3 samples during condensation at 12 K produced six new infrared bands: 3552, 3217, 3152, 3133, 1523, and 1496 cm−1. The 3552 cm−1 band has been assigned to OH−, the 3133 cm−1 band to NH, and the 3217 and 1496 cm−1 bands to NH2 radical by earlier workers. The 3152 and 1523 cm−1 bands exhibit proper 15N shifts to be assigned to the NH−2 anion isolated in solid argon.

Notes: The two-photon absorption spectra of the 3sE' states of cyclopropane-h6 and cyclopropane-d6 are obtained by two-photon resonant three-photon ionization spectroscopy. Bands are vibronically resolved by virtue of a free-jet expansion. The hot bands of these spectra suggest that the excited state is distorted in both the carbon ring deformation coordinate and the CH2 wag coordinate. These are the lowest e' modes, and thus their activity can be interpreted as arising from Jahn–Teller interactions. A satisfactory fit of both line positions and intensities is achieved within this model. The dynamics and structure predicted by the Hamiltonian used are described. It is concluded that, in the ground vibronic state, nonadiabatic interactions are small, but the electronically excited molecule is nevertheless very fluxional, sampling a wide range of geometries. Morever, while only minimal barriers to pseudorotation exist in each coordinate individually, there is some locking of the phases of the two modes, so the pseudorotations are not completely independent.

Notes: Absolute cross sections for electron-impact ionization of the SiF3 free radical from threshold to 200 eV are presented for formation of the parent SiF+3 ion and the fragment SiF+2, SiF+, and Si+ ions. A 3 keV beam of SiF3 is prepared by near-resonant charge transfer of SiF+3 with 1,3,5-trimethylbenzene. The beam contains only ground electronic state neutral radicals, but with as much as 1.5 eV of vibrational energy. The absolute cross section for formation of the parent ion at 70 eV is 0.67±0.09 A(ring)2. At 70 eV the formation of SiF+2 is the major process, having a cross section 2.51±0.02 times larger than that of the parent ion, while the SiF+ fragment has a cross section 1.47±0.08 times larger than the parent. Threshold measurements show that ion pair dissociation processes make a significant contribution to the formation of positively charged fragment ions.

Notes: Absolute cross sections are measured for electron impact ionization and dissociative ionization of SiF2 from threshold to 200 eV. A fast (3 keV) neutral beam of SiF2 is formed by charge transfer neutralization of SiF+2 with Xe; it is primarily in the ground electronic state with about 10% in the metastable first excited electronic state (a˜ 3B1). The absolute cross section for ionization of the ground state by 70 eV electrons to the parent SiF+2 is 1.38±0.18 A(ring)2. Formation of SiF+ is the major process with a cross section at 70 eV of 2.32±0.30 A(ring)2. The cross section at 70 eV for formation of the Si fragment ion is 0.48±0.08 A(ring)2. Ion pair production contributes a significant fraction of the positively charged fragment ions.

Notes: We have studied the electronic decay of an oxygen 1s core hole in N2O and the subsequent fragmentation into ions. Coincidence studies between the energy resolved Auger electrons and the ions reveal the individual decay channels for the various doubly charged ionic configurations populated in the Auger decay of the core hole. The site specificity in these events is not absolutely confined to breaking only the O–NN bond, even though the excitation of a core hole at the oxygen site in the initial absorption event is localized. This localization is to some extent preserved in the Auger decay, which favors the participation of valence electrons having wave functions with a large overlap at the site of the initial oxygen core hole. However some of these electrons are also crucial in forming the ON–N bond or the bonding structure of the molecule as a whole, such that removal of these electrons in the Auger decay results in the total destruction of the molecular bond.

Notes: A theoretical force field for in-plane vibrations of benzene has been determined from ab initio calculations at both the Hartree–Fock level with 4-21G, 6-31G, and 6-31G* basis sets and the MP2 level with 4-21G and 6-31G basis sets. The average error of the calculated frequencies at the MP2 level is between 2% and 3%. The reliability of the force field and vibrational frequency predictions of the calculations are analyzed. All diagonal stretching force constants obtained at the MP2 level are in quantitative agreement with Ozkabak–Goodman experimental force field, while the diagonal force constants involving ring deformation and CH rock are somewhat overestimated by the theory. Most of the off-diagonal force constants agree with the Ozkabak–Goodman results in sign but there are some significant quantitative differences in magnitude. Comparisons are made with other force fields, including results obtained by scaling ab initio calculations or introducing modified Hamiltonians. A simple extrapolation method for introducing correlation corrections into Hartree–Fock force constants gives excellent results for benzene.

Notes: Crystalline benzene has been investigated at room temperature as a function of pressure up to 25 GPa in diamond anvil cells by Raman scattering and powder x-ray diffraction techniques. The concomitant spectroscopic and crystallographic results show the existence of numerous pressure-induced phases. Changes in the profiles of the Raman spectra and in the x-ray diffraction patterns, as well as changes in the variations of the Raman frequencies and the cell parameters with pressure indicate two first-order phase transitions at 1.4±0.1 and 4±1 GPa and a second-order one at 11±1 GPa. At 24 GPa the x-ray diffraction pattern seems to indicate the existence of a new phase. Two monoclinic structures are proposed for the phases above 1.5 GPa, in addition to the already known one. From these data, molar volume has been determined as a function of pressure and the Grüneisen parameters have been inferred in the different phases. Their pressure dependences are analyzed in the light of theoretical predictions. Arguments are given for a phase transformation at normal pressure and below 140 K or at room temperature below 1 GPa. A schematic P–T phase diagram is suggested and a controversy on the nature of the triple points located on the melting curve is clarified.

Notes: Absorption spectra of YAG:Nd3+ in the region 28 000–39 500 cm−1 at liquid helium temperature (LHeT) and liquid nitrogen temperature (LNT) have been studied. The whole absorption spectra of YAG:Nd3+ at room temperature (RT) has been measured. Based upon the present and earlier measurements the data at three temperatures are listed. By using the irreducible tensor method, we diagonalized the parametrized D2 Hamiltonian in a free-ion wave function space involving the whole J multiplets, fitting the eigenvalues to the experiment data at LNT, we assigned the whole Stark levels by using the eigenvectors obtained.

Notes: Several fluorescence line groups in the infrared, visible, and ultraviolet regions are observed by exciting YAG:Nd3+ crystal with Ar+ laser (5145 A(ring)) and x-ray at liquid nitrogen temperature (LNT) and room temperature (RT), respectively. All of these line groups are assigned properly. Some new Stark levels are determined from the fluorescence spectra. The excited state absorption process is observed.

Notes: The opto-thermal detection method has been used to obtain rotationally resolved infrared spectra for both (14N14N16O)2 and (15N14N16O)2. The vibrational band observed in each case correlates with the ν1+ν3 band of the N2O monomer. A large number of A- and B-type transitions have been assigned and fit for both species yielding accurate molecular constants for both the ground and excited vibrational states. From intensity alternations associated with nuclear spin statistics and the small inertia defect [Δ‘=0.611 amu A(ring)2 for (14N14N16O)2], it is clear that the nitrous oxide dimer has a slipped parallel geometry which possesses a center of symmetry. Isotopic substitution shows that of the two such structures, only the one having the oxygen atoms close to one another is experimentally observed. The structural constants are determined to be RNN=3.493(4) A(ring) and θNNO=59.2(5)°.

Notes: The effect of pressure on the conformation of biphenyl in carbon disulfide was measured by using high pressure infrared spectroscopy with a diamond anvil up to 13 kbar. Pressure effects on the nonplanar vibrational modes indicate that the dihedral angle decreases at the rate of 2° kbar−1. Under high pressure the frequency of the C–H stretching vibrational modes shifts to blue so that the repulsive force becomes more predominant in the solute–solvent interactions between CH and CS2 molecule. The decrease in the dihedral angle of biphenyl induced by compression is related to the change of the intrinsic volume of the biphenyl molecule.

Notes: The structure and vibrational predissociation dynamics of HeCl2 are studied by pump–probe spectroscopy and by three-dimensional quantum mechanical calculations. Parity selected excitation spectroscopy is used to confirm the essential features of the previous analysis of the HeCl2 B←X laser excited fluorescence spectra. Product vibrational and rotational state distributions are measured for the v'=6, 8, 12, 20, and 24 levels of HeCl2 in the B state. For the v'=6 and 8 levels the dependence of the product state distribution on the initially excited rotational state is also measured. Although the dissociation dynamics are dominated by Δv=−1, V→T energy transfer, several interesting effects are revealed by monitoring the product rotational degrees of freedom. Due to the symmetry of the HeCl2 potential, the parity of the initially excited HeCl2 rotational state is conserved during the dissociation dynamics. Even when a single initial rotational state is excited, the observed product rotational state distribution is bimodal. The product rotational distribution is nearly independent of the amount of kinetic energy released to the product degrees of freedom. Three-dimensional quantum mechanical calculations using a simple potential energy surface are remarkably successful at reproducing the details of the experimental measurements. Only five parameters of the potential were adjusted to calculate the excitation spectrum, the vibrational predissociation product state distributions, and the lifetimes of the excited states. Analysis of the dissociation mechanism in terms of simple models, however, is not straightforward. In particular, the impulsive, quasiclassical half-collision model is not compatible with the observed independence of the product rotational state distribution from the amount of kinetic energy which is released. The close agreement between state-to-state experiment and quantum theory on the HeCl2 dynamics shows that the shortcomings of more approximate theories are fundamental and cannot be attributed to lack of knowledge of the true potential energy surface.

