Solvation dynamics for an ion pair in a polar solvent: Time-dependent fluorescence and photochemical charge transfer

The results of a molecular dynamics (MD) computer simulation are presented for the solvation dynamics of an ion pair instanteously produced from a neutral pair, in a model polar aprotic solvent. These time-dependent fluorescence dynamics are analyzed theoretically to examine the validity of several linear response theory approaches, as well as of various theoretical descriptions (e.g., Langevin equation) for the solvent dynamics per se. It is found that these dynamics are dominated for short times by a simple inertial Gaussian behavior, a feature which is absent in many current theoretical treatments, and which is related to the approximate validity of linear response theory. Nonlinear aspects, such as an overall spectral narrowing, but a transient initial spectral broadening, are also discussed. A model photochemical charge transfer process is also briefly considered to elucidate aspects of the connection between solvation dynamics and chemical kinetic population evolution.