Electron spectroscopy (UPS(HeI and II) and metastable impact electron spectroscopy (MIES)) applied to molecular surfaces: the interaction of atoms and molecules with solid water

Abstract

We report studies of the interaction of atoms and molecules with solid molecular surfaces, water in particular, by combining photoelectron spectroscopy, UPS with HeI and II, and metastable impact electron spectroscopy (MIES). In MIES charge exchange processes of the Auger-type taking place between metastable He atoms and the surface under study are utilized to gain information on their electronic structure. The MIES spectra give a rather direct image of the surface DOS. We concentrate on the following processes taking place on water films produced at 80 K: (1) Interaction of Na Atoms with Amorphous Solid H 2O Films: emphasis was on the role of the 3sNa electrons in the water dissociation process. In order to make a detailed comparison with density functional theory (DFT), DOS (density of states) information is compared with the MIES spectra. Our results are consistent with the theoretical prediction that the 3s-electron is delocalized from the Na-core and trapped (solvated) between the Na-core and water molecules of the surrounding water shell. (2) Ionization and Solvation of NaCl Interacting with Amorphous Solid Water: at 90 K there is no interpenetration of H 2O and NaCl. However, ionic dissociation of NaCl takes place when H 2O and NaCl are in direct contact. At 105 K the solvation of the ionic species Cl − and Na + becomes significant. The desorption of H 2O from the mixed film takes place between 145 and 170 K; those species bound ionically to Na + and Cl − are removed last. (3) The Interaction of PBTs (persistent, bio-accumulative, and toxic substances), chlorobenzene and chlorophenyl, with amorphous solid water: the organic layers produced at 80 K were annealed up to 200 K under in situ control of MIES and UPS. The different behaviour of the interfaces for the three studied cases is traced back to the different mobilities of the molecules with respect to that of water. The interaction between H 2O and the benzene derivatives is discussed on the basis of qualitative free energy profiles.