Notes: Vibrational energy relaxation time T1 of nitrogen in highly compressed mixtures of D2–N2 has been measured at room temperature as a function of gas density and mixture ratio. The first vibrational state of N2 has been populated by a rapid V–V transfer from D2 which has been vibrationally excited by collision-induced absorption. The decay of the N2 vibrational excited population has been time resolved by monitoring the infrared collision-induced fluorescence. The data are compared with attractive hard-sphere models developed for energy relaxation in dense fluids.

Notes: The dynamics of a state undergoing decay to a manifold of states coupled to a dissociative continuum is investigated in the weak coupling limit. The decay rate is found to be composed of terms symmetric and asymmetric in the relative energy difference between the initial state and the states in the manifold. The well-known symmetric terms arise from resonantly enhanced decay of the initial state through each state in the manifold to the continuum. The newly identified asymmetric terms arise from interfering couplings of the initial state to the continuum through different states in the manifold, i.e., from off-diagonal elements of the width matrix Γkk' of the effective Hamiltonian describing dissociation of the states in the manifold. The general line shape derived here is used to fit asymmetric features in Stark level-crossing spectra of D2CO. Statistical properties of the width matrix are summarized, and the relation between diagonal and off-diagonal matrix element magnitudes is derived.

Notes: A full quantum mechanical calculation is carried out on the first excited state of CH2I2 to model the absorption and emission spectra and examine the photodissociation dynamics from a time dependent point of view. The dissociation at 355 nm is direct in the sense that the wave packet does not revisit the Franck–Condon region. The initial motion of the excited molecule is mainly along the CI2 symmetric stretch coordinate while simultaneously spreading in the antisymmetric stretch coordinate. The molecule then dissociates along a C–I "local'' mode; no I2 can be formed in this energy region. Vibrationally hot CH2I radical in the C–I mode is predicted. The model is in good agreement with available experimental results. A simple and intuitive method is presented to construct model potential energy surfaces for two chromophore systems from the potential energy surface and information known for the corresponding single chromophore. CH3I and CH2I2 are used as numerical illustrations.

Notes: The Maxwell–Schrödinger equations describing the ultrashort laser pulse propagation in multilevel molecular media are shown to be analytically solvable and to reduce to nonlinear multisine-Gordon equations for certain models of the field–molecule system. Special cases of level schemes are shown to lead to soliton propagation. A numerical study is presented in order to examine the effect of nonresonant (virtual) transitions on pulse propagation and amplification in a three-level molecular system as a function of intensity and detuning.

Notes: It is well known that the correlated ground state energy can be derived from a knowledge of the single particle Green's function of a system, even though the two density cannot be obtained from it. In this paper it is shown that the single particle Green's function in fact contains more detailed information than the total energy alone, to the extent that the local Slater‘xnLöwdin correlation potential can be obtained from it. This potential can be used as a more detailed criterion to judge the quality of approximate Green's functions than the total energy by itself. The formalism leads moreover to a natural partitioning of the correlation energy into terms depending on the correlation correction to the one density alone and a remaining "true'' correlation contribution. In this paper we calculate the single particle Green's function using the second order approximation to the self-energy for a series of small model systems (He, Be, H2, LiH, and H2O). The correlation potential and the partitioning of the correlation energy are used to analyze this approximation and to assess its accuracy in these systems.

Notes: We have performed a study of the water dimer interaction using larger basis sets and higher levels of theory than have been previously applied to this system. For the minimum geometry we have used spdf basis sets containing up to 212 orbitals. Our most accurate SCF interaction energy for the minimum is −3.73±0.05 kcal/mol. We have shown that this energy can be reproduced to within 0.1 kcal/mol using much smaller basis sets containing proper (diffuse) exponents. Accounting for the basis set superposition error is shown to be essential. We computed the dispersion energy with neglect of the intramolecular correlation using basis sets of various sizes. The best value obtained in a large spdf basis set with exponents which optimize this quantity is −1.93 kcal/mol and it is expected to be accurate to 0.1 kcal/mol or better. Using some of these basis sets we have performed supermolecular many-body perturbation theory (MBPT) and coupled-cluster (CC) calculations including triple excitations. We have shown that if the correlated supermolecular interaction energies are not corrected for the basis set superposition error, the correlated part of the interaction energy varies widely with the basis set. In contrast, the corrected values converge smoothly. On the basis of the dispersion energy and the MBPT/CC results we predict the correlated part of the interaction energy to be −1.0±0.3 kcal/mol, which leads to the total interaction energy of −4.7±0.35 kcal/mol. Thus, our work favors the lower limit of the experimentally predicted interaction energy of −5.4±0.7 kcal/mol.

Notes: The Ewald method, which is known as a method for evaluating electrostatic interactions in periodic systems, is extended to nonperiodic systems. A general way of applying the Ewald method to nonperiodic systems is described and a detailed application is demonstrated for the treatment of ions immersed in dipolar solvent molecules. As a preliminary test, electric fields produced by point dipoles forming a simple cubic lattice are calculated, both by the new Ewald treatment and by the conventional cutoff (truncation) method. These test calculations not only demonstrate the advantage of the new Ewald method but also reveal inherent problems associated with the cutoff method.

Notes: We propose a model based on separating the anharmonic vibrations from the stable packing configurations that elucidates how the loss of configurational entropy Sc gives rise to kinetic sluggishness as the temperature of a glass forming liquid is lowered towards T2, where Sc vanishes. The shear viscosity, the self-diffusion coefficient, and the structural relaxation time of a cooperative rearranging region are evaluated. We calculate the time necessary to establish an equilibrium distribution of subcritical clusters in the supercooled liquid. A comparison of the induction time with the structural relaxation time sheds light into the nature of the transition at T2.

Notes: A theoretical approach to the problem of the marked irreproducibility of certain chemical reactions studied by Epstein's group at Brandeis University is presented. The model is based on the use of a Langevin equation with a multiplicative noise. Stirring and finite volume effects are incorporated in our approach. Analytical results are provided for the mean first passage time (MFPT), which is the appropriate quantity we associate here with the reaction time.

Notes: The electric microfield distribution in fluids of particles of arbitrary size, shape, charge, and charge distribution is studied. The Morita–Iglesias formalism is extended to include angular dependent interactions. The results of the mean spherical approximation are discussed using an exact relation for the second moment of the microfield distribution, obtained in this work. In the Onsagerian strong coupling limit the sum rule for the second moment simplifies, and becomes identical to the mean spherical approximation result for the second moment.

Notes: We investigate the partition of ionic species in a system of two phases that are immiscible even in the limit of vanishing ionic concentrations. An analytic expression for the solvation free energy and for the standard free energy of transfer is obtained for dipolar hard-sphere solvents in the mean spherical approximation. We find that both dipolar hard-sphere solvents and continuum solvent models yield an electric potential difference (equilibrium junction potential) between two pure solvents in contact that is identically zero. Simple analytic expressions for partition coefficients and junction potentials are obtained in the ideal-solution limit for these solvent models. We note that for the junction potential the ideal solution limits differ in general from the pure-solvent values. For nonideal solutions, approximations based on the mean spherical approximation can be applied conveniently to calculate the activities of ionic species and to obtain the partition coefficients.

Notes: The path integral formulation of the Kirkwood diffusion equation is used to derive the perturbation expansion of the intrinsic viscosity in powers of the hydrodynamic interaction. Within the preaveraging approximation the perturbation expansion yields the Zimm formula for the intrinsic viscosity. The renormalization of the dynamical viscosity [η(ω)] has been explicitly carried out for both zero and large frequencies. At intermediate frequencies the renormalization of [η(ω)] has been performed in an approximate manner. A simple analytical expression for the dynamical viscosity is given. For low ω the real part of [η(ω)] has a plateau, while for large ω the Zimm behavior ([η(ω)]∼ω−1/3) is obtained.

Notes: Numerical methods are described for the calculation of time correlation functions and the stress relaxation moduli for the model developed in the preceding paper [J. Chem. Phys. 89, 3892 (1988)]. Illustrative results are presented for the storage and loss modulii, the shear compliance, and the translational diffusion constant, with parameters appropriate to polybutadiene.

Notes: The gas phase optical spectrum of jet-cooled Pt2 has been investigated over the range of 11 300 to 26 300 cm−1 using resonant two-photon ionization spectroscopy in combination with time-of-flight mass spectrometry. Numerous vibronic bands are observed. Analysis of the data gives the location of some 26 excited electronic states, which are characterized by the frequencies of their origin bands, vibrational frequencies, and anharmonicities. Variation of the second color in a two-color resonant two-photon ionization scheme has determined the ionization threshold of Pt2 to be 8.68±0.02 eV. The observation of the onset of predissociation, characterized by a sharp drop in excited state lifetime, places the dissociation energy of Pt2 at 3.14±0.02 eV. In combination with the Pt atomic ionization potential of 8.8±0.2 eV, these results give the bond strength of Pt+2 as D0(Pt−Pt+)=3.26±0.24 eV. The strength of the chemical bond in Pt2, as compared to Au2, demonstrates that there are significant 5d contributions to the chemical bonding in Pt2.

