Chapter 103 - Heterogeneous electron transfer probed with femtosecond two-photon photoemission spectroscopy

Time-resolved measurements of photo-induced electron injection are reported. Electron transfer occurred from the excited state of the molecular chromophore perylene into the TiO2(1 10) surface of a rutile single crystal in ultra-high vacuum. Femtosecond two-photon photoemission spectroscopy (fs-2PPE) was employed since it has a much higher sensitivity than fs transient absorption. A long rigid spacer-cum-anchor group (TriPod) was attached to the aromatic chromophore to keep the latter at a distance greater than I nm from the crystal surface, Correspondingly, electron transfer was slow with a time constant of 2 ps and could easily be separated from fast energy relaxation with time constant of 40 fs that the injected hot electrons showed in TiO2. From UPS and XPS data it was deduced that electrons were mainly injected into electronic levels about 0.8 eV above the lower edge of the conduction band.