Probing the Microscopic Molecular Environment in Liquids:  Intermolecular Dynamics of CS₂ in Alkane Solvents

The femtosecond optical-heterodyne-detected optical Kerr effect/Raman-induced Kerr effect (OHD OKE/RIKE) dynamics of CS 2 dissolved in a series of alkane solvents are reported. The data reveal that the nondiffusive (subpicosecond) dynamics of simple molecular liquids are determined largely by the details of the local, microscopic environment, rather than by the bulk solution properties, with no correlation observed between the short-time, nondiffusive dynamics and the bulk solution viscosity. For each solvent investigated, the vibrational spectral density is observed to narrow and shift to lower frequency with increasing dilution. While the same general trend is observed for each solvent, deviations of magnitude are observed for the longer-chain n -alkanes. This, coupled with the markedly nonexponential decay of the orientational anisotropy observed for the higher-alkane dilutions, suggests the presence of two distinct environments in which isolated pockets of CS 2 exist. The observed spectral evolution is discussed in terms of a simple damped harmonic oscillator model in which the vibrational spectral densities are inhomogeneously broadened. This model allows for contributions from underdamped, critically damped, and overdamped oscillators that arise from a single vibrational degree of freedom. Analysis of the data in terms of this model reveals an exchange of spectral density from the higher-frequency underdamped oscillators to lower-frequency overdamped and critically damped oscillators, with the degree of inhomogeneity decreasing with increasing dilution.