Notes: Multiphoton ionization (MPI) in conjunction with photoelectron spectroscopy (PES) has been used to examine Rydberg states of OCS in the energy range from 70 547 to 75 472 cm−1. The combination of techniques allows us to correct previous assignments for several vibronic bands, and to probe vibronic mixing in the Rydberg levels. In the course of this study we have found routes for preparing OCS+ with control over both vibrational mode and fine structure state.

Notes: A systematic study of the dependence of the band gap, the linear optical susceptibility, the polarizability α, and the second hyperpolarizability γ, on the number of repeat unit is conducted for the thiophene series from monomer to hexamer. The linear optical susceptibilities for oligomers have been determined from the refractive index measurements on vacuum deposited films using the m lines technique. The orientationally averaged polarizabilities 〈α〉 have been measured from refractive index measurements of THF solutions. The orientationally averaged second hyperpolarizabilities 〈γ〉 have been measured by degenerate four-wave mixing studies of THF solutions. The validity of the Lorentz–Lorenz approximation is tested and found to be satisfactory. The experimental values of 〈α〉 and 〈γ〉 for thiophene and 〈α〉 for bithiophene are found to be in qualitative agreement with those obtained by a recent ab initio calculation which used the finite field method and included diffuse polarization functions. The experimentally observed dependence of 〈α〉 and 〈γ〉 on the number N of the thiophene repeat unit is compared with that predicted by a free electron model, PPP methods, and the ab initio calculations. For the case of polarizability, the repeat unit dependence is in good agreement with that predicted by the ab initio calculations but for the case of second hyperpolarizability, the agreement is not as good.

Notes: The 13C dipolar powder spectra of 13CH3 31PO(OH)2 at room temperature and 13CH3 19F at low temperature (T=25 K) are reported. An intense peak is observed at the center of the Pake doublet. This phenomenon is explained by the transverse cross relaxation mechanism between the 13C and either the 31P or 19F nuclei. This theory shows the importance of the nonsecular terms in the Redfield relaxation theory. The chemical shielding tensors and dipolar coupling constants of these two molecules are obtained by fitting the experimental spectra.

Notes: The acoustic waves generated by a relaxing gas have a shape which depends on the ratio of the relaxation rate to the acoustic propagation rate. It is shown how the relaxation time constant can be extracted from this waveform for a variety of relaxation rate laws, using a Green's function to solve the linearized wave equation. The accuracy of the method is confirmed by measuring the relaxation rate constants for OCS diluted in Ar and He. These, plus a previous measurement of the relaxation rates of highly excited SF6 and C6F5H in Ar, are in good agreement with the literature.

Notes: The mixed-mode oscillations observed at high flow rates in the Belousov–Zhabotinskii (BZ) reaction are considered and comparison is made between these oscillations and the dynamics of three different mathematical models based on slow manifolds. It is shown that the model proposed by Rössler for the generation of complex behavior in nonequilibrium chemical reactions is in conflict with the behavior of the BZ reaction. It is also shown that a slow-manifold model based on the hysteresis-Hopf normal form fails to accurately reproduce the oscillations found at high flow rates in the BZ system. A model of the type first proposed by Boissonade is presented; the model consists of the coupling of two simple systems. It is shown that this model naturally generates mixed-mode oscillations like those observed in the BZ reaction.

Notes: Self-broadened nitrogen isotropic Q(J) Raman linewidths have been inverted to obtain effective rotation–translation (R–T) state-to-state rate constants using the energy corrected sudden (ECS) formalism. These rate constants are discussed as a function of the rotational levels J and temperature T. Collisional Q(J) line shifts have been investigated by high-resolution inverse Raman spectroscopy (IRS) over a wide temperature range. Semiclassical calculations lead to a clear understanding of their J and T dependence. This exhaustive study of both diagonal and off-diagonal relaxation matrix elements has allowed us to calculate the collisionally narrowed Q branch at high pressure. New measurements of N2 Q branch at high pressure have been performed by IRS. The good agreement of ECS profiles with IRS data, for various pressures and temperatures, underlines the consistency of the present R–T ECS scaling analysis.

Notes: We present here a calculation of nuclear magnetic resonance (NMR) spectral line shapes of spin 3/2 nuclei exchanging between sites with different quadrupole interactions and different reorientational correlation times. The calculation, which is based on the solution of the semiclassical Liouville equation, is performed for the whole range of exchange rates and reorientational correlation times including very slow motions. It is shown that the linewidth at half-height does not vary in a simple way as a function of the exchange rate, but rather passes through two maxima.

Notes: The time-resolved infrared (IR) double-resonance method has been applied to the study of collision-induced vibrational energy transfers in CD3Cl. The fast energy transfer between the Coriolis-coupled levels of the v2=1 and v5=1 vibrational states of CD3Cl was investigated both in the pure gas and in mixtures with argon yielding rate constants, respectively, equal to (1.18±0.08)107 and (2.12±0.78)106 s−1 Torr−1. This fast intermode transfer is found to occur only between rovibrational levels of the same symmetry type. The V–V transfer between the A and E symmetry levels has also been observed, as well as the V–V transfer between the two isotopic species CD335Cl and CD337Cl. The measured rate constants for these processes are very close and equal to (6.1±0.4)105 s−1 Torr−1. The rate constant for the vibrational thermalization was also determined and found to be (2.5±0.5)103 s−1 Torr−1, in agreement with previous studies.

Notes: We examine the relation between photodissociation dynamics and the form of the total absorption spectrum for H2O in the first absorption band. Application of an exact time dependent formalism gives a direct and intuitive relation between molecular motion and spectral features. We show that the series of weak structures on top of the broad continuum is due to symmetric stretch motion in the excited state. The spacing between the structures is, essentially, given by the frequency associated with this motion. In addition, we present some excited state eigenfunctions, which provide a direct connection between the structured spectrum, the final product distributions, and the dynamics.

Notes: The bending corrected rotating linear model is used to study the angular distributions for the reactions F+H2, F+HD, and F+D2. The calculations are performed using two new surfaces (No. T5 and T5A) that were recently reported by Steckler et al. (Ref. 3). Analysis of the factors determining the shapes of these angular distributions indicates that the angular momentum dependence of the vibrationally adiabatic barriers in the entrance and exit channels plays a central role in determining the characteristics of the vibrational state-selected, reactive product angular distributions. While the distributions predicted for the reactions F+H2(v=0)→H+HF(v'=3), F+D2(v=0)→H=DF(v'=4), and F+DH(v=0)→H+DF(v'=4) are in good agreement with the high resolution molecular beam studies of these same reactions, the angular distributions predicted for the reaction F+HD(v=0)→D+HF(v'=3) do now show the forward scattering of products derived from high resolution molecular beam studies. This discrepancy appears to be due to a shortcoming in the T5 and T5a surfaces (i.e., the angular momentum dependent, vibrationally adiabatic barriers in the exit channel are too high in energy to permit forward "resonant'' scattering at the low scattering energies corresponding to the experimental studies).

Notes: Experiments were carried out using a time-resolved atmospheric pressure ionization mass-spectrometer (TRAPI) in N2–H2O (∼1 ppm) system at temperatures from 233 to 543 K and at pressures from 167 to 760 Torr. The title reaction showed temperature and pressure dependences which were explained by the following scheme: N+3+H2O(arrow-right-and-left)(N+3⋅H2O)* (forward and backward rate constants ka and kb; (N+3⋅H2O)*→H2NO++N2 (forward rate constant kp; (N+3⋅H2O)* +N2→H2NO++2N2 (forward rate constant ka. Assuming that kd is equal to the collision rate constant of 7.1×10−10 cm3 s−1, the individual rate constants were determined as ka =2.8×10−9 cm3 s−1 (302 K), kb =17T3.6 s−1 where T is temperature in K, and kp =2.0×109 s−1 (302 K). The product H2NO+ ion changed by successive reactions with H2O into H2NO+⋅H2O and subsequently to H3O+.

Notes: A pulsed laser photolysis technique has been employed to study the kinetics of the important stratospheric reaction O+ClO→k1Cl+O2 in N2 buffer gas over the temperature and pressure ranges 231–367 K and 25–500 Torr. 351 nm pulsed laser photolysis of Cl2/O3/N2 mixtures produced Cl atoms in excess over O3. After a delay sufficient for the reaction Cl+O3→ClO+O2 to go to completion, a small fraction of the ClO was photolyzed at 266 nm to produce O(3P). The decay of O(3P) in the presence of an excess, known concentration of ClO was then followed by time-resolved resonance fluorescence spectroscopy. We find that k1 is independent of pressure, but that k1(T) increases with decreasing temperature. Our results suggest that the Arrhenius expression k1(T)=(1.55±0.33)×10−11 exp{(263±60)/T} cm3 molecule−1 s−1 is appropriate for modeling stratospheric chemistry. Errors in the Arrhenius expression are 2σ and represent precison only. The absolute accuracy of k1 at any temperature within the range studied is estimated to be ±20%. Our results agree with other recent measurements of k1 at 298 K but give significantly faster rate coefficients at stratospheric temperatures. A few measurements of the rate coefficient for the reaction ClO+ClO→k7products were also carried out. These measurements were necessary to assess the time dependence of [ClO].

