Picosecond studies of the temperature dependence of homogeneous and inhomogeneous vibrational linewidth broadening in liquid acetonitrile

Abstract

The temperature dependence of homogeneous and inhomogeneous vibrational linewidth broadening is reported for the symmetric CH₃‐stretching vibration in acetonitrile over its entire liquid range at P=1 atm. A selective excite‐and‐probe vibrational dephasing experiment based on transient stimulated Raman scattering in high laser depletion is used to measure the homogeneous dephasing times T₂. The separation of homogeneous and inhomogeneous broadening processes is accomplished using the combined results of isotropic spontaneous Raman studies and selective picosecond vibrational dephasing experiments. As a function of temperature, the relative contributions of homogeneous and inhomogeneous broadening are shown to change significantly in opposing directions. Agreement between experiment and theory supports previous suggestions that homogeneous broadening is caused by rapidly varying processes which affect the vibration via short range repulsive forces. The results also suggest that inhomogeneous broadening is caused by slowly varying local density sites which interact with the vibration through long range attractive forces.