Non-equilibrium dynamics at the gas-liquid interface: State-resolved studies of NO evaporation from a benzyl alcohol liquid microjet.

First measurements of internal quantum-state distributions for nitric oxide (NO) evaporating from liquid benzyl alcohol are presented over a broad range of temperatures, performed by liquid-microjet techniques in an essentially collision-free regime, with rotational/spin-orbit populations in the Π manifolds measured by laser-induced fluorescence. The observed rotational distributions exhibit highly linear (i.e., thermal) Boltzmann plots but notably reflect rotational temperatures (T) as much as 30 K lower than the liquid temperature (T). A comparable lack of equilibrium behavior is also noted in the electronic degrees of freedom but with populations corresponding to spin-orbit temperatures (T) consistently higher than T by ∼15 K. These results unambiguously demonstrate evaporation into a non-equilibrium distribution, which, by detailed-balance considerations, predict quantum-state-dependent sticking coefficients for incident collisions of NO at the gas-liquid interface. Comparison and parallels with previous experimental studies of NO thermal desorption and molecular-beam scattering in other systems are discussed, which suggests the emergence of a self-consistent picture for the non-equilibrium dynamics.