Notes: A general method for analyzing the results of classical trajectory calculations to obtain the details of intramolecular energy transfer is described. The method is based on the determination of the time dependence of the normal mode velocities by projection of the instantaneous Cartesian velocities onto the normal mode vectors. It is shown that the method obviates the need to arbitrarily define a "bond'' or "mode'' energy as a means of following the energy flow. Average mode energies are computed using the virial theorem. For a given potential surface, the results are exact within the framework of the classical approximation. The method is applied to a study of intramolecular energy transfer in 1,2-difluoroethane. Decay rates and pathways of energy flow for initial excitation of each of the 18 vibrational modes are reported. The results obtained from the time variation of the normal mode velocities are used to extract a first-order, mode-to-mode energy transfer rate coefficient matrix. The mode-to-mode coefficients are shown to provide an excellent means of collating the energy transfer information. Their values yield a quantitative description of the energy transfer rates and a clear picture of the relative importance of the available pathways for energy flow in the system.

Notes: The steady state rate constant for passage over a fluctuating potential barrier is derived from the Fokker–Planck equation using the stable states picture. The time scale of fluctuations in barrier height and position do not influence the reactive frequency; however, the reactive frequency does depend upon the time scale of fluctuations in the barrier curvature. If these are fast relative to [ω2m+(ξ/2)2]1/2, where ωm is the mean barrier frequency and ξ is the friction for motion along the reaction coordinate, then the crossing takes place on the potential of mean force. If the barrier fluctuations are slow on the same time scale, then the rate is an average over that for the distribution of barriers. In both of these limits the form of the Kramers solutions is recovered, whereas in the intermediate case, the rate also depends explicitly on the relaxation time scale of the fluctuations. A physically reasonable model for the decay of fluctuations in the potential is proposed and used to obtain an analytic expression for the rate that covers the range between these two limits.

Notes: As in a prior article (Ref. 1), we consider an oscillatory dissipative system driven by external sinusoidal perturbations of given amplitude Q and frequency ω. The kinetic equations are transformed to normal form and solved for small Q near a Hopf bifurcation to oscillations in the autonomous system. Whereas before we chose irrational ratios of the frequency of the autonomous system ωn to ω, with quasiperiodic response of the system to the perturbation, we now choose rational coprime ratios, with periodic response (entrainment). The dissipative system has either two variables or is adequately described by two variables near the bifurcation. We obtain explicit solutions and develop these in detail for ωn/ω=1; 1:2; 2:1; 1:3; 3:1. We choose a specific dissipative model (Brusselator) and test the theory by comparison with full numerical solutions. The analytic solutions of the theory give an excellent approximation for the autonomous system near the bifurcation. The theoretically predicted and calculated entrainment bands agree very well for small Q in the vicinity of the bifurcation (small μ); deviations increase with increasing Q and μ. The theory is applicable to one or two external periodic perturbations.

Notes: A careful semiclassical analysis of "above-barrier'' localization is provided for the case of one-dimensional random analytic potentials. The result is a practical semiclassical formula for the localization length, which shows exponential unboundedness in the classical limit. The algebraic, (h-dash-bar)→0, asymptotic series for the density of states is also determined. In the process of deriving the latter result an interesting link between the density of states and the localization length (via the Lyapunov exponent) is revealed. In addition, the results of this paper provide some of the groundwork for the semiclassical analysis of "nonadiabatic transport'' associated with time-varying potentials.

Notes: An efficient method is presented for evaluating two-electron Cartesian Gaussian integrals, and their first derivatives with respect to nuclear coordinates. It is based on the recurrence relation (RR) of Obara and Saika [J. Chem. Phys. 84, 3963 (1986)], and an additional new RR, which are combined together in a general algorithm applicable to any angular momenta. This algorithm exploits the fact that the new RR can be applied outside contraction loops. It is shown, by floating point operation counts and comparative timings, to be generally superior to existing methods, particularly for basis sets containing d functions.

Notes: Accurate local density-functional calculations of the electron density distribution in the H2O dimer are performed and, in order to distinguish intramolecular charge shifts from intermolecular charge transfer, analyzed in terms of an expansion in atom-centered multipole moments. The dependence on basis set and basis set superposition error of the electron density redistribution upon forming the hydrogen bonded complex has been examined. A model study reveals that only strong hydrogen bonds induce electron density redistributions large enough to be observable by means of x-ray diffraction.

Notes: A new internally contracted direct multiconfiguration–reference configuration interaction (MRCI) method is described which allows the use of much larger reference spaces than any previous MRCI method. The configurations with two electrons in the external orbital space are generated by applying pair excitation operators to the reference wave function as a whole, while the singly external and internal configurations are standard uncontracted spin eigenfunctions. A new efficient and simple method for the calculation of the coupling coefficients is used, which is well suited for vector machines, and allows the recalculation of all coupling coefficients each time they are needed. The vector H⋅c is computed partly in a nonorthogonal configuration basis. In order to test the accuracy of the internally contracted wave functions, benchmark calculations have been performed for F−, H2O, NH2, CH2, CH3, OH, NO, N2, and O2 at various geometries. The deviations of the energies obtained with internally contracted and uncontracted MRCI wave functions are mostly smaller than 1 mH and typically 3–5 times smaller than the deviations between the uncontracted MRCI and the full CI. Dipole moments, electric dipole polarizabilities, and electronic dipole transition moments calculated with uncontracted and contracted MRCI wave functions also are found to be in close agreement. The efficiency of the method is demonstrated in large scale calculations for the CN, NH3, CO2, and Cr2 molecules. In these calculations up to 3088 reference configurations and up to 154 orbitals were employed. The biggest calculation is equivalent to an uncontracted MRCI with more than 78 million configurations.

Notes: The MP-QMC procedure, which is a combination of the model potential (MP) and quantum Monte Carlo (QMC) method, is employed to determine the electron affinity of Cl, together with the fixed-node approximation. Only valence electrons are treated explicitly, while core electrons are replaced by an effective potential constructed with Gaussian-type functions. The expectation value of electron affinity of Cl atom is obtained from subtracting the valence energy of Cl− from that of Cl. The result is in very good agreement with experiment.

Notes: Mass spectrometry was used to identify and to thermochemically characterize the gaseous fluorides RuF4 , RuF3 , RuF2 , and RuF generated by fluorination of Ru powder in a platinum effusion cell. Reaction enthalpies and bond dissociation energies were derived from equilibrium measurements involving these species, yielding the following results in kJ mol−1 : D°(F3 Ru–F)=361; D°(F2 Ru–F)=339; D°(FRu–F)=464; and D°(Ru–F)=402. The results show that the +4 oxidation state dominates the high temperature chemistry of the Ru–F system. Results are compared with the properties of the neighboring Mo–F system.

Notes: A three-body potential suitable for molecular dynamics (MD) simulations has been developed for vitreous silica by adding three-body interactions to the Born–Mayer–Huggins (BMH) pair potential. Previous MD simulations with the BMH potential have formed glassy SiO2 through the melt-quench method with some success. Though bond lengths were found to be in fair agreement with experiment, the distribution of tetrahedral angles was too broad and the model glass contained 6%–8% bond defects. This is indicative of a lack of the local order that is present in the laboratory glass. The nature of the short range order is expected to play an important role when investigating defect formation, surface reconstruction, or surface reactivities. An attempt has been made to increase the local order in the simulated glass by including a directional dependent term in the effective potential to model the partial covalency of the Si–O bond. The vitreous state obtained through MD simulation with this modified BMH potential shows an increase in the short range order with a narrow O–Si–O angle distribution peaked about the tetrahedral angle and a low concentration of bond defects, typically ∼1%–2%. The static structure factor S(q) is calculated and found to be in good agreement with neutron scattering results. Intermediate range order is also discussed in reference to the distribution of ring sizes.

Notes: A kinetic theory for a fluid of particles interacting via a pair potential with hard-core plus truncated tail is described and used to derive a transport theory for the Lennard-Jones fluid as well as the square-well fluid. Numerical results for shear viscosity, thermal conductivity, and the self-diffusion coefficient are given for the Lennard-Jones fluid and compared with simulation and experimental results. Our Lennard-Jones theory proves quantitatively useful over a wide range of states.

Notes: The hypernetted-chain (HNC) and Percus–Yevick (PY) approximations are solved numerically for isotropic fluids of hard spherocylinders with length-to-breadth ratios ranging from 2 to 6. The theoretical results are compared with the available Monte Carlo data for the equation of state and the pair correlation function. The HNC theory was found to predict the existence of a nematic phase at densities in reasonable agreement with recent computer simulations.

Notes: This paper describes a comparative theoretical study of the infinite dilution properties of solutions of univalent ions in different water-like solvents. The theoretical results are obtained by solving the reference hypernetted-chain (RHNC) approximation as described in the first article of this series [J. Chem. Phys. 88, 7715 (1988)]. It is shown that the inclusion of the octupole moment of the water molecule in the model solvent leads to strong preferential solvation of negative ions, and that this has important effects upon certain thermodynamic properties and upon the ion–solvent and ion–ion structure. Questions concerning the definition and physical interpretation of individual ion partial molar volumes are raised and considered in detail. It is argued that in general the individual ion results obtained experimentally may not accurately reflect the true microscopic ion–solvent structure.

