Dephasing of individual rotational states in liquids

We continue our previous examination of the fate of individual rotational q uantum states in liquids by an analysis of the rotational Raman spectrum of solutions of H-2 and D-2 in Ar-(l). Rewriting the conventional Kubo treatm ent allows us to show how the character of the spectrum is an immediate con sequence of the rotational friction felt by the solutes. On evaluating that friction via classical molecular dynamics we find that the spectra should consist of well-resolved, homogeneously broadened lines, reflecting the pic osecond-long dephasing times. We find, in particular, that the rotational s tates in H-2 should relax predominantly by pure dephasing, whereas D-2, wit h its smaller rotational quanta, should exhibit significant energy relaxati on as well. The linewidths predicted for H-2 are nicely in accord with thos e computed by more involved nonadiabatic, mixed quantum-classical simulatio ns. (C) 2000 American Institute of Physics. [S0021-9606(00)51448-2].