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p-GaAs(100) surface and its relation to negative electron affinity
A241 Surface Science 283 (1993) 213-216 North-Holland
An unusual adsorption state of hydrogen on the Pd( 100)-p(2 × 2)-p4g-A1 bimetallic surface Hiroshi Onishi, Tetsuya Aruga and Yasuhiro Iwasawa * Department of Chemistry, Faculty of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan Received 23 April 1992; accepted for publication 11 May 1992 A Pd(100) surface modified by aluminum was investigated with AES, LEED and H2-TPD. The AI/Pd(100) surface gives an ordered surface alloy with p(2 × 2)-p4g symmetry upon annealing at 700 K. Molecular hydrogen dissociates spontaneously on the p(2× 2)-p4g surface alloy, whereas the AI monolayer deposited at 300 K prevents the adsorption of H 2. A recombinative desorption (6-H 2) process starts on the alloy at 360 K with first-order kinetics. The activation energy and the pre-exponential factor for the 6 desorption state are evaluated to be 144 +_ 10 kJ/mol and 4 x 1020 ±1 s - t , respectively. These unusually large values are ascribed to bimetallic ensembles on the ordered surface alloy.
Surface Science 283 (1993) 217-220 North-Holland
Bonding of 0 2, CO 2, and CO on a Cs/p-GaAs(100) surface and its relation to negative electron affinity J. Sakai, G. Mizutani and S. Ushioda Research Institute of Electrical Communication, Tohoku University, Sendai 980, Japan Received 21 April 1992; accepted for publication 15 May 1992 The electron affinity of the negative electron affinity (NEA) surface of p-GaAs, created by coadsorption of Cs and 0 2, increases (becomes less negative), when it is exposed to CO2 or excess 0 2, while CO has no effect. To find the cause for this difference, we have investigated the bonding structure of these molecules (or atoms) at the Cs/p-GaAs surface by thermal desorption spectroscopy (TDS). The TDS data show a common feature that is shared by the (Cs + O 2) and (Cs + CO 2) coadsorbed surfaces but not by the (Cs + CO) coadsorbed surface. From the