Notes: Large local energy fluctuations in liquid water and their physical origin are investigated by using classical molecular dynamics (MD) calculation and quenching techniques. Performing a trajectory calculation of 100 ps, it is found that large rotational motions of individual water molecules, which are always associated with potential energy destabilization of 10–20 kcal/mol, occur once in about 10 ps. The stabilization and destabilization of the individual water molecules are induced by cooperative motions. In order to analyze these cooperative motions in the liquid water, the water structures are quenched to their local minima (called the inherent structures). Comparing the inherent structures successively visited by the system, it is found that collective motions of about 10–40 molecules localized in space occur in unstable regions. The potential energy fluctuation of an individual molecule can reach up to 15 kcal/mol even in the inherent structures. The strong potential energy correlation among neighboring molecules indicates these cooperative motions cause the "flip–flop''-type energy exchanges; as a molecule is stabilized, another is to be unstabilized and vice versa. A flip-flop motion does not involve a (large) energy barrier but causes large energy fluctuations of the individual molecules. A large portion of potential energy fluctuations of the individual water molecules is accounted for as the superposition of fluctuations in the inherent structures and those in the normal modes build upon these structures.

Notes: The hydrodynamic radii of two types of aggregates, diffusion-limited aggregation clusters (DLA) and bond-percolation clusters (BPC), are calculated by numerically solving the hydrodynamic interaction between different particles in the cluster. Though they have almost the same fractal dimensionality, DLA and BPC clusters exhibit different effective hydrodynamic scaling behaviors. For BPC, the ratio between the hydrodynamic radius and the radius of gyration, Rh/Rg, remains almost constant (1.14) for clusters of up to 900 particles; while for DLA the hydrodynamic radius Rh increases faster than the radius of gyration Rg, with Rh∼N0.45 for the same range of N.

Notes: We present exact calculations of the quantum efficiency in a general kinetic model for charge separation in artificial photosynthetic systems. The kinetic models we employ consist of a light absorber covalently linked to a linear chain of N sites. The electron is initially excited at the light absorber and subsequently migrates along the molecular chain by virtue of a random distribution of hopping rates. Decay processes are included at each site which simulate the recombination of the electron with the hole at the light absorber. An exact expression is obtained for the quantum yield for this general model of photosynthetic charge separation. From this expression we show that (1) redox biases play a sensitive role in determining the quantum yield of electrons, (2) because the e−-hole recombination rates fall off exponentially with distance, e-hole recombination beyond the nearest neighbor to the light absorber is negligible, and (3) the distribution of electron hopping rates may cause the quantum yield to decay faster than 1/N even when e−-hole recombination at multiple sites is included. The implications of these results on the experimental design of molecular electronic devices is discussed. Further, we show how the expression for the quantum yield can be used to construct the exact transit time, the moments of the transit time, and the exact expressions for the site probabilities for an arbitrarily disordered one-dimensional system when the particle is placed initially at the origin.

Notes: The transition metal trimer anions Ni−3, Pd−3, and Pt−3 are prepared in a flowing afterglow ion source with a cold cathode dc discharge. The low-lying electronic states of the neutral trimers are probed by 488 nm negative ion photoelectron spectroscopy at an electron kinetic energy resolution of 5–12 meV. Each trimer exhibits multiple low-lying electronic states. Vibrational progressions are observed in tripalladium and triplatinum. The adiabatic electron affinities are found to be EA(Ni3)=1.41±0.05 eV, EA(Pd3)(approximately-less-than)1.5±0.1 eV, and EA (Pt3)=1.87±0.02 eV.

Notes: The adiabatic elastic constants of single crystals of ice Ih from the Mendenhall Glacier, and the longitudinal and transverse speeds of sound of polycrystalline ice aggregates, have been measured in the range 0–2.8 kbar at −35.5 °C by Brillouin spectroscopy. The ice was held in a 10 kbar optical cell that allowed the sample to be rotated under loads of up to 10 kbar about one axis from outside the cell. The elastic constants C11, C12, C13, C33, and C44, and the bulk modulus are quadratic in the pressure range 0–2.8 kbar.

Notes: Two new intermolecular potentials for Ar–HCl are obtained by least-squares fitting to molecular beam microwave and far-infrared laser spectra of the Ar–HCl van der Waals complex. In agreement with previous potentials, the equilibrium geometry is found to be linear Ar–H–Cl, with a well depth around 175 cm−1. However, the far-infrared spectra allow the potential in the region of the linear Ar–Cl–H geometry to be determined as well, and a secondary minimum is found at this geometry, with a depth around 140 cm−1. The secondary minimum arises because the Ar atom can approach closer to the Cl atom at the linear geometry than at a T-shaped geometry. The fitted potential is used to calculate additional bound states of the Ar–HCl van der Waals complex, which have not yet been observed spectroscopically.

Notes: The germyl (GeH3) radical has been observed by resonance enhanced multiphoton ionization (REMPI) spectroscopy in the region of 370–430 nm. The spectrum arises from two-photon resonances with the 5p 2A‘2 (D3h) Rydberg state that possesses an origin at 419.1 nm (ν0–0 =47 705 cm−1). A vibrational progression of 756 cm−1 was assigned to the "umbrella'' mode, ν2. The observed X˜ 2A1 (C3v) v‘2=2 to v‘2=0 vibrational interval is 663 cm−1 which leads to an estimated barrier to inversion of 1530 cm−1.

Notes: Vibronic spectra of three band systems of 40Ar2 and 36Ar40Ar at 108, 107, and 105 nm were recorded and analyzed. Isotope shifts of bandheads were used to establish vibrational numbering of the observed bands. Rovibronic structure was examined at resolving powers up to 5×105, yielding band centers, rotational constants, and providing evidence of Hund's case (c) coupling for Ar2. From band system II at 107 nm, with bands involving v'=20–27 and v‘=0–5, improved constants for the ground X 0+g state were obtained: ω‘e=30.68(8), ωex‘e=2.42(5), ωey‘e=−0.062(13), ωez‘e=0.010(2), and D‘e=99.2(10) cm−1, B‘e=0.059 65(8) cm−1, and Re=3.761(3) A(ring). Similarly, constants for v'=20–27 of the B 0+u were evaluated, with estimates of T'e=88 210(400) and De=5640(400) cm−1. Band system I at 108 nm, led to constants for the v'=23–30 levels of the A1u state and to estimates of T'e=87 458(500) and De=5786(500) cm−1. Analysis of band system III at 105 nm yielded new and improved data for levels v'=0–9 of the C 0+u state, including T'e=95 033.6(3), De=465.8(4), ω'e=68.16, ωexe=4.631, ωey'e=0.0479, ωeze=0.0070, and B'e=0.065 24(20) cm−1, the latter resulting in Re=3.596(5) A(ring). Potential energy curves based on these data were calculated for the ground and three excited states.

Notes: Two-color multiphoton ionization has been used to study phenol–water complexes containing one phenol and one, two, and three water molecules. Photoionization efficiency spectra show that phenol–(H2O) possesses a sharp ionization threshold, giving an adiabatic ionization potential of 63 980 cm−1 (1300 cm−1 lower than previously reported). This is followed by a vibrational progression in the ion of about 240 cm−1 that has been assigned to the hydrogen bond stretching motion. Spectra obtained with the ionization wavelength set so as to avoid fragmentation of the complexes have resulted in a reassignment of the S1 state spectra of phenol–(H2O)2 and phenol–(H2O)3. Spectra taken with the ionization laser delayed by 300 ns reveal that both phenol–(H2O)2 and phenol–(H2O)3 lose two water molecules after intersystem crossing from the vibrationless level of the S1 state. However, it is known that the triplet species obtained from phenol–(H2O)3 is much longer lived than that obtained from phenol–(H2O)2. This large difference in the triplet lifetimes provides clues as to the nature of the bonding in the parent complexes.

Notes: We study the thermodynamics and structure of a model monomolecular film of pentadecanoic acid on water. In this model the water is treated as a uniform polarizable continuum and the pentadecanoic acid molecules as chains of 15 pseudoatom with internal bond constraints, angle bending and torsional intramolecular interactions, and Lennard-Jones atom–atom intermolecular interactions. The only low pressure phases exhibited by this model at temperatures between 300 and 400 K are the very low density vapor phase and a well ordered condensed phase with a reciprocal density ≤23 A(ring)2 per molecule. This is in sharp contrast with experimental studies which show the existence of a stable liquid-expanded phase with a density of about 35 A(ring)2 per molecule at pressures 〈15 dyn cm−1.

Notes: The Herman infrared bands of nitrogen have been recorded in rotationally cold emission from a supersonic jet discharge. The rotational analysis confirms their assignment to the transition C‘ 5Πu→A' 5Σ+g. Rotational constants have been derived for the lower state, as well as term values for the lowest rotational levels of the upper state.

