Femtosecond electron injection from optically populated donor states into the conduction band of semiconductors

Hot electron injection from the aromatic chromophore perylene into TiO ₂ was measured with transient absorption signals for different rigid anchor-cum-spacer groups revealing 15 fs as the shortest and 4 ps as the longest injection time. The energetic position of the donor orbital of the chromophore with respect to the conduction band edge was determined at about 0.8 eV employing ultraviolet photoelectron spectroscopy (UPS) and simple absorption spectroscopy. It is not clear in the case of rutile or anatase TiO₂ whether unoccupied surface states are involved in the electron injection process as acceptor states. Since the surface reconstruction of TiO₂ is difficult to control electron scattering between a well-defined surface state and isoenergetic unoccupied bulk states was studied with InP(100). Electron scattering was time-resolved employing two-color two-photon-photoemission (2PPE). Scattering from isoenergetic bulk states to the empty C₁ surface state was found to occur with a 35 fs time constant, and the reverse process showed a time constant in the range of 200 fs. The latter was controlled by energy relaxation in bulk states, i.e. via the emission of longitudinal optical phonons in InP. In general, the injection of a hot electron from a molecular donor into electronic states of a semiconductor has to be distinguished from consecutive electron scattering processes between surface states and bulk states. Distinguishing between the different processes may become difficult, however, if the electronic interaction becomes large for a small chromophore directly attached to the semiconductor.