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An instantaneous normal mode (INM) analysis of the short-time solvation dynamics of the B-state (200 nm) Rydberg excitation of methyl iodide in high pressures of Ar (ρ*=0.08, 0.3, and 0.8) is presented. Solute–solvent interaction potentials for this system have been determined by previous absorption and resonance scattering studies. The B-state transition energy correlation function (ECF), also known as the solvation correlation function, calculated by the linear coupling INM theory is in good agreement with the ECF given by molecular dynamics simulation at short times (≤150 fs) that are well beyond the so-called inertial regime (≤100 fs). The shape and peak frequency of the solvation spectra are relatively constant over the wide range of bath densities considered here in contrast to the INM total density of states. This is attributed to the relative density independence of the first peak in the solute–solvent pair distribution function. Similarly, the ECFs are also only modestly dependent on solvent density. A cancellation of the density dependence of the solvation spectrum area and the second moment of the absorption spectrum line shape, and the nearly constant solvation spectrum shape, accounts for the relatively weak density dependence of the ECF decay. A computationally fast, semianalytical method for calculating the weighted density of states incorporating both two- and three-body correlations is shown to be in reasonable agreement with the total INM weighted density of states. A participation ratio analysis of the eigenvectors contributing to the solvation spectrum reveals that single solvent–solute interactions are responsible for the solvation response of the ρ*=0.08 and 0.3 solutions. More collective, totally symmetric solvent motions involving just a few solvent particles, in addition to single solvent interactions, contribute to the solvation response at the liquidlike density of ρ*=0.8. The effects... [ABSTRACT FROM AUTHOR]

The polarization of the resonance fluorescence of symmetric top rigid rotors is described by a third-order density matrix treatment of resonance emission and a sum-over-all-rovibronic states scattering-tensor invariant framework. Within this theoretical approach the resonance fluorescence depolarization is a function of the excited electronic state population and rovibronic coherence decay rates, as well as the electronic absorption/emission line shapes. This description of the depolarization of resonance fluorescence is contrasted with that of resonance Raman in terms of angular momentum selection rules and dependence on material relaxation parameters. In contrast to resonance Raman emission in solution, the accompanying resonance fluorescence polarization is found to be most sensitive to the resonant excited state lifetime when this population decay time is of the order or less than rotational periods. These effects are demonstrated for excitation resonant with the B-state origin of CH3I vapor in high pressures of CH4 and CO2. The solute–solvent interaction responsible for the pure dephasing of the resonant optical coherence does not appear to cause orientational redistribution of the excited chromophore, at least on the time scale of the CH3I B-state origin lifetime. The influence of excited electronic B-state rovibrational pure-dephasing effects on the resonance fluorescence polarization measurements are discussed. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The recently demonstrated technique of optically phase-locked pulse-pair (PLPP) excited spontaneous emission is described by a third-order perturbative density matrix approach. A nonlinear polarization description shows how PLPP spectroscopy depends on all the relevant material dephasing time scales. The time and frequency integrated resonance spontaneous emission consists entirely of resonance fluorescence, and is derived exclusively from excited-state population decay terms, i.e., diagonal second-order density-matrix elements. These third-order polarization results are proportional to the previously derived linear polarization expressions found to describe the observed PLPP I2 vapor emission. The nonlinear treatment allows a comparison of this technique to other forms of ultrafast pump–probe spectroscopies such as transient absorption and photon echo techniques. The role of impulsively prepared coherences is clearly described by this analysis. The effect of pulse duration, relative to material dephasing times, is explored for relaxation given by the optical Bloch equations. The most significant differences between a linear and nonlinear polarization treatment occur for pulse durations greater than the optical dephasing time or when excited state population and coherence decays are of the order of rovibrational periods. A fluorescence line narrowing effect, due to short pulse excitation, is predicted. [ABSTRACT FROM AUTHOR]

The analysis of Raman excitation profiles of rotational scattering transitions is shown to be a sensitive measure of rotationally specific intramolecular dynamics in the subpicosecond time regime. O, P, Q, R, and S Raman REPs of ammonia are reported and theoretically treated using previously derived resonance rotational scattering intensity expressions. Resonance rovibrational Raman spectra are obtained with UV Raman excitation continuously resonant with the v’2 =2 vibronic band (207–210 nm) of the NH3 X → A absorption spectrum. The resulting REP analysis reveals that the rates of photodissociation on the A state surface have a significant rotational quantum dependence in the 22 band. Rovibronic lifetimes decrease from 140 to 70 fs as J increases from 2 to 8 in this resonant A state vibrational level. These lifetime effects are largely ascribed to centrifugal effects. [ABSTRACT FROM AUTHOR]

The spontaneous resonance emission of CH3I in high pressures (800–1600 psi) of CH4 and Ar excited in the region of the Rydberg B-state origin (∼201 nm) are reported. These emission spectra consist of narrow Raman-like (RL) and broad fluorescence-like (FL) spectral features. The observed ratio of the Raman/fluorescence intensity in these high pressure solutions is a function of the excitation wavelength as the incident radiation is tuned through the pressure broadened electronic origin band. Molecular dynamics simulations are implemented for the analysis of the observed emission spectral shapes and their excitation frequency dependence. The four-time dipole correlation functions required for the calculation of this nonlinear polarization derived signal are successfully approximated by a product of two two-time dipole correlation functions for these chromophore-bath systems (factorization approximation). The complex emission band shapes and their excitation frequency dependence are captured by this approach. The dispersion in the RL/FL emission redistribution is due to the multiple time scales inherent to the decay of the resonant optical coherence of these pressure-broadened absorptions. The wavelength dependent pure-dephasing rate is determined by the nonlinear shape of the solute–solvent difference potential. The observational time scale dependence of the spectroscopic homogeneous and inhomogeneous line broadening labels is clearly demonstrated and contrasted here for absorption and Raman scattering. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The observed electronic line broadening of the X→B (6s Rydberg) absorption spectrum of CH3I vapor in moderate to high pressures (55–140 atm, reduced densities ∼0.08–0.14) of Ar and CH4 is analyzed via molecular dynamics simulations. Good fits to the absorption line shapes are found in this pressure/density range for a given set of ground and excited state solute–solvent potential parameters in the static limit, i.e., the absorption linewidths and shapes are dominated by inhomogeneous broadening on the time scale of the decay of the dipole correlation function. The pressure dependence of these absorption line shape changes is explained in terms of the shape of the solute–solvent ground-excited state difference potential. Consistent with the static limit description at these moderate to high pressures, the corresponding transition energy correlation function, a quantity of central importance in stochastic and Brownian oscillator line shape theories, decays on a much longer time scale than the inverse absorption widths. At moderate to high pressures, simulations find relatively long-lived solvent clusters surrounding the CH3I solute. The slow decay of the energy correlation functions, and hence the validity of the static approximation, is attributed to these cluster dynamical time scales. At bath pressures lower than observed here, MD simulations reveal that the static limit is no longer valid and satellite bands, due to an underdamped solvent response, are found. The evolution of a Gaussian optical line shape at higher densities is discussed with respect to the difference potential shape, the number density, and the central limit theorem. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]