Notes: It is shown that there exists a set of relations between position moments rk (with k being a nonnegative integer) linearly averaged over the wave function. The true wave function must satisfy all of these relations, and therefore they can be used as criteria to assess the accuracy of approximate wave functions. The zero momentum energy formula proposed previously is the simplest case of the present results.

Notes: This study provides the first quantitative interpretation of the intensity pattern of spinning sidebands observed in the magic-angle spinning (MAS) NMR spectra of paramagnetic solids. The 200 MHz 1H MAS-NMR spectrum of copper chloride dihydrate and its deuterated analog are reported. The inhomogeneous interactions predominantly responsible for the sidebands are the magnetic dipolar couplings between the nucleus and the thermally-averaged magnetic moments due to the unpaired electrons on copper atoms. It is demonstrated that even in the presence of many such couplings to a given nucleus, a g anisotropy of the unpaired electron, and a chemical shift anisotropy of the nucleus, the net inhomogeneous interaction responsible for the sideband intensities is formally equivalent to a chemical shift tensor. However, inhomogeneous dipolar couplings to other nuclei give rise to subspectra corresponding to individual spin states of the other nucleus; the resultant composite spectrum no longer resembles that arising from a chemical shift tensor. The 1H MAS-NMR spectrum calculated using the known structure of copper chloride dihydrate correctly predicts the overall spectral width and sideband intensity pattern experimentally observed for the deuterated compound, and indicates that the unpaired electron density on the copper atom is partially delocalized (∼15%) onto the neighboring chlorine atoms. Two comparable sources of line broadening in deuterated copper chloride dihydrate are demonstrated to be the magnetic susceptibility anisotropy and T2 relaxation. The isotropic proton chemical shift is shown to be influenced by a small pseudocontact shift (∼10 ppm upfield) and a larger Fermi contact shift (∼76 ppm downfield).

Notes: Fluorescence excitation and dispersed fluorescence spectra are reported for 1:1 and 2:1 complexes of naphthalene with perylene under supersonic jet conditions. Confirming preliminary results, the fluorescence spectra of the 1:1 complex following excitation of an ag (in-plane) mode at 353 cm−1 and its first overtone show unusually weak vibrational coupling. Although excitation of combination levels of 3531 with out-of-plane modes at 74, 79, and 93 cm−1 gives rise to emission which is predominantly "relaxed,'' the residual "unrelaxed'' component indicates a significant degree of mode-selective vibrational coupling. It is notable that the vibrational coupling for 3532 excitation (i.e., at (approximate)700 cm−1) is substantially less extensive than for excitation into the 3531 combination bands nearly 300 cm−1 lower in energy. A similar comparison has been made between a second ag mode, at 550 cm−1, and a perturbed b3g (out-of-plane) mode, at 540 cm−1. In this case, the data indicate a difference in coupling, which is less obvious than for the 353 cm−1 case, but still indicates a significant dynamic difference in the picosecond domain. Higher-energy excitation is seen to give rise to a large amount of spectral broadening, to (approximate)700 cm−1, yet without any appreciable shift. This indicates that a single equilibrium conformation is present. Comparison with data for uncomplexed perylene and for other perylene complexes at a similar excitation energy (1300–1400 cm−1) suggests that the broadening is due to vibrational coupling involving combinations and overtones of Franck–Condon active low-frequency modes in perylene/naphthalene. Fluorescence excitation data for 2:1 complexes show that the three-band combination of the 1:1 complex at 74–93 cm−1 is replaced by a single, strongly Franck–Condon active mode at 62 cm−1. The corresponding ground state frequency is about 56 cm−1 and, overall, this mode shows harmonic behavior. Like the 1:1 complex, the fluorescence spectrum following excitation into v=2 of the out-of-plane mode, at (approximate)125 cm−1, shows little evidence of vibrational coupling. At and above 353 cm−1, perylene/(naphthalene)2 shows extensive vibrational coupling, since no structure persists in the fluorescence spectra. A comparison has been made between the naphthalene and some other 1:1 complexes of perylene. At 353 cm−1, the Ar1 complex shows less vibrational coupling than perylene/naphthalene, although the presence even of an argon dimer generates a greater degree of coupling. The benzene complex shows extensive IVR at this energy, by comparison with the weakly coupled naphthalene case. Further, while the naphthalene/perylene out-of-plane modes show predominantly resonant Franck–Condon emission profiles, up to excitation energies of 170 and 124 cm−1 for the 1:1 and 2:1 complexes, respectively, perylene/benzene shows extensive IVR even for 70 cm−1 excitation. These data strongly indicate that intermolecular modes, especially weakly hindered internal rotation, are responsible for the pronounced differences in the degree of vibrational coupling.

Notes: In this paper we report the results of one-dimensional simulations of vibrational relaxation in a dense classical molecular fluid. The novel feature of the simulations is that we collect statistics on the energy transfer from vibrational to translational modes in individual collisions, instead of just monitoring the total vibrational energy of the system. This gives much more information on the microscopic processes involved, and in particular reveals that under a wide range of conditions the vibrational relaxation rate is dominated by an effect which we refer to as "thermal softening'' of the vibration frequency. This effect is very important in defining and calculating the vibrational relaxation rate.

Notes: A kinetic theory of recombination and decay due to Lowry and Snider [J. Chem. Phys. 61, 2320 (1975)] is used as the basis for obtaining the equations of change for a molecular system undergoing recombination and decay. Because of the differing localizations of the hydrodynamic variables for the monomers and the dimers (the dimer center of mass is located halfway between the two monomers making up the dimer), the recombination and decay processes contribute explicitly to how the hydrodynamic fluxes are defined. This is particularly the case for the stream velocity appearing in the equation of continuity. The superoperators describing the recombination and decay collision processes naturally involve both reactive and collisional transfer contributions to the hydrodynamic fluxes. A sum rule for the collision superoperators is introduced which allows a logical separation of these effects to be made. Colliding molecules are allowed to be at different positions during a collision and this implies that kinetic energy is not conserved. An equation for the rate of change of potential energy density is obtained treating the collision processes in a manner analogous to the way the rates of change for the single molecule hydrodynamic variables are calculated. This is shown to give a consistent description for energy conservation.

Notes: The method of Ryckaert, Ciccotti, and Berendsen [J. Comp. Phys. 23, 327 (1977)] for integrating the Cartesian equations of motion of a system with holonomic constraints, has been extended to allow the independent constraint of arbitrary internal coordinates. To illustrate this new methodology, and to investigate the effects of dihedral angle constraints on the equilibrium and dynamical properties of macromolecules, we have carried out parallel sets of molecular dynamics simulations and normal mode analyses of a small dipeptide: one without constraints, and one with a single backbone dihedral angle constrained. We find that the averages and the fluctuations of the energies, and of the internal degrees of freedom are not significantly modified by the constraint. However, in the region between 100 and 1400 cm−1 of the normal mode spectrum, the constraint shifts the frequencies of the modes, and modifies their contributions to the spectra of the internal coordinates. Except for the lowest frequency torsional modes, in which anharmonic effects are significant, the behavior of the molecular dynamics power spectra is similar to that of the normal mode spectra. We also illustrate the use of a dihedral angle constraint, in conjunction with a recently developed thermodynamic perturbation method, to calculate the torsional potential of mean force for gas phase n-butane. These results are in good agreement with those obtained in previous studies, and they suggest that the combined internal coordinate constraint/thermodynamic perturbation method is well suited for computing free energy surfaces. As an application of this combined approach, we investigate the role of thermal bond and angle fluctuations on the relative free energies of the three lowest energy conformers of N-methylalanylacetamide. The results indicate that the contribution to the free energy from these fluctuations can be significant, and that care must be taken when models with rigid bonds and angles are used to calculate free energy surfaces.

Notes: The core-level photoexcitation and photoionization of SF6 were studied in the vicinity of the resonances below and above the S 2p threshold. The decay channels of the S 2p→6a1g discrete excitation were characterized, with decay leading mostly to valence-shell satellites. The S 2p continuum data show an oscillatory asymmetry parameter β(S 2p) near threshold that is virtually identical to β(Si 2p) in SiF4. It also resembles—but differs from—theoretical curves for β(S 2p) in atomic sulfur and in SF6. Data at the feature assigned as an eg shape resonance indicate strong multielectron properties for this state, because a resonance in the S 2p satellite is observed at the same photon energy as the main-line resonance. We propose a unified model which generally includes configuration interaction both in the continuum-state manifold and between discrete doubly excited states and the continua, to explain this unexpected satellite behavior. Finally, the S(L2,3VV) Auger electron asymmetry parameter shows no significant deviation from zero near the t2g and eg shape resonances.

