INTERMOLECULAR DYNAMICS IN ACETONITRILE PROBED WITH FEMTOSECOND FOURIER-TRANSFORM RAMAN-SPECTROSCOPY

We present femtosecond optical heterodyne detected optical Kerr effect measurements of the transient birefringence of neat acetonitrile liquid. Using a recently developed Fourier transform analysis, the transient data are presented in both the time- and frequency-domain representations. The nuclear contributions to the transients, free from any distortions introduced by the electronic response, are generated from the frequency-domain data. The focus of this work is on the high-frequency dynamics associated with intermolecular vibrational degrees of freedom. We observe markedly nonexponential dynamics at short times, with the long-time relaxation characterized by an approximately 1.4-ps exponential time constant. The femtosecond dynamics exhibit characteristics of rapid inhomogeneous dephasing, which accounts for approximately 80% of the signal decay, as well as evidence for contributions from lower frequency, overdamped oscillators. The NLO intermolecular vibrational spectrum of acetonitrile exhibits a broad resonance with a band maximum of approximately 55 cm-1 and a bandwidth of approximately 100 cm-1. The femtosecond transients emphasize the role of the (microscopic) intermolecular potential energy surfaces in shaping the short-time vibrational aspects of the intermolecular dynamics in this liquid. The possible role of similar considerations in the short-time aspects of dynamic solvation phenomena is discussed. The manifestations of intermolecular vibrational motion in solvation transients have not yet been observed experimentally but are revealed clearly in recent molecular dynamics simulations. The correspondence between certain aspects of the two experiments is noted, and experimental questions associated with the observation of intermolecular vibrational modulation of solvation transients are discussed.