The electronic excitation induced by ultrashort laser pulses and the subsequent photodissociation dynamics of molecular fluorine in an argon matrix are studied. The interactions of photofragments and host atoms are modeled using a diatomics-in-molecule Hamiltonian. Two types of methods are compared: (1) quantum-classical simulations where the nuclei are treated classically, with surface-hopping algorithms to describe either radiative or nonradiative transitions between different electronic states, and (2) fully quantum-mechanical simulations, but for a model system of reduced dimensionality, in which the two most essential degrees of freedom are considered. Some of the main results follow: (1) The sequential energy transfer events from the photoexcited Fz into the lattice modes are such that the ``reduced dimensionality'' model is valid for the first 200 fs. This, in turn, allows us to use the quantum results to investigate the details of the excitation process with short laser pulses. Thus, it also serves as a reference for the quantum-classical ``surface hopping'' model of the excitation process. Moreover, it supports the validity of a laser pulse control strategy developed on the basis of the ``reduced dimensionality'' model. (2) In both the quantum and quantum-classical simulations, the separation of the F atoms following photodissociation does not exceed 20 bohr. The cage exit mechanisms appear qualitatively similar in the two sets of simulations, but quantum effects an quantitatively important. (3) Nonlinear effects are important in determining the photoexcitation yield. In summary, this paper demonstrates that quantum-classical simulations combined with reduced dimensionality quantum calculations can be a powerful approach to the analysis and control of the dynamics of complex systems.

The vibrational self-consistent field method is used to analyze the inhomogeneous spectral distribution of transitions caused by vacancies and thermally populated phonons, specializing to molecular iodine isolated in an Ar matrix. At experimentally relevant temperatures, for a vacancy concentration of 1.4%, both defect-induced and phonon-induced spectral shifts contribute to the spectral distribution. Both contributions scale linearly with vibrational overtone number. The predicted widths are consistent with reported resonant Raman spectra. In time-resolved coherent anti-Stokes Raman scattering (TRCARS) measurements, spectral indistinguishability implies that all members of the inhomogeneous ensemble contribute coherently to the detectable homodyne signal. The connection between spectral distribution and the observable in TRCARS is derived. The predicted polarization beats and free induction decay due to the inhomogeneous ensemble are in qualitative agreement with experiments. (C) 2001 American Institute of Physics.

Anharmonic vibrational frequencies and intensities are calculated for 1:1 and 2:2 (HCl)(n)(NH(3))(n) and (HCl)(n-)(H(2)O)(n) complexes, employing the correlation-corrected vibrational self-consistent field method with ab initio potential surfaces at the MP2/TZP computational level. In this method, the anharmonic coupling between all vibrational modes is included, which is found to be important for the systems studied. For the 4:4 (HCL)(n)- (H(2)O)(n) complex, the vibrational spectra are calculated at the harmonic level, and anharmonic effects are estimated. Just as the (HCl)(n)(NH(3))(n) Structure switches from hydrogen-bonded to ionic for n = 2, the (HCl)(n)-(H(2)O)(n) switches to ionic structure for n = 4. For (HCl)(2)(H(2)O)2, the lowest energy structure corresponds to the hydrogen-bonded form. However, configurations of the ionic form are separated from this minimum by a barrier of less than an O-H stretching quantum. This suggests the possibility of experiments on ionization dynamics using infrared excitation of the hydrogen-bonded form. The strong cooperative effects on the hydrogen bonding, and concomitant transition to ionic bonding, makes an accurate estimate of the large anharmonicity crucial for understanding the infrared spectra of these systems. The anharmonicity is typically of the order of several hundred wavenumbers for the proton stretching motions involved in hydrogen or ionic bonding, and can also be quite large for the intramolecular modes. In addition, the large cooperative effects in the 2:2 and higher order (HCl)(n)(H(2)O)(n) Complexes may have interesting implications for solvation of hydrogen halides at ice surfaces.

