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) 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 At bath pressures lower than observed here, MD simulations reveal that the static limit is no longer valid and satellite bands, due to an underdamped solvent response, are found.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.