PROBING THE MICROSCOPIC MOLECULAR ENVIRONMENT IN LIQUIDS - INTERMOLECULAR DYNAMICS OF CS2 IN ALKANE SOLVENTS

The femtosecond optical-heterodyne-detected optical Kerr effect/Raman- induced Ken effect (OHD OKE/RIKE) dynamics of CS2 dissolved in a serie s of alkane solvents are reported, The data reveal that the nondiffusi ve (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 bet ween the short-time, nondiffusive dynamics and the bulk solution visco sity. For each solvent investigated, the vibrational spectral density is observed to narrow and shift to lower frequency with increasing dil ution. While the same general trend is observed for each solvent, devi ations of magnitude are observed for the longer-chain n-alkanes. This, coupled with the markedly nonexponential decay of the orientational a nisotropy observed for the higher-alkane dilutions, suggests the prese nce of two distinct environments in which isolated pockets of CS2 exis t. The observed spectral evolution is discussed in terms of a simple d amped harmonic oscillator model in which the vibrational spectral dens ities are inhomogeneously broadened. This model allows for contributio ns from underdamped, critically damped, and overdamped oscillators tha t arise from a single vibrational degree of freedom. Analysis of the d ata in terms of this model reveals an exchange of spectral density fro m the higher-frequency underdamped oscillators to lower-frequency over damped and critically damped oscillators, with the degree of inhomogen eity decreasing with increasing dilution.