A 2-PHOTON PHOTOEMISSION-STUDY OF GAAS(100)

Two broad spectral features are identified in the direct two-photon ph otoemission spectra for oxidized GaAs(100) for photon energies between 3.1 eV and 3.5 eV. The double structure is consistent with the optica lly allowed direct two-photon transitions from the valence bands near the GAMMA point. This interpretation includes intermediate-state relax ation within the time period of a single laser pulse, namely 180 fs. F or annealed GaAs(100) surfaces the work function is approximately 0.6 eV larger, than for the oxidized sample and the double structure is no longer visible .