Notes: The interface layer in front of an ice crystal growing into the melt has been studied simultaneously by Raman and Rayleigh scattering. The Raman spectra of the light scattered from the interface layer material give evidence for intermolecular dynamics which cannot be detected in either bulk phase. The Rayleigh scattering data are in agreement with the data of Böni et al. [Phys. Rev. A 28, 2953 (1983)], who have discovered the interface layer. For the first time it has been possible to measure the thickness of the layer as a function of the growth rate. At high growth rates enhanced Rayleigh scattering originating from the interface layer of a thickness of about 3 μm has been observed. With decreasing growth rate the thickness of the layer increases, reaching up to 200 μm at zero growth rate, and at melting the layer stays back in the water. From the linewidth of the Rayleigh scattered light we deduce a diffusion constant Di(approximate)2×10−8 cm2/s. Raman spectra show that the intermolecular structure in the interface layer is essentially water-like. Compared to water additional intensity has been observed in the polarized Rayleigh wing, indicating a new polarized and unshifted line with a linewidth that corresponds to a relaxation time τ0(approximate)4×10−13 s. It is tentatively assigned to the decay of a symmetric, intermolecular structure occurring in the interface layer. Slight differences found in the O–H stretching band can be interpreted as a small increase of the average number of H bonds per water molecule in the interface layer as compared to bulk water. The O–H stretching band looks as if the water in the interface layer has a temperature which is about 3 deg colder as it actually is.

Notes: The kinetics of photochemical spectral hole burning are modeled considering low temperature photochemical mechanisms requiring both one and two photons. The calculated results are compared with experimental results on photochemical hole burning in the anthracene–tetracene photoadduct (AT) using single color irradiation. Better agreement is obtained for the two photon mechanism while assuming the second photon is absorbed by the lowest triplet state of AT. The effects of irradiation source bandwidth and polarization are included in the analysis. Photochemical and transient saturation effects are evident in the calculated hole shapes.

Notes: The spectroscopically parametrized CNDO/S3 model is used to provide a quantitative description of the valence electron photoemission spectra of a series of substituted anilines. The CNDO/S3 density of valence states was used to determine the twist angles in these molecules via analysis of gas-phase photoemission spectra. These twist angles are compared with prior estimates. Finally, a discussion of the use of photoelectron spectroscopy in the determination of molecular geometries is presented.

Notes: T←S phosphorescence excitation spectra of benzaldehyde and its aldehyde deuterium derivative in methylcyclohexane at 4.2 K are analyzed in terms of canonical mixture of 105 Born–Oppenheimer vibronic wave functions of the nπ* and ππ* triplet states. Two perturbation terms are employed; one accounting for vibronic interactions, and the other for direct environmental coupling interactions between T1 and T2. A good simulation of the benzaldehyde-h6 T←S spectrum is thus obtained with the zeroth order origin level of T2(ππ*) lying 675 cm−1 above the zeroth order origin of T1(nπ*). The T←S spectrum of benzaldehyde-ld1 is also approximately simulated (the zeroth order ππ* origin is located 625 cm−1 above the zeroth order nπ* origin) but with some disparities from the observed vibronic profile. For comparison, vapor phase T←S spectra are also calculated as a function of the zeroth order energy separation.

Notes: Two broad continuous bands are observed in the visible region following electron impact ionization of a He-seeded molecular beam of SiCl4 or GeCl4. By using tunable vacuum UV radiation from a synchrotron source to measure the threshold energy at which the fluorescence bands occur, it is shown that the bands are related to the initial formation of the C˜ 2T2 state of the parent ion SiCl+4/GeCl+4. By comparison with photoelectron data, the bands are assigned to bound–free transitions in SiCl+4/GeCl+4 C˜ 2T2–A˜ 2T2 and X˜ 2T1. The C˜ state of CCl+4 does not fluoresce. The results are compared with the electronic emission spectra of the three tetrafluoro molecular ions.

Notes: UV resonance Raman excitation within the S2, S3, and S4 π→π* electronic transitions of pyrene results in strong enhancement of totally symmetric ring vibrations. The Raman modes most strongly enhanced within these electronic transitions derive from vibrations exhibiting Franck–Condon vibronic structure in the absorption spectrum. The 1597 cm−1 (b3g) mode shows selective enhancement between the S3 and S4 transitions, and between the S2 and S3 transitions due to Herzberg–Teller coupling between these symmetry-allowed states. The experimentally observed S4 resonance Raman excitation profiles of the totally symmetric pyrene fundamentals agree closely with those calculated using resonance Raman transform theory if non-Condon contributions are included. We see an increased non-Condon contribution with an increasing vibrational frequency, indicating nonadiabatic vibronic interactions. The high incident energy fluxes of the Nd-YAG laser-based excitation source cause saturation of the pyrene Raman intensities since molecular absorption depletes the ground state population. The long-lived S1 singlet excited state bottlenecks relaxation back to the ground electronic state. Formation of pyrene phototransients is also observed with high incident energy fluxes at particular excitation wavelengths.

Notes: Multiplet structure was observed at 4.2 K in the 35,37Cl ligand electron nuclear double resonance (ENDOR) spectra of single crystals of Cu(II)–doped histidine hydrochloride monohydrate. By using second-order perturbation theory we show clearly that the observed multiplet structure arises from the 63Cu and 35,37Cl indirect nuclear spin–spin interaction. Our theoretical analysis in general because it encompasses the anisotropies of both the g and hyperfine tensors, and also allows for the quantization of the nuclear spins along the effective field at each nucleus.

Notes: An ab initio calculation of the fluorescence polarization including autoionization is made in the case of O+2, using the multichannel quantum defect approach. If only the states belonging to Rydberg series converging to the b 4∑−g state of O+2 are included, due to their narrow ab initio calculated autoionization widths, the values of the polarization of the A 2Πu −X 2Πg fluorescence is found to be negative in the 73–69 nm wavelength region, in contradiction with recent experiments [Keller et al., J. Chem. Phys. 87, 3299 (1987)]. Good agreement with experimental results is obtained only in taking into account the influence of the 3∑−u Rydberg state (B 2∑−g) 4pσu, which enlarges the autoionized peaks either by perturbation or by predissociation. There results are compared to the qualitative theoretical interpretation of Keller et al. in terms of two 3∑−u valence states.

Notes: This paper describes experiments to probe the dynamics and decay pathways of the C˜ 2T2 and D˜ 2A1 excited electronic states of Group IV tetrahalide molecular ions MX+4 (M=C, Si, Ge; X=F, Cl) in the gas phase. Tunable vacuum UV radiation from a synchrotron source is used to ionize MX4 into these electronic states of MX+4. Flouorescence from ions initially produced or from fragments is monitored undispersed by a suitable pm tube. When the synchrotron is operated cw, such synchrotron-induced fluorescence spectra give energy thresholds for fluorescence, and in favorable cases an estimate of the fluorescence quantum yield (of MX+4 C˜ or D˜) can be made. When the synchrotron is operated pulsed, radiative lifetimes can be measured as a function of excitation energy. The fluorides and chlorides display very different decay properties. The dynamical behavior of these states is rationalized (a) with respect to their spectroscopic properties, and (b) with respect to the dissociation channels energetically "open'' to them.

Notes: The electronic and structural characterization of the title charge transfer (CI) complex is carried out mainly by the exploitation of the infrared powder spectrum and its temperature dependence from room temperature to 15 K as well as of the Raman powder spectrum at 15 K. The infrared spectrum shows that the complex has a fully ionic electronic ground state (ρ(approximately-equal-to)1) and the analysis of the spectroscopic effects due to the radical electron–intramolecular vibration interactions reveals a structure built up of segregated stacks of TMPD and TCNQF4. Moreover, this analysis shows that the TCNQF4 stacks are nonregular in the whole studied temperature range, whereas, by lowering temperature, the TMPD stacks undergo a phase transition from a regular to a nonregular structure with onset at about 180 K. The effective on-site electronic correlation parameter, the hopping integral, and the magnetic gap of both stacks in the nonregular phase are estimated by adopting a cluster model made up of a dimeric unit to describe the stacks and by comparison with known materials. On the same basis, the temperature dependence of the hopping integrals is also estimated. The collected information, considered in the light of the Peierls–Hubbard model, suggests that TMPD–TCNQF4 can be considered a compound with strong electronic correlations.

Notes: Absorption and total fluorescence spectra of NO transitions perturbed by argon and krypton at various densities from the dilute gas to the low temperature solid are reported for photon energies 6≤hν≤9 eV. The valence transitions shift monotonically to lower photon energies with increasing perturber number density. The Rydberg transitions to n=3 (A 2Σ+, C 2Π, D 2Σ+) exhibit a moderate red shift at low perturber densities followed by an increasing blue shift with increasing density. Both valence and Rydberg transition energies vary smoothly and continuously with perturber number density even upon the liquid–solid phase transition. For fluid argon the experimental energy shifts are analyzed employing the semiclassical theory for line broadening.

Notes: Subnanosecond-time-resolved fluorescence spectra of 1-methylindole and 2,3-dimethylindole in n-butanol were recorded at temperatures of 118–293 K in order to elucidate the unusually large Stokes shift of indoles in polar solvents. Above 200 K the presence of an isoemissive point in the emission spectra indicates interconversion between a partially relaxed excited state (B* state) and a completely relaxed one (R* state). The B*→R* process is characterized by a rate parameter identical with the inverse of the longest longitudinal dielectric relaxation time of the solvent and thus due to the collective reaction of butanol tetramers far removed from the solute. Analysis of the time evolution of the spectral shift below 200 K, reveals that the B* emission spectrum undergoes a Bakhshiev-type continuous red shift characterized by a relaxation time which compares well with the Debye relaxation time of the second dielectric dispersion region and may therefore be attributed to the rotation of a butanol monomer close to the solute molecule. As concerns the nature of the emitting states, neither the 1Lb–1La level inversion interpretation nor the 1La/CT (1La state with increased charge–transfer character) interpretation is wholly consistent with the results of this investigation.

