Multiphoton photoemission spectroscopy of TiO₂and Au/TiO₂ surfaces

TiO₂ is a widely investigated metal oxide due to its photocatalytic and photovoltaic applications. Until recently, the ability to probe the ultrafast electron dynamics of this system has been limited to transient absorption experiments. Understanding the photoexcited electron dynamics in TiO₂ is vital to extend its applications through a thorough understanding of its electronic structure. TiO₂ is known to catalyze reactions upon UV excitation. Photocatalysis in TiO₂ can be extended into visible by coupling it with plasmonic metal nanoparticles, which can be very efficient light absorbers in the visible through excitation of their localized plasmon resonance. Au nanoclusters coupled to TiO₂(110) have been shown to enhance photocatalytic properties upon visible excitation. By probing the rutile TiO₂(110) and Au/TiO₂ interface using multiphoton photoemission (mPP) with a tunable ultrashort (~20 fs) laser pulse, we investigate the dynamics of electrons excited in its conduction band. mPP spectra reveal resonances where TiO₂ defect state electrons absorb energy to reveal new optical energy levels and transitions between them. Decoration of the surfaces with Au overlayer introduces new resonances, but does not show a plasmonic enhancement of the optical response of Au/TiO₂ surfaces.