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 TiO 2 (110) surface by multiphoton photoemission (mPP) spectroscopy with wavelength tunable ultrafast (~20 fs) laser pulse excitation. Tuning the mPP photon excitation energy between 2.9 and 4.6 eV reveals a nearly degenerate pair of new unoccupied states located at 2.73±0.05 and 2.85±0.05 eV above the Fermi level, which can be analyzed through the polarization and sample azimuthal orientation dependence of the 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 Ti 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 Ti 3+ states of t 2g symmetry populated by formation oxygen vacancy defects, which exist within the band gap of TiO 2 . Furthermore, we studied the electron dynamics within the conduction band of TiO 2 by three-dimensional (3D) time-resolved pump-probe interferometric mPP measurements. The spectroscopic and time-resolved studies reveal competition between 2PP and 3PP processes where the t 2g -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 TiO 2 .

Non-Band-Gap Photoexcitation of Hydroxylated TiO2

“…The intermediate state in the 2PPE experiment was also recently attributed to a d–d transition from the BGS with t 2 g character to either t 2 g or e g states in the bulk conduction band; 7,10 however, that interpretation is also inconsistent with our data that show the peak intensity is correlated with the density of OH b . We are aware of recent density functional theory calculations that indicate that the adsorption of OH b does indeed give rise to additional densities of state with p character that lies ∼2 to 3 eV above E F , 34 and these may be responsible for the intermediate states observed.…”

Hot electron lifetimes in metals probed by time-resolved two-photon photoemission

“…[23]. Further examples are, for instance, the work by S. Ramakrishna et al [24], who modeled TR-2PPE data using the Heisenberg equations of motion, and very recent works on the ultrafast carrier dynamics at the ZnO(1 0 1 0) surface [25] and within the conduction band of reduced rutile TiO 2 (1 1 0) [26]. However, most of the time-resolved photoemission experiments addressing the carrier dynamics in semiconductors focused not on single electron excitations as typically addressed in TR-2PPE experiments but on the ensemble dynamics after very intense optical excitation (on this point see, i.e., pioneering work by R. Height [3,27] and J. Bokor [28]).…”

Ultrafast Multiphoton Thermionic Photoemission from Graphite

“…The photoemitted electron energy E vs parallel momentum k ∥ distributions, EðkÞ, are analyzed and recorded with a Specs Phoibos 100 hemispherical analyzer with a 2D delay line detector. The analyzer acceptance angle is AE15° [78].…”

“…TR-mPP measurements [38,78] are performed on a highly oriented pyrolytic graphite (HOPG). The sample surface is cleaved in air and degassed at 900 K for 10 hours in the ultrahigh vacuum chamber (base pressure <1 × 10 −10 mbar) for photoemission measurements.…”