The microscopic dynamics of classical and quantal rotors in liquids

The rotational dynamics of various solutes in liquids are studied by using molecular dynamics (MD) simulation and instantaneous normal mode (INM) theory. To study the dynamics of classical rotors in liquids, a short-time rotational Generalized Langevin equation is derived by using the INM description of the solvent dynamics. The, resulting INM friction captures the essential features of the exact friction quite well. Using the projections of the INMs, the microscopic details of the friction are extracted for various homonuclear diatomics in a dense supercritical argon fluid. The solvent atoms nearest to the solute strongly influence the friction and give rise to the remarkable similarity in the solvation and rotational and vibrational friction spectra. The fixed-orientation friction evaluated by freezing the orientation of the solute is found to be virtually identical to the exact friction. The rotational energy relaxation and dephasing of H₂ and D₂ in argon liquid are studied on a level-to-level basis. Perturbation theory is implemented in the level-to-level energy relaxation of a quantal rotor in liquids to show that the relaxation rate is directly proportional to the rotational friction at the frequency relevant to the transition considered. The nonlinear coupling mechanism crucial in the high frequency friction is successfully taken into account by the rotor-nearest atom pair dynamics. A microscopic theory for rotational Raman spectra in liquids is developed by using a cumulant expansion. The theory describes the rotational dephasing in terms of rotational friction and naturally divides the dephasing into pure dephasing and population relaxation. The relative importance of the two processes in dephasing is studied by MD simulation for the rotational Raman spectra of H₂ and D ₂ in argon liquid. Our purely classical MD is in excellent agreement with the mixed quantum/classical MD previously reported in the literature.