Steven T. Govoni and Gilbert M. Nathanson*

Department of Chemistry, University of Wisconsin I 101 University Avenue, Madison, Wisconsin 53706-1322

Received September 9, 1993

Controlled collisions between gas-phase D20 molecules and liquid sulfuric acid can provide valuable insights into hydrogen bonding, solvation, and protonation at the water vapor-acid interface.' When D20 molecules strike a low vapor pressure liquid like H2SO4, they may scatter impulsively from the surface or bind momentarily to the acid molecules before desorbing or dissolving. Each incident D20 may also undergo proton exchange, either during the first few collisions or after solvation. These pathways can be probed by directing a beam of D20 molecules at a fresh surface of H2SO4 in vacuum and monitoring the identities and velocities of the scattered products. We find that, despite concentrated sulfuric acid's extreme protonating and desiccating powers, D20 Molecules can survive collisions with the acid at incident energies spanning 6.4-66 kJ/mol. Almost all D20 that survive scatter impulsively from the surface: those D20 which interact strongly enough with the liquid to accommodate on the surface dissolve into the acid rather than desorb into the vacuum. The data also imply that solvation accompanies proton exchange and prevents the products from desorbing within the 0.05-s time scale of the experiment. We conclude that D20 molecules either strike the acid in nonreactive trajectories, undergoing one or a few impulsive collisions before departing, or suffer enough collisions to dissipate their energy and dissolve for long times.