OXYGEN ON MO(110) - LOW-TEMPERATURE ADSORPTION AND HIGH-TEMPERATURE OXIDATION

High-resolution electron energy loss spectroscopy (HREELS), Auger electron spectroscopy (AES) and thermal desorption mass spectrometry (TDS) have been used to study the low-temperature and high-temperature oxidation of a Mo(110) single crystal. Oxidized overlayers prepared at 90 and 1000 K were studied at temperatures up to 1400 K. A low-oxygen coverage on Mo(110) at 90 K produces only one vibrational loss at 530 cm-1, indicating dissociative oxygen adsorption into the long-bridge site on this surface. Upon increased exposure, oxygen undergoes conversion to a quasi-threefold adsorption site, causing two characteristic losses at 395 and 605 cm-1 to become spectroscopically visible. An additional loss at 990 cm-1 appears at saturation oxygen exposure. An overlayer prepared by exposure of Mo(110) at 90 K to a low coverage of oxygen followed by heating to 1200 K regenerates a ''clean'' surface, as observed in HREELS; this is caused by oxygen diffusion into the subsurface region. Heating an oxygen-saturated overlayer to 1200 K regenerates a vibrational spectrum that is similar to a low coverage of oxygen on Mo(110). Heavy oxidation of Mo(110) at 1000 K produces an oxide layer characterized by the appearance of a fourth vibrational mode at 740 cm-1 and a twofold increase in the O(KLL)/Mo(LMM) Auger peak-to-peak ratio. Heating this O/Mo(110) layer to 1400 K regenerates a surface that is similar to the oxidized molybdenum overlayer produced by a saturation-oxygen exposure at 90 K.