Quantum-State-Resolved Scattering of OCS at the Gas–Liquid Interface: Hyperthermal versus Thermal Vibrational Equilibration Dynamics in Polyatomics

Abstract

Shot-noise-limited direct absorption spectroscopy with a high-resolution quantum cascade laser is used to explore translational-to-rovibrational (T → V, R) energy transfer in hyperthermal collisions (E inc ≈ 21(1) kcal/mol) of polyatomic OCS at the gas–liquid interface. Such data provide first evidence for rovibrational quantum state-dependent branching into trapping–desorption (TD) and impulsive scattering (IS) pathways for a polyatomic molecule, with unexpected behavior evident due to novel resolution of vibrational degrees of freedom. On the one hand, the rotationally hot IS channel reveals negligible excitation of the OCS bending vibration beyond populations present in the initial supersonic beam (T vib ≈ 220 K), consistent with a more “spectator” role for polyatomic vibrations (T vib(IS) ≪ T S). On the other hand, however, the data sampled at hyperthermal energies exhibit evidence of notably greater vibrational energy transfer, yielding vibrational distributions nearly thermally accommodated with the liquid (T vib(TD′) ≈ T S), but quite surprisingly only for the rotationally thermalized TD scattering channel at high energy. This is in stark contrast with previous gas–liquid studies of OCS energy transfer at low E inc ≈ 2(1) kcal/mol, for which complete rotational (T rot(TD) ≈ T S) but negligible vibrational (T vib(TD) ≪ T S) accommodation occurs, which is in excellent agreement with Landau–Teller–Rapp predictions. Specifically, the results indicate that high energy rovibrational scattering of polyatomics at gas–liquid interfaces involves nonequilibrium dynamics more complex than simple branching into “fully thermalized” (TD) and “nonthermalized” (IS) pathways. To help interpret these results, classical molecular dynamics (MD) is explored for OCS rovibrational excitation on a model liquid surface, which indeed confirms that rovibrational energy transfer at the gas–liquid interface is influenced by both (i) surface interaction time and (ii) translational energy dependence of the vibrational excitation rate. The results highlight that the heretofore simply labeled “TD” channel does not necessarily imply complete loss of collisional memory but instead contains incident energy/internal quantum-state-differentiated pathways exhibiting both equilibrium and nonequilibrium dynamics.