INITIAL-STAGE OF TITANIUM OXIDATION STUDIED BY DIRECT RECOIL SPECTROMETRY

The initial stage of oxidation of polycrystalline titanium for low oxygen exposures has been studied by time-of-flight (TOF) direct recoil spectrometry (DRS). The DRS signals from surface hydrogen, oxygen, and titanium obtained by use of 4-kev Na** plus and K** plus and two different recoil angles are monitored as a function of exposure to O//2. Desorption of this oxygen induced by 3-kev Ar** plus is also monitored by oxygen DRS. The results show that there are two distinct forms of oxygen below 2 langmuirs. The initial form at low dose ( less than 0. 25 langmuir) exhibits increasing O(DR) and decreasing Ti(DR) signals as a function of exposure, a rapid O//2 uptake rate, and a high Ar** plus -induced oxygen desorption rate, suggesting top-layer sites. The second form at doses of 0. 25-2. 0 langmuirs has slowly increasing O(DR) and constant Ti(DR) signals and lower rates of O//2 uptake and Ar** plus -induced desorption, suggesting subsurface sites. Hydrogen migrates to the surface upon heating and is displaced by oxygen upon chemisorption.