A molecular dynamics analysis of resonance emission: Optical dephasing and inhomogeneous broadening of CH₃I in CH₄ and Ar

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.