Selectivity in Gas-Liquid Interactions: Molecular Beam Scattering of CD₄ and ND₃ from an Aqueous Flat Liquid Jet

The dynamics of polar and nonpolar molecules colliding with an aqueous surface are characterized by scattering molecular beams of deuterated methane and ammonia, CD4 and ND3 (Ei = 28.9 and 30.3 kJ mol−1, respectively), from a flat liquid jet of cold salty water (8 m LiBr, 230 K). Translational energy distributions of scattered species collected as a function of collision geometry probe both impulsive scattering (IS) and thermal desorption (TD) mechanisms. We find that CD4 scattering is dominated by IS and exhibits a super-specular angular distribution. The fraction of TD scattering events is notably smaller for cold salty water than for dodecane, consistent with a higher free energy of solvation for CD4 in the water jet. In contrast, no scattering signal is seen for ND3 from the water jet, a result attributed to the high solubility and efficient protonation of ND3 in liquid water. The IS channel for CD4 was analyzed using a soft-sphere model, yielding a higher internal energy (Eint) and lower effective surface mass (meff) than was seen for Ne/water; the higher value of Eint is attributed to rotational excitation of the scattered CD4. These findings demonstrate that the outcomes of a gas-liquid collision - scattering trajectory, surface adherence, and energy transfer - are directed at the molecular level by both the gaseous scatterer and liquid surface.