Raman free-induction-decay measurements in low viscosity and supercooled toluene: Vibrational dephasing by shear fluctuations

Total dephasing decay profiles of the $ν12$ (1002 $cm−1)$ ring-stretching mode of toluene have been measured in the time domain in a range extending from the low viscosity, normal liquid (380 K, 0.26 cP) to the high viscosity, supercooled liquid (140 K, 4600 cP). In the region from 1 to 5 cP (250 to 200 K), the profile makes a transition from exponential to partially Gaussian, consistent with a loss of motional narrowing. In contrast to many interpretations of dephasing in terms of collisional dynamics, these data clearly indicate an important role for diffusive dynamics in vibrational dephasing. Above 10 cP, oscillations appear, and their period decreases with increasing viscosity. An analysis of the low to moderate viscosity region with a Kubo model shows that the frequency modulation time is dependent on the viscosity. The dephasing decay profiles are used to test a recent viscoelastic theory of dephasing [Chem. Phys. 233, 257 (1998)], which attributes the perturbation of the vibrational frequency to shear fluctuations in the liquid. A second dephasing process must be included to obtain reasonable fits in the viscosity range below 10 cP. The second process is consistent with a Gaussian frequency modulation whose modulation time is weakly viscosity dependent or has no viscosity dependence. We speculate that this process is due to rapid rotation about the toluene z axis and that the same process also causes the oscillations seen at higher viscosity. There is a good correspondence between the value of the infinite frequency shear modulus found from fitting the Raman free-induction-decay data and the value recently found by Larsen, Ohta, and Fleming by fitting photon-echo-peak-shift measurements in toluene [J. Chem. Phys. 111, 8970 (1999)]. This correspondence supports the idea that both vibrational dephasing and nonpolar electronic solvation are caused by the same physical mechanism.