Application of the metastable impact electron spectroscopy (MIES), in combination with UPS and TPD, to the study of processes at ice surfaces

Abstract

We report studies of the interaction (80–200 K) of atoms and molecules with ice films, deposited on tungsten, by combining photoelectron spectroscopy, UPS with HeI and II, metastable impact electron spectroscopy (MIES), and temperature programmed desorption (TPD). The focus is on the following issues: (1) Interaction of Na Atoms with films of solid NH 3 and CH 3OH: the delocalization of the 3sNa electrons and their role in the deprotonation of ice molecules (seen for CH 3OH, but not for NH 3) is studied. DOS (density of states) information from density functional theory (DFT) is compared with the MIES spectra. It is concluded that the 3s electron, delocalized from its Na core and trapped between the core and surrounding molecules, triggers the CH 3OH deprotonation; for NH 3, Na dimers appear to play an important role in the delocalization process. (2) Interaction of salt (NaCl) and acid (HCOOH) molecules with H 2O ice: neither NaCl nor HCOOH penetrate into the ice film during adsorption below 90 K. However, ionic dissociation of NaCl occurs as a consequence of the NaCl–ice interaction. At 105 K the solvation of ionic species becomes significant. The desorption of H 2O from the mixed film takes place between 145 and 170 K; those species bound to Na + and Cl − are removed last. No dissociation of HCOOH is observed as a consequence of the interaction with ice. Partial solvation of formic acid species takes place above about 120 K whereby these species become embedded into the rim of the water film. However, no deeper penetration of formic acid molecules into the water film can be detected before H 2O desorption becomes significant. The TPD spectra of HCOOH display structure, around 160 K, close to or coinciding with the sublimation temperature of water, and around 180 K, caused by species chemisorbed on the tungsten substrate supporting the ice film. The interaction of both NaCl and HCOOH with ice is discussed on the basis of qualitative free energy profiles.