A New Mechanism For XPS Line Broadening: The 2p-XPS of Ti(IV)

There is almost a factor of 2 increase in the full width at half maximum (FWHM) of the main Ti 2P(1/2) XPS peak compared to the 2P(3/2) in the closed-shell Ti(W) compounds TiO2 and strontium titanate, STO. Although the spectra of anatase and rutile forms of TiO2 differ slightly from STO, the 2p(1/2) broadening over 2P(3/2) is very similar. For STO, we show that ascribing this differential broadening to a short 4112 lifetime is unphysical. For STO, rigorous and fully relativistic electronic structure calculations have been carried out for both the initial state and the 2p core-hole states; these calculations include many-body effects as well as the one-body effects of spin orbit and ligand field splittings. The agreement of the theoretical and measured XPS data for the main 2p(1/2) and 2P(3/2) peaks indicate that the necessary one- and many-body effects have been included. They show that the broadening is due to the presence of XPS intensity distributed over many unresolved final states for a 2p(1/2) core-hole, whereas the 2p(3/2) core-hole has the expected single symmetric peak. The many-body effects for the core hole states involve mixing of the normal, single-hole, configurations with shake up configurations where valence electrons are promoted from filled orbitals into empty orbitals. This configuration mixing allows configurations with a 2p(1/2) core-hole to mix with those that have a 2p(3/2) core-hole, an effect which, to our knowledge, has not been previously considered. It is the mixing of XPS allowed 2p(3/2) excitations with XPS forbidden 2p(3/2) shake configurations that leads to the distribution of the 213112 XPS intensity over several final states, and the 2p(1/2) XPS broadening is reproduced using a realistic lifetime for the 2p(3/2) core-hole. This novel mechanism for the broadening of XPS features might be more general than solely for the 2p XPS of Ti(W) oxides. The calculations also show the presence of major covalency for the STO orbitals, which is larger for the 2p core-hole configuration than for the ground state, proving that the change in covalency is a major contribution to the core-hole screening.