Critical Test of Vibrational Dephasing Theories in Liquids by Use of Selective, Coherent, Picosecond Stokes Scattering

A picosecond coherent probing technique is used to determine the homogeneous vibrational linewidth in liquids. Isotope effects on the symmetric C${\mathrm{H}}_3$-stretching vibration in methanol and acetone provided a sensitive test for various vibrational dephasing theories. Theories based on binary-collision or hydrodynamic models do not correctly account for the observed isotope effect. Exchange theory is shown to be qualitatively consistent with the results.