Structural relaxation of vapor-deposited water, methanol, ethanol, and 1-propanol films studied using low-energy ion scattering.

Glassy thin films of water, methanol, ethanol, and 1-propanol were prepared by deposition from the gas phase at 20 K. Relaxation of their surface and bulk structures was investigated by measuring temperature evolutions of H(+), H(-), and total yields in low-energy H(2)(+) scattering. The surface structure of a methanol film changes at temperatures of about 20 K below the glass transition temperature (T(g) = 103 K) because of enhanced diffusivity of molecules at the surface. Both surface and bulk structures change at around T(g) for ethanol, but the structure of the 1-propanol film is unchanged at temperatures higher than T(g), indicating that the vapor-deposited 1-propanol glass is more stable than the liquid-quenched one. These behaviors strongly suggest that the configurations of molecules in the vapor-deposited glass differ significantly from those in the liquid-quenched glass and that the aliphatic-chain length plays an important role in structural relaxation. The surface (bulk) structure of the water film changes gradually (is invariant) across T(g), suggesting that the relaxation and glass transition processes differ significantly between those of water and alcohols.