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 2p1/2 XPS peak compared to the 2p3/2 in the closed-shell Ti­(IV) compounds TiO2 and strontium titanate, STO. Although the spectra of anatase and rutile forms of TiO2 differ slightly from STO, the 2p1/2 broadening over 2p3/2 is very similar. For STO, we show that ascribing this differential broadening to a short 2p1/2 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 2p1/2 and 2p3/2 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 2p1/2 core-hole, whereas the 2p3/2 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 2p1/2 core-hole to mix with those that have a 2p3/2 core-hole, an effect which, to our knowledge, has not been previously considered. It is the mixing of XPS allowed 2p1/2 excitations with XPS forbidden 2p3/2 shake configurations that leads to the distribution of the 2p1/2 XPS intensity over several final states, and the 2p1/2 XPS broadening is reproduced using a realistic lifetime for the 2p1/2 core-hole. This novel mechanism for the broadening of XPS features might be more general than solely for the 2p XPS of Ti­(IV) 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.