Notes: Assignments of the C–H stretching fundamental of fluoroform, CHF3, have been obtained from a spectrum recorded near 3035 cm−1 at sub-Doppler resolution (∼10 MHz FWHM) and low effective temperature (∼4 K) in an adiabatically cooled molecular beam using bolometric detection of molecules excited by a color-center laser. This fundamental band is highly perturbed and has resisted analysis at higher temperatures, even at Doppler-limited resolution, whereas its overtones have been the subject of several studies of intramolecular vibrational relaxation. Under molecular beam conditions, the central Q branch, which dominates the spectrum at room temperature, almost vanishes, while the bulk of the intensity is equally shared by two "sidebands'' with Q branches symmetrically displaced by ∼±5 cm−1 from the suppressed central Q branch. This peculiar structure is due to a close anharmonic and Coriolis resonance between the fundamental ν1 (species A1) and the combinations ν±4+ν±5+ν±6 (A1+A2) located only 1.40 cm−1 above ν1, whose origin is at 3033.55 cm−1. The coupling constant of the resonance is k1456/2=3.68 cm−1. Additional severe perturbations by the E components of ν4+ν5+ν6 as well as by ν2+ν4+ν6 have been identified in the spectrum.

Notes: A Fourier transform infrared (FTIR) isotopic matrix study of the C2H3 vinyl radical produced by the vacuum UV photolysis of ethylene has been carried out in the midinfrared from 250 to 4800 cm−1. A variety of carbon-13 and deuterium substituted ethylene parent molecules were used to form various isotopomers of C2H3. On the basis of its isotopic behavior and by comparison with Dupuis and Wendoloski's GVB ab initio calculations, an absorption at 900 cm−1 has been assigned to the ν7, out-of-plane bending mode of C2H3. The following values for ν7 have been obtained for vinyl isotopomers: 887 cm−1 for 12CH2=12CD; 797 cm−1 for 12CD2=12CH; 704 cm−1 for 12CD2=12CD; 889 cm−1 for 13CH2=12CH; 895 cm−1 for 12CH2=13CH; and 894 cm−1 for 13CH2=13CH.

Notes: The results of the first rotational analysis of NiO are reported. Several bands in the green spectra region were resolved and analyzed using laser induced fluorescence (LIF). It was found that the symmetry of the well behaved ground electronic state is 3Σ− as was predicted by ab initio calculations. Moreover, the analysis showed that the angular momentum coupling scheme can be described as intermediate between Hund's coupling cases (b) and (c). A least-squares fit of the ground state combination differences gave re=1.627 A(ring).

Notes: A Møller–Plesset energy functional (Lagrangian) which is variational in all variables (the Lagrange multipliers, the orbital rotation parameters, and the orbital energies) is constructed. The variational property ensures that the responses of the orbitals and orbital energies to order n in geometrical perturbations determine the energy derivatives to order 2n+1. The Lagrange multipliers satisfy the somewhat stronger 2n+2 rule. The multipliers, orbital rotations, and orbital energy responses are determined from coupled perturbed Hartree–Fock-type equations using an exponential parametrization of the orbitals. This ensures that the orbital rotations and energy responses are treated in the same way and calculated from a single set of linear equations. Explicit expressions for energy derivatives up to third order are derived for the second-order Møller–Plesset energy.

Notes: A comprehensive theoretical investigation of the third-order susceptibilities for the nonlinear optical processes dc Kerr and dc electric-field-induced second-harmonic generation is reported for H2 and D2. Accurate wave functions of the James–Coolidge-type, which account for electronic correlation, are utilized and particular attention is given to the effects of vibration. Results for both the parallel and perpendicular components of the susceptibilities are given for λ=∞, 10 640, 6943, 6328, 5900, 5145, and 4880 A(ring). These are sufficient to enable a direct comparison with certain experimental data and, in general, the agreement is within the expected error bounds.

Notes: A molecular dynamics technique is introduced for the simulation of the adiabatic dynamics of an excess electron coupled to a classical many-body system. The instantaneous ground state wave function of the electron is represented by a superposition of distributed Gaussian basis functions, each with equal amplitude. We present generalized equations of motion for the coupled system, which optimize the positions and widths of the Gaussians by simulated annealing. The condition of equal amplitude ensures the aggregation of the Gaussians in regions of finite electron probability density and hence yields a particularly efficient representation of localized ground states. The method is applied to an electron solvated in liquid ammonia and results for equilibrium properties are compared to quantum path integral calculations. New results for the dynamics are discussed in the light of mobility measurements.

Notes: The homogeneous (single-cluster) and inhomogeneous contributions to the low temperature electronic absorption spectrum of 35–50 A(ring) diameter CdSe clusters are separated using transient photophysical hole burning. The clusters have the cubic bulk crystal structure, but their electronic states are strongly quantum confined. The inhomogeneous broadening of these features arises because the spectrum depends upon cluster size and shape, and the samples contain similar, but not identical, clusters. The homogeneous spectrum, which consists of a peak 140 cm−1 (17 meV) wide, with a phonon sideband and continuum absorption to higher energy, is compared to a simple molecular orbital model. Electron–vibration coupling, which is enhanced in small clusters, contributes to the substantial broadening of the homogeneous spectrum. The inhomogeneous width of the lowest allowed optical transition was found to be 940 cm−1, or seven times the homogeneous width, in the most monodisperse sample.

Notes: From a combined theoretical and experimental study of 1,2,4,5-tetrachlorobenzene (TCB) we conclude that this crystal shows many interesting effects. In agreement with earlier optical measurements, which probe the complete vibron band structure of TCB, we calculate that several of the vibron modes have the dispersion of a one-dimensional crystal with stacks of molecules along the a axis. The inclusion of fractional atomic charges in the atom–atom potential used in the calculations is absolutely necessary to obtain the correct vibron bandwidths. Also the sign of the vibrational coupling matrix elements, which is given correctly by the calculations, is determined by these charges. For other properties, such as the crystal stability, the phonon frequencies, the site splitting in the vibron bands, and also the dispersion of the lower frequency vibrations, the interactions between the "one-dimensional'' stacks are essential, however. The calculations predict further, in qualitative agreement with the Raman spectra, that the splitting between the pairs of inequivalent vibron bands in α-TCB (the site splitting) is significantly larger than the factor group splitting in β-TCB. Finally we have found that, for those vibrations where the conditions are shown by the calculations to be favorable, the 35Cl/37Cl isotope effects are clearly visible in the Raman spectra.

Notes: The cross section for the formation of H(n=3) or H(n=4) by photodissociation of H2 has been measured for the first time, using synchrotron radiation between 500 and 300 A(ring). The comparison with computed cross section curves allows the identification of the doubly excited molecular dissociating states. The importance of the diabatic behavior of these states is discussed.

Notes: The reaction of Cl2 with the (100) face of a GaAs single crystal was studied in the temperature range from 25 to 150 °C. The reaction was found to be first order in Cl2 at low pressures with an activation energy of 23.6 kcal. At pressures above 10 Torr it was found to reach a limiting rate with an activation energy of 14.2 kcal, attributable to the enthalpy of desorption of the GaCl3 product from this surface.

Notes: Rotational–vibrational distributions are reported for Cl2 product states of the vibrational predissociation of the v=6 through v=12 levels of the ArCl2 molecule in the B electronic state. For v=6 and 7, the Δv=−1 dissociation channel dominates the predissociation dynamics, and the observed rotational distribution appears to be slightly inverted. This inversion does not appear to be due to "momentum gap'' constraints but may be due to kinematic symmetry selection rules. Above v=7 the molecule decays via the Δv=−2 channel. Unlike the rotational distributions observed for HeCl2 and NeCl2, those of the ArCl2 Δv=−2 channel are highly structured and have a strong dependence on the initially excited vibrational level. It is argued that the dependence of the observed rotational distribution on the initial vibrational state (for the dissociation of ArCl2) is due to intermediate resonances in a sequential, IVR mechanism, whereas the dissociation of HeCl2 and NeCl2 occurs by a direct mechanism. Although this conclusion cannot be proved at this time, experiments and calculations are suggested that would directly measure the participation of intermediate levels.

Notes: Rotational energy distributions of the desorbing NO product from the NH3+O2 reaction on a Pt(111) single crystal have been measured using the laser-induced fluorescence technique in conjunction with an UHV apparatus. Over the surface temperature range 800–1300 K, the rotational temperature of NO was found to remain virtually constant near 400 K.

Notes: Absolute cross sections for all ionic products formed in reactions of B+1–13 with oxygen have been measured under single collision conditions, at collision energies from 0.25 to 10 eV. Three main reaction mechanisms appear to be important: oxidative fragmentation, collision induced dissociation, and boron atom abstraction. The dominant oxidation process are exoergic for all cluster sizes, but appear to have bottlenecks or activation barriers for the larger cluster ions. Clusters smaller than B+6 have similar chemistry, then there is a sharp transition in chemistry for clusters larger than B+6 . Correlations are explored between cluster reactivity and cluster stability, and the oxidation chemistry is compared to the similar results found for aluminum cluster ion oxidation.