Investigation of dynamical processes at liquid surfaces by molecular scattering

Abstract

We report the results of a laser–molecular beam scattering study of liquid surfaces. I₂ is used as a probe molecule and its internal quantum-state distribution is determined both before and after scattering from the surface of liquid samples. Time resolution of the laser-induced fluorescence signal allows us to distinguish the processes of direct inelastic scatter and trapping followed by desorption. Marked differences in the spectral patterns, are observed as laser timing is varied and also when the liquid sample is changed. The effect of the chemical nature of the surface is particularly marked in gallium for which reactive channels are likely. An analysis of surface-dependent trapping and desorption is presented based on the availability of acceptor and promoter modes of molecules in the liquid surface. Modes associated with the CH₃ group appear to be particulary efficient in absorbing the translational energy of small gas-phase molecules whilst those of the CF₃ group are less effective. Polydimethylsiloxane has a range of acceptor modes and hence is an efficient trap species. Furthermore it possesses effective promoter modes and torsional motions of the Si—O—Si link may be closely involved in this role.