An extension of the vibrational self-consistent field (VSCF) method is developed for quantitative calculations of molecular vibrational spectroscopy in a crystalline solid environment. The approach is applicable to fields such as matrix-isolation spectroscopy and spectroscopy of molecular crystals. Advantages of the method are that extended solid vibrations and their coupling to intramolecular modes are incorporated, and that the treatment includes anharmonic effects, both due to the intrinsic property of individual modes and due to coupling between modes. Suitable boundary conditions are adopted in treating the solid environment. In applications, e.g., molecules in rare-gas crystals, hundreds of coupled molecular and matrix modes can be handled computationally. The method is applied to the vibrational matrix-shift of iodine in an argon matrix, and the calculated overtone frequencies are compared to experimental values obtained from both time-domain coherent Raman and frequency-domain Resonance Raman measurements. The physical origin of the shifts is interpreted in detail, and the properties of the iodine-argon interactions essential to obtain the correct sign and magnitude of the shift are elucidated. An I-2-Ar potential, based on anisotropic atom-atom interactions and fitted to ab initio calculations, gives the best agreement with experiment. The results show that the VSCF solid-state approach is a powerful tool for matrix spectroscopy. (C) 2001 American Institute of Physics.

{{Vibrational energy levels and infrared absorption intensities of several neutral and ionic hydrogen-bonded clusters are computed directly from ab initio potential energy surfaces, and the results are compared with experiment. The electronic structure method used to compute the potential surfaces is MP2, with Dunning's triple-zeta + polarization basis set. The calculation of the vibrational states from the potential surface points is carried out using the correlation corrected vibrational self-consistent field (CC-VSCF) method. This method includes anharmonicity and the coupling between different vibrational modes. The combined electronic structure/vibrational algorithm thus provides first-principles calculations of vibrational spectroscopy at a fairly accurate anharmonic level and can be useful for testing the accuracy of electronic structure methods by comparing with experimental vibrational spectroscopy. Systems treated here are (H(2)O)(n)

Potential energy surface points computed from variants of density functional theory (DFT) are used to calculate directly the anharmonic vibrational frequencies of H2O, Cl-H2O, and (H2O)(2). The method is an adaptation to DFT of a recent algorithm for direct calculations of anharmonic vibrational frequencies using ab initio electronic structure codes. The DFT calculations are performed using the BLYP and the B3LYP functionals and the results are compared with experiment, and also with those calculated directly from a potential energy surface obtained using ab initio Moller-Plesset second-order perturbation theory (MP2). The direct calculation of the vibrational states from the potential energy points is performed using the correlation-corrected vibrational self-consistent field (CC-VSCF) method. This method includes anharmonicity and correlations between different vibrational modes. The accuracy of this method is examined and it is shown that for the experimentally measured transitions the errors in the CC-VSCF calculations are much less than the errors due to the potential energy surface. By comparison with the experimentally measured frequencies the CC-VSCF method thus provides a test for the quality of the potential energy surfaces. The results obtained with the B3LYP functional, in contrast to those of the BLYP functional, are of comparable quality to those obtained with MP2. The B3LYP anharmonic frequencies are in good agreement with experiment, showing this DFT method describes well the anharmonic part of the potential energy surface. The BLYP results systematically underestimate both the harmonic and anharmonic frequencies and indicate that using this functional for the description of hydrogen-bonded systems may cause significant errors. (C) 2000 American Institute of Physics. [S0021-9606(00)30105-2].

A combination of experimental, molecular dynamics, and kinetics modeling studies is applied to a system of concentrated aqueous sodium chloride particles suspended in air at room temperature with ozone. irradiated at 254 nanometers to generate hydroxyl radicals. Measurements of the observed gaseous molecular chlorine product are explainable only if reactions at the air-water interface are dominant. Molecular dynamics simulations show the availability of substantial amounts of chloride ions for reaction at the interface, and quantum chemical calculations predict that in the gas phase chloride ions will strongly attract hydroxl radicals. Model extrapolation to the marine boundary Layer yields daytime chlorine atom concentrations that are in good agreement with estimates based on field measurements of the decay of selected organics over the Southern Ocean and the North Atlantic. Thus, ion-enhanced interactions with gases at aqueous interfaces may play a more generalized and important role in the chemistry of concentrated inorganic salt solutions than was previously recognized.

