Ultrafast multiphoton pump-probe photoemission excitation pathways in rutileTiO2(110)

Abstract

We investigate the spectroscopy and photoinduced electron dynamics within the conduction band of reduced rutile $\mathrm{Ti}{\mathrm{O}}_{2}(110)$ surface by multiphoton photoemission $(\mathrm{mPP})$ spectroscopy with wavelength tunable ultrafast $(\ensuremath{\sim}20\phantom{\rule{0.16em}{0ex}}\mathrm{fs})$ laser pulse excitation. Tuning the $\mathrm{mPP}$ photon excitation energy between 2.9 and 4.6 eV reveals a nearly degenerate pair of new unoccupied states located at $2.73\ifmmode\pm\else\textpm\fi{}0.05$ and $2.85\ifmmode\pm\else\textpm\fi{}0.05$ eV above the Fermi level, which can be analyzed through the polarization and sample azimuthal orientation dependence of the $\mathrm{mPP}$ spectra. Based on the calculated electronic structure and optical transition moments, as well as related spectroscopic evidence, we assign these resonances to transitions between $\mathrm{Ti}\phantom{\rule{0.16em}{0ex}}3d$ bands of nominally ${t}_{2g}$ and ${e}_{g}$ symmetry, which are split by crystal field. The initial states for the optical transition are the reduced $\mathrm{T}{\mathrm{i}}^{3+}$ states of ${t}_{2g}$ symmetry populated by formation oxygen vacancy defects, which exist within the band gap of $\mathrm{Ti}{\mathrm{O}}_{2}$. Furthermore, we studied the electron dynamics within the conduction band of $\mathrm{Ti}{\mathrm{O}}_{2}$ by three-dimensional time-resolved pump-probe interferometric $\mathrm{mPP}$ measurements. The spectroscopic and time-resolved studies reveal competition between 2PP and 3PP processes where the ${t}_{2g}\text{\ensuremath{-}}{e}_{g}$ transitions in the 2PP process saturate, and are overtaken by the 3PP process initiated by the band-gap excitation from the valence band of $\mathrm{Ti}{\mathrm{O}}_{2}$.