Photo-induced Valence Changes Observed by Photoelectron Spectroscopy of Thin Films of l -Cysteine on Ag(100), (110), and (111) Surfaces Grown in Saliva-emulated Aqueous Solution

Infrared laser-induced valence changes of sulfur, oxygen, and nitrogen atoms of l-cysteine molecules attached to the surfaces of crystalline silver are investigated through photo-electron spectroscopy. The obtained spectra depend on the Miller index of the surface of the silver substrate. Molecular dynamics simulation reveals that the alignment of l-cysteine molecules shows some patterns characteristic of the surface index. Sulfur atoms can be adsorbed either on hollow site or bridge site. The simulation suggests that the valence change is possible only for atoms on the hollow site. The simulation also suggests zwitterionic ion form of the adsorbed l-cysteine molecules and hydrogen-bond formation between adjacent molecules under certain conditions. The observed valence changes of oxygen and nitrogen atoms can be understandable on the basis of the simulation.