Anharmonic correlation-corrected vibrational self-consistent-field (CC-VSCF) calculations are reported for the neutral HXeI molecule. Fundamental, overtone and combination frequencies and their absorption intensities are computed, and compared with previous and new experimental data from FTIR matrix isolation measurements. Agreement between experiment and calculations extend the identification of the HXeI molecule, and the calculations prove useful in aiding assignment of new observed transitions. The results show that especially the Xe-H bond of HXeI is highly anharmonic. While the agreement between theory and experiment is useful for assignment, quantitative discrepancies still remain due to the deficiency of MP2 theory to describe the highly anharmonic surface. (C) 2000 Elsevier Science B.V. All rights reserved.

We study the coverage and temperature dependences of the tracer diffusion coefficient for a chemisorbed layer of interacting hydrogen atoms on a model of a bcc metal (110) surface. The surface is rigid, and the short bridge barriers between adjacent chemisorption wells are sufficiently low that hydrogen atoms diffuse actively across the surface, on a time scale compatible with our molecular dynamics simulations. We deduce the coverage dependence of the effective activation barrier and the prefactor. We also examine, as a function of coverage, the percentage of jumps from singly occupied to either empty or occupied chemisorption wells, and from doubly occupied wells to empty or singly occupied wells. Although the effective activation barrier deduced from the numerical data exhibits a weak dependence on coverage, as found in data on H diffusion on the W(110) surface, the percentage of jumps of the types mentioned varies dramatically. The prefactor in the diffusion constant extracted from the simulations agrees well with elementary expectations for the rigid surface, but is much larger than that found experimentally. Finally, the low coverage tracer diffusivity is found to be appreciably anisotropic. The anisotropy decreases substantially as coverage increases. (C) 2000 Elsevier Science B.V. All rights reserved.

A first-principles calculation of vibrational spectroscopy of HXeH, HXeCl, HXeBr, and HXeOH molecules is performed by combining ab initio codes with the vibrational self-consistent field (VSCF) method, and with its extension by perturbation theory (CC-VSCF). The MP2/CC-VSCF method is anharmonic, and it is able to reproduce the experimentally observed spectral features of HXeH, HXeCl, HXeBr, and HXeOH. The most intense bands of the HXeY molecules, the Xe-H stretching modes, are found to be highly anharmonic. In general, the other fundamental modes presented anharmonic effects to a lesser extent. New predictions of overtone and combination vibrations are made to help experimental investigations of these molecules. It is shown that vibrational spectroscopy calculations are reliable and useful for analyzing the spectral features of rare-gas-containing molecules. While the results of the MP2/CC-VSCF calculations are in much better agreement with experiments than the corresponding harmonic frequencies, substantial discrepancies remain. These are mostly due to the large electronic correlation effects in these systems, which are not sufficiently well presented at the MP2 level.

The second-order Moller-Plesset ab initio electronic structure method is used to compute points on the potential energy surface of glycine. Some 50 000 points are computed, covering the spectroscopically relevant regions, in the vicinity of the equilibrium structures of the three lowest-lying conformers of glycine. The vibrational states and spectroscopy are computed directly from the potential surface points using the correlation corrected vibrational self-consistent field (CC-VSCF) method, and the results are compared with experiment. Anharmonic effects and couplings between different vibrational modes that are included in the treatment are essential for satisfactory accuracy. The following are found: (1) The spectroscopic predictions from the ab initio potential are in very good accord with matrix experiments. (2) Theory agrees even more closely with spectroscopic data for glycine in He droplets, where environmental effects are much weaker than in the matrix. This suggests that errors in the ab initio potential are smaller than rare-gas matrix effects. (3) The accuracy of the ab initio potential is. by this spectroscopic test, much superior to that of OPLS-AA, a state-of-the-cut empirical potential. The relative failure of the empirical potential is due to its inability to describe: details of the hydrogen-bonded interactions, and is most critical in one of the glycine conformers where such interactions play an especially important role.

