Nonadiabatic Spin–Orbit Excitation Dynamics in Quantum-State-Resolved NO(2Π1/2) Scattering at the Gas–Room Temperature Ionic Liquid Interface

Abstract

Room temperature ionic liquids (RTILs) offer an extremely promising new class of solvents with chemical control of bulk gas solubility, but surprisingly little is known about detailed molecular scale interactions at the gas–liquid interface. In this work, quantum state-to-state resolved collision dynamics at the gas–liquid interface are studied by scattering a jet-cooled molecular beam of ground state NO(2Π1/2; N = 0) molecules from 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (i.e., [bmim]+[Tf2N]−) RTIL, with the resulting rovibronic state distributions probed via laser-induced fluorescence as a function of incident collision energy (Einc) and surface temperature (Ts). Significant excitation is observed from ground (2Π1/2) to excited (2Π3/2) spin–orbit states, highlighting the presence of electronically nonadiabatic effects at the gas–RTIL interface sensitive to both Einc and Ts. At low collision energies (Einc = 2.7(9) kcal/mol), the two spin–orbit manifold rotational distributions are well described by a single temperature, but with (i) Trot(2Π1/2) consistently 30 K lower than Trot(2Π3/2), and (ii) both temperatures lower than Ts. At high collision energies (Einc = 20(6) kcal/mol), the rotational populations are well fit to two-temperature “trapping-desorption” (TD) and “impulsively scattered” (IS) distributions, with the branching ratio into the TD channel (α) for 2Π1/2 consistently higher than that for the spin–orbit excited 2Π3/2 state. From detailed balance considerations these rotational temperatures, in both the low collision energy and TD component of the high collision energy scattered flux, imply the presence of electronic and rotational state dependent trapping-desorption probabilities and provide new theoretical challenges to high level modeling of collision dynamics at the gas–RTIL interface.