UHV-Cleaved NiO (100), CoO (100), and MnO (100) by He II UPS

The rocksalt transition-metal oxides whose HeII UPS spectra are presented here have the same crystal structure but different cation d-electron configurations: Mn2+ is 3d5; Co2+ is 3d7; and Ni2+ is 3d8. There is currently a great deal of interest in the growth of thin films of these oxides, and it is important to determine the stoichiometry of such films. These UPS spectra are for ultrahigh vacuum-cleaved single-crystal surfaces having bulk stoichiometry. While interpretation of the spectra is complicated by final-state screening effects [V. E. Henrich and P. A. Cox, see The Surface Science of Metal Oxides (Cambridge University Press, Cambridge, 1994)], the increase in the number of occupied 3d orbitals in going from MnO to NiO is clearly visible in the size of the peak at low binding energy (which is the 0 eV reference for the abscissa in the accompanying figure).