Electron-stimulated desorption and thermal desorption spectrometry of H 2O on nickel (111)

Abstract

The adsorption of H 2O at low temperature on clean and oxygen dosed nickel (111) has been studied using thermal desorption spectrometry (TDS) and electron-stimulated desorption (ESD). Both the single and multilayer H 2O regimes have been investigated. For first layer chemisorbed water, the thermal desorption data can be modeled well assuming first order kinetics with a desorption energy of about 9.8 kcal mol . For ice multilayers, a leading edge analysis gives a desorption energy of 11.5 ± 0.5 kcal mol . ESD measurements detected both H + and H 2O + ions, and several types of ESD measurements were made: determination of ion yield versus electron beam energy, determination of ion kinetic energy distribution curves (lEDC's), and determination of the temperature dependence of the ion yield (TDIY) at fixed excitation energy. The latter measurements show a correlation with the thermal desorption data: TDIY of H 2O + during desorption of multilayer H 2O films gives 11.0 ± 0.5 kcal mol for the desorption energy, in excellent agreement with the TDS results. Additional features are resolved in the temperature dependence of the ESD signal which are absent in the TDS spectra. Characteristic ion yield and ion kinetic energy distribution curves are given for adsorbed layers on the clean surface in both the single and ice multilayer regime. For the oxygen predosed surface, TDS was used to isolate a H 2O desorption state at higher temperature which is due to dissociative recombination, and ESD was used to characterize the ion yields and IEDC's of H + from hydroxyl groups adsorbed on this type of surface. Comparison of the data for H + ions from adsorbed water and hydroxyls indicates that ESD in this case is sensitive primarily to the local parent O-H bond, and insensitive to secondary chemical bonds.