Dynamics of electron injection from the excited state of anchored organic molecules into rutile (110)TiO2

Hot electron injection from the excited electronic singlet state of perylene chromophores into the (110) surface of rutile TiO2 single crystals was measured with femtosecond two-photon photoemission (2PPE) for different anchor/bridge groups attached to the perylene chromophore. Femtosecond 2PPE probes the time and energy dependence of the population of firstly the excited state of the chromophore and secondly of the hot electrons injected into the surface layer of the semiconductor. Measuring both these contributions gives a complete picture of the ultrafast photo-induced injection process and bridges the gap to conventional measurements of the rise time of the corresponding photocurrent. Studying the system in ultra-high-vacuum (UHV) makes all the tools of surface science available. Impurities on the surface were studied with XPS, the alignment of the occupied and unoccupied electronic levels at the interface with UPS and with 2PPE, respectively. The orientation of the elongated chromophores with respect to the crystal surface was deduced from angle and polarization dependent 2PPE signals making use of the known orientation of the dipole moment for the optical transition, the energy distribution of the injected hot electrons was determined with 2PPE from the energy distribution of the photoemitted electrons, and finally the escape of the injected electrons from the surface to bulk states of the semiconductor was obtained from femtosecond 2PPE transients.