Ultrafast H2 and D2 rotational Raman responses in near critical CO2: an experimental and theoretical study of anisotropic solvation dynamics.

The optical heterodyne detected anisotropic rotational Raman responses of H(2) and D(2) (22 mol %) in a near critical CO(2) (rho(*) = rho/rho(c) = 0.8, T = 308 K) solution are reported. J-specific rotational Raman correlation functions (RCFs) for the S(J) transitions of H(2) (J = 0,1,2) and D(2) (J = 0,1,2,3) in this CO(2) solution are determined from these measurements. A mixed classical-quantum simulation methodology results in RCFs that are in excellent agreement with the experimentally derived J-specific responses. The observed S(J) coherence decay time scales, J-dependence, rotor mass dependence, and solvent-induced transition frequency shifts are well captured by these simulations. Pure dephasing of these rotational Raman transitions is shown to be close to the homogeneous limit of the standard Kubo line shape analysis and attributable to the rotor center-of-mass translation in an anisotropic solvent cage. Rotor translational motion in the vicinity of a single CO(2) appears to dominate this dephasing mechanism. Mixed classical-quantum simulations, incorporating the effects of solution fluctuation driven nonadiabatic coupling of instantaneous adiabatic states, including full J-mixing, are required for the agreement between theory and experiment obtained here. Simulations of the classically excited angular kinetic energy of D(2) rotors are used as an estimate of T(1) relaxation rates and are found to be negligible compared to the D(2) rotational Raman coherence time scale. These results are discussed in the context of previous mixed classical-quantum and rotational friction calculations of the dephasing and energy relaxation contributions to H(2) rotational Raman coherence decays. Advantages of time domain acquisition of these rotational Raman responses as compared to spontaneous Raman measurements are illustrated here.