Effects Of Permanent Dipoles And Static Fields On Molecular Spectra

An analytic expression for the absorption spectra of a two-level molecule or atom is derived, within the rotating wave approximation (RWA), which includes the effects of permanent dipole moments and static electric fields. The derivation is for the interaction of the system with a plane-polarized sinusoidal electromagnetic field (EMF) in the semi-classical electric dipole approximation. The RWA resonance profile, and a series of exactly calculated two-level model spectra, are used to investigate some single- and multi-photon spectral effects due to permanent dipoles and static fields, relative to the atomic problem (no permanent dipoles). These effects include the occurrence of even as well as odd photon transitions. Permanent dipole moments can cause narrowing of the resonances, oscillatory fringes around the resonances as a function of frequency, and decreases in the molecule-EMF coupling, relative to the atomic results. Comparisons with exact two-level model spectra are used to study the validity of the RWA results and indicate that several features, such as dynamic backgrounds and shifts of the resonance frequencies from the weak EMF limits of {dollar}\omega\sbsp{lcub}\rm res{rcub}{lcub}\rm N{rcub}{dollar} = {dollar}\Delta{dollar}E/N, N = 1,2,3, dots, are missing in the RWA spectra as the coupling increases.;Perturbative corrections to the RWA absorption spectra and the full widths at half maxima for the resonances are derived, neglecting static fields, and used to investigate and explain the effects missing in the RWA; the RWA resonance profile is a zeroth plus first order result obtainable from a time-independent Floquet Hamiltonian secular equation. The usefulness and validity of the perturbative corrections are investigated and it is concluded that these corrections are not useful computationally past second order. However, the corrections are useful in understanding some of the deficiencies of the RWA. These include the shifts of the N-photon resonance positions to low frequency, with respect to {dollar}\Delta{dollar}E/N, that can occur for molecules with non-zero permanent dipoles; the shift is always to high frequency for atoms. A series of exact model calculations, for giant dipole molecules, is also used to discuss the effects of permanent dipole moments on spectra and to establish comparisons with the recent literature on the subject

SPECTROSCOPIC DIAGNOSTICS FOR TEMPERATURE, CLUSTER SIZE AND DEGREE OF SOLVATION IN INHOMOGENEOUS VAN DER WAALS CLUSTERS

Author Institution: Guelph-Waterloo Centre for Graduate Work in Chemistry, University of WaterlooRecent experiments by Scoles et $al.^{1}$ on the spectroscopy of rare gas clusters containing an infrared active chromophore such as $SF_{6}$ or $Si_{4}$, suggest that for different solvent/solute combinations, the impurity may find itself preferentially either buried inside the cluster, or sitting on its surface. Previous theoretical work suggests that a characteristic doublet they observe in the infrared spectra of moderately large ($n \geq 10^{3}$) $SF_{6}-(Ar)_{n}$ clusters is due to the $SF_{6}$ being only partially immersed in the bath of perturbing Ar atoms. The present paper reports the results of recent simulations designed to associate features of such spectra with the degree to which the solute chromophore molecule is immersed in the rare gas solvent, and the temperature and size of these clusters.${^{1}}$G. Scoles, private communication (1991)

SOLUTE FREQUENCY SHIFTS AS A PROBE OF STRUCTURE AND DYNAMICS IN HETEROGENEOUS VAN DER WAAS CLUSTERS

$^{1}$ D. Eichenauer and R.J. La Roy, J. Chem. Phys. 88, 2898 (1988). $^{2}$ M.Y. Hahn and R.L. Wheten, Phys. Rev. Lett. 61. 1190 (1988).Author Institution: Guelph-Waterloo Centre for Graduate Work in Chemistry, University of WaterlooMolecular dynamics simulations of $SF_{6}.(Rg)_{n}$ clusters $(n=5-12)$, where $Rg =Ar$ and Kr, have been used to determine the minimum energy structures and study dynamical properties, such as transitions between liquid- and solid-like structures. A model, developed by Eichenauer and $Le Roy,^{1}$ which predicts the frequency shifts of the $\nu_{3}$ band of the infrared-active $SF_{6}$ chromophore, has been used to characterize these clusters. The magnitude of the frequency shift has proved to be a sensitive probe of the distinct ``catchment regions'' on the potential energy surface which are accessed in the course of a trajectory. Abrupt changes in the frequency shift have been found to be associated with spontaneous iso-energetic isomerizations between structures consisting of a liquid-like monolayer of Ar atoms coating the $SF_{6}$, and others in which a solid-like ``cap'' of (Ar)n forms on one face of the $SF_{6}$ molecule. It has been observed that ``melting'' is accompanied by a broadening of the simulated $\nu_{3}$ band, behaviour similar to that observed experimentally by Hahn and $Whetten_{2}$ in studies of a benzene chromophore in Ar clusters, Illustrative examples of the use of the infrared-active $\nu_{3}$ band of the $SF_{6}$ chromophore in probing the structure and dynamical properties of some impure rare gas clusters will be presented and discussed