Electron-energy-loss and secondary-electron emission spectroscopies of clean and hydrogen-covered Ni (100) surfaces

The clean and hydrogen-covered Ni (100) surfaces at 300 K have been studied using electron-energy-loss and secondary-electron emission spectroscopies (EELS and SES, respectively). The clean-surface SES spectrum has peaks at 7.4, 9, 10.4, 11.4, 13.4, and 22.8 eV above ${E}_{F}$, which correspond to maxima in the calculated density of unoccupied states. The EELS spectrum is characterized by peaks at 3.4, 6.9, 8.9, 18, and 26 eV in agreement with optical data. The peaks at 3.4, 6.9, and 26 eV are attributed to one-electron transitions from the $3d$ bands to empty states of high $\mathrm{sp}$-like partial density of states. No hydrogen-induced peaks are observed in the EELS spectrum: Only the 6.9- and 8.9-eV peak intensities are strongly reduced. These results may indicate that the Ni-H bonding occurs predominantly via $\mathrm{sp}$ electrons and that $d$ electrons play a minor role.