COLLISIONS OF PROTIC AND APROTIC GASES WITH HYDROGEN-BONDING AND HYDROCARBON LIQUIDS

We explore collisions of Ne, CH4, NH3, and D2O with glycerol, a hydrogen bonding liquid, and with squalane, a liquid hydrocarbon. The experiments are carried out by directing a molecular beam at a continuously renewed liquid surface in vacuum and monitoring the identity and velocity of the scattered products with mass spectroscopy. We observe both direct inelastic scattering and trapping desorption when the gases strike the liquids. The polyatomic gases thermalize readily at low collision energies but rebound more frequently as the incident energy increases. We find that impulsive energy transfer is extensive and depends only weakly on the type of gas or liquid; for encounters leading to direct scattering, the gases appear to undergo hard spherelike collisions with the CH2 and CH3 groups of squalane and the OH and CH2 groups of glycerol. The gases accommodate differently on the two liquids, however: Neon and methane equilibrate more efficiently on squalane, ammonia thermalizes equally well on each liquid, and water is trapped more frequently by glycerol. The differences in trapping probabilities are smaller than expected from their solubilities, but they roughly follow trends in the free energies and enthalpies of solvation. Our results suggest that thermal accommodation in gas-liquid collisions reflects both the mechanical roughness and softness of hydrocarbons and the strong attractive forces between protic gases and the OH groups of glycerol.