Enhanced Electron-Stimulated-Desorption Ion Angular Distribution Method for Imaging Molecular Orientations of Adsorbed Species.

摘要

A striking improvement has been made in the quality of positive ion angular distribution data obtained in electron-stimulated-desorption ion angular distribution (ESDIAD). The improved method has been used to study configurational changes in chemisorbed CO on Ni(110) which are induced by intermolecular interactions at high coverage. CO molecules adsorbed with normal C-O bond orientations on ridge Ni sites tilt away from the normal by + or - 19 degs in directions perpendicular to the ridges for CO coverages above 0.75 CO/Ni. Evidence for the C-O tilting has also been studied as a function of electron energy. This has demonstrated that an O(1s) excitation process leads to intense and sharp O(+) ESDIAD beams. The improved ESDIAD method involves the removal of a soft x-ray background signal which originates from electron impact on the metal surface, and is therefore present in all photographic ESDIAD measurements made to date. The angular distribution of the background effect is independent of electron energy and current density. The resultant pure ESDIAD patterns show more quantitative details in high and low signal regions than has been seen before.