An embedded-atom type potential for liquid indium is developed by fitting bulk liquid thermodynamic and structural data. An empirical pairwise Ar-In interaction is also proposed. Molecular-dynamics simulations of argon scattering from liquid indium are carried out and compared with molecular beam scattering data. Very good agreement is found between the experimental and theoretical angular and energy scattering distributions. This supports the potential functions used. Implications for the atomic-scale structure of liquid In and for gas-surface energy transfer are briefly discussed. (C) 2000 American Institute of Physics. [S0021-9606(00)70733-1].

MP2 and CCSD(T) calculations are used to analyse the structures and vibrational spectra of HRgF molecules, where the rare gas atom is He, Ne, Ar, Rr, Xe or Rn. We extend the analysis of the vibrational spectra of these molecules to include anharmonic corrections for the most likely candidates for experimental detection, i.e., HArF, HKrF, HXeF, and their deuterated isotopomers. The anharmonic correlation-corrected vibrational self-consistent-field (CC-VSCF) calculations are used for this, and fundamental, overtone and combination frequencies and their absorption intensities are computed. (C) 2000 Elsevier Science B.V. All rights reserved.

The anharmonic vibrational frequencies of H2O, Cl-H2O and (H2O)(2) are calculated using potential energy surfaces computed from DFT, specifically the generalized-gradient approximation (GGA) functional HCTH and the hybrid functional B97(2c). HCTH gives reasonable agreement with experiment and can be recommended in situations where the use of a hybrid functional would be difficult. B97(2c) is found to be superior in accuracy to all other functionals tested and should be the functional of choice when the anharmonic potential energy surfaces of polyatomic systems are required for spectroscopic or similar applications. This point is illustrated by the excellent agreement with experiment obtained in calculations on formic and acetic acid using the B97(2c) functional. (C) 2000 Elsevier Science B.V. All rights reserved.

Photodissociation and recombination of an F-2 molecule embedded in an Ar cluster is investigated. The electronic states involved are described by the valence bond approach for the F(P-2)+F(P-2) interaction, with spin-orbit coupling included and the anisotropic interactions between F and Ar atoms described by the diatomics-in-molecules (DIM) approach. The potential energy surfaces for 36 electronic states and the nonadiabatic couplings between them are constructed in this basis. The surface hopping method is used for dynamical simulations. The main results are: (i) Spin nonconserving transitions play a crucial role both in the dissociation and in the recombination dynamics. (ii) The ratio between the population of the triplet states and the population of the singlet states reaches the statistical equilibrium value of 3:1 60 fs after the photoexcitation, but the population of specific singlet and triplet states remains nonstatistical for at least 1.5 ps. (iii) Recombination on the only bound excited state ((3)Pi (u)) becomes significant within 100 fs and builds up to 40% of the trajectories within 1 ps after excitation of the cluster with 4.6 eV. This is in accord with recent experiments on ClF/Ar solid, where strong emission from this state was found. (iv) 3% of recombination on the ground (1)Sigma (g) state is found as well. (v) For excitation energy of 4.6 eV, the dissociation can be direct or delayed. In delayed dissociation the F photofragments hit the Ar cage more than once before escaping the cage. (vi) For excitation energy of 6.53 eV the yield of dissociation was found to be 100%, and the dissociation is direct only. (C) 2000 American Institute of Physics. [S0021-9606(00)01240-X].

