Investigation of hydrogen chemisorption on the tungsten (100) surface

[摘要] A new apparatus for surface studies has been constructed and placed in operation. The instrument incorporates a monoenergetic beam of electrons, a high resolution, high sensitivity electrostatic ene'rgy spectrome.terfor electrons and ions, plus other hardware and electronics necessary toperform work function measurements, Auger electron spectroscopy, electronstimulated desorption of ions, low energy electron diffraction, and electronscattering energy loss measurements. It has been used to investigategeometric, energetic, and kinetic properties associated with the chemisorptionof hydrogen on a single crystal tungsten (100) surface over a temperaturerange from 600c to -1500C. A new hydrogen binding state, labeled the fast state, has been observed and characterized using ESD techniques. This state is desorbable if the undosed crystal is cooled below room temperature. It is populated to-4a very low coverage, ~10 monolayer, by adsorption from the background ofsome . other than H T e tota esorpt10n cross . cm2spec1es 2' hId' sect1on, 10-17 , and energy distribution of the fast state are markedly different than for the S states.LEED pattern changes, observed as a function of hydrogen coverage, are in good agreement with results published in the literature. The intensity of the c(2x2) pattern associated with the S2 hydrogen state maximizes at a work function change with adsorption of 0.15 eV. LEED elastic intensity profiles have been obtained for several low index beams. These are compared to theoretical, microscopic model calculations and to published experimental profiles. A c(2x2) LEED pattern is obtained if the crystal is cooled below room temperature. Attempts have been made to correlate the appearance of this pattern to changes in measurements taken in this low temperature range with other techniques. No significant bulk to surface diffusion of hydrogen was found. No evidence was found from work function and electron scattering experiments to support the hypothesis that the low temperature, c(2x2) pattern results from hydrogen on the surface. In addition, the fast hydrogen state is not believed to be responsible for the c(2x2) pattern. LEED intensity profiles from the undosed, cold surface c(2x2) pattern and from the c(2x2) pattern associated with the ~2 hydrogen state are not identical, an indication that the atomic geometry responsible for the two patterns may not be the same.Electron scattering energy loss spectra have been obtained as a function of hydrogen coverage for loss energies of 0 eV ~ w < 40 eV. Hydrogen adsorption has a marked effect on the spectra. Fine structure in the data has been discussed and compared to photoemission data. Cooling the clean crystal below room temperature causes only a slight change in the energy loss spectrum.

[效力级别] tungsten crystal

[关键词] crystal surface studies;tungsten crystal