A MOLECULAR-DYNAMICS STUDY OF ELECTRONIC ABSORPTION-LINE BROADENING IN HIGH-PRESSURE NONPOLAR GASES

The observed electronic line broadening of the X-->B (6s Rydberg) abso rption spectrum of CH3I vapor in moderate to high pressures (55-140 at m, reduced densities similar to 0.08-0.14) of Ar and CH4 is analyzed v ia molecular dynamics simulations. Good fits to the absorption line sh apes are found in this pressure/density range for a given set of groun d and excited state solute-solvent potential parameters in the static limit, i.e., the absorption linewidths and shapes are dominated by inh omogeneous broadening on the time scale of the decay of the dipole cor relation function. The pressure dependence of these absorption line sh ape changes is explained in terms of the shape of the solute-solvent g round-excited state difference potential. Consistent with the static l imit description at these moderate to high pressures, the correspondin g transition energy correlation function, a quantity of central import ance in stochastic and Brownian oscillator line shape theories, decays on a much longer time scale than the inverse absorption widths. At mo derate to high pressures, simulations find relatively long-lived solve nt clusters surrounding the CH3I solute. The slow decay of the energy correlation functions, and hence the validity of the static approximat ion, is attributed to these cluster dynamical time scales. At bath pre ssures lower than observed here, MD simulations reveal that the static limit is no longer valid and satellite bands, due to an underdamped s olvant response, are found. The evolution of a Gaussian optical Line s hape at higher densities is discussed with respect to the difference p otential shape, the number density, and the central limit theorem. (C) 1995 American Institute of Physics.