Ultraviolet (UV) photodissociation experiments are carried out for Ar-n(HBr) clusters in which the HBr is adsorbed on the surface of the Ar-n, and also on isomers of these systems in which HBr is embedded within the rare-gas cluster. The mean size of the cluster distribution in the experiments is around (n) over bar=130. The kinetic energy distribution (KED) of the hydrogen atoms that left the clusters is measured. Molecular dynamics (MD) simulations of the photodissociation of the chemically similar clusters Ar-n(HCl) are used to provide a qualitative interpretation of the experimental results. The clusters with embedded HBr give a very cold H-atom KED. The clusters with the surface-adsorbed HBr give a KED with two peaks, one corresponding to very low energy H atoms and the other pertaining to high energies, of the order of 1.35 eV. The theoretical simulations show that already for n=54, there is a strong cage effect for the ``embedded'' molecule case, resulting in slow H atoms. The surface-adsorbed case is interpreted as due to two types of possible adsorption sites of HX on Ar-55: for a locally smooth adsorption site, the cage effect is relatively weak, and hot H atoms emerge. Sites where the HBr is adsorbed at a vacancy of Ar-n lead to ``encapsulation'' of the H atom produced, with a strong cage effect. A weak tail of H atoms with energies well above the HBr monomer excess energy is observed for the embedded case. Simulations support that this is due to a second photon absorption by recombined, but still vibrationally hot, HBr. The results throw light on the differences between the cage effect inside bulk structure and at surfaces. (C) 2000 American Institute of Physics. [S0021-9606(00)00925-9].

Laser pulse-induced photodissociation of molecules in rare-gas solids is investigated by representative quantum wavepackets or classical trajectories which are directed towards, or away from, cage exits, yielding dominant photodissociation into different neighbouring cages. The directionality is determined by a sequence of reflections inside the relief provided by the slopes of the potential energy surface of the excited system, which in turn depend on the initial preparation of the matrix isolated system, e.g, by laser pulses with different frequencies or by vibrational pre-excitation of the cage atoms. This reflection principle is demonstrated for a simple, two-dimensional model of F-2 in Ar. (C) 2000 Elsevier Science B.V. All rights reserved.

An Amber-type of force field, based on experimental vibrational frequencies which is suitable for monosaccharides. is presented. In the present force field, the atomic partial charges and some torsional parameters are derived from a fit of calculated vibrational energy levels to known experimental spectra for alpha-D-glucose. The vibrational spectra were calculated using the vibrational self-consistent field (VSCF) method, which includes contributions from both anharmonic and mode-coupled terms. We find that with a reparametrization of the force field the agreement between the experimental and calculated vibrational spectra is +/-3.3 cm(-1) for alpha-glucose and +/-5.1 cm(-1) for beta-glucose. Using the VSCF method, we an also able to lend support to the idea that the COH bending motion is strongly coupled to the methylene and methine modes, as well as the other internal modes. We then test our spectroscopically derived force field by calculating the anomeric effect for alpha –> beta glucose. Molecular dynamics simulations are performed separately for both anomers (alpha and beta) in order to evaluate configurational entropy, and hence free energy. We find that out of six simulations. half correctly predict the anomeric free energy Delta G(alpha–>beta) = -0.3 kcal/mol, while two simulations yield a Delta G(alpha–>beta) = +0.2 kcal/mol, and in one simulation Delta G(alpha–>beta) similar to 0.0 kcal/mol. We also calculate the atomic pair distribution function, g(r), and show that in most simulations, the beta-conformer is slightly more adept at structuring the surrounding water molecules, resulting in better hydrogen bonding fur this anomer. However, we believe that our force field, which is static, is unable to represent adequately the dynamic interactions between the pyranose sugar and the surrounding water molecules. This resulted in a large fluctuation about the average calculated anomeric free energy of Delta G(alpha–>beta) similar to -0.1 kcal/mol. Thus, while the current all atom force field is well suited for spectroscopic studies of monosaccharides, it is not yet well suited for dynamical studies.

Diffusion quantum Monte Carlo (DQMC) is a powerful method for calculating the vibrational ground state of large systems but is inapplicable in general for excited states. We propose a general method for excited states, based on combining DQMC with the approximate vibrational self-consistent field (VSCF) approach; the latter is used to obtain the nodes of the excited state wavefunctions. The combined DQMC-VSCF is approximate but found high accuracy in test calculations of Ar-3. DQMC-VSCF is also applied to collective mode excitations of Ar-13. The method provides full spectroscopic assignment for the computed excited states through VSCF. (C) 1999 Elsevier Science B.V. All rights reserved.