CHARACTERIZATION OF MOLECULAR MODIFIED SURFACE-STATES BY WAVELENGTH-DEPENDENT AND TIME-DEPENDENT 2-PHOTON PHOTOEMISSION SPECTROSCOPY

Wavelength-dependent two-photon photoemission (WD-TPPE) spectroscopy w as used to investigate the surface state properties of CdTe crystals b efore and after the adsorption of specially designed organic molecules . One photon was used to modify the population of the surface states a nd a second photon to eject electrons from the substrate. We measured the dependence of the photoemission signal on the energy of the first photon and on the delay between the two light pulses. The energy of su rface states, relative to the bands, was found to correlate with the r elaxation time of the semiconductor surface after being photoexcited. This is explained in terms of a simple kinetic model for electron tran sfer. These findings demonstrate that the properties of surface states of semiconductors can be manipulated by adsorbing suitable organic mo lecules on the semiconductor surface and that the WD-TPPE method is a useful tool for optoelectronic characterization of semiconductor surfa ces, with sensitivity exceeding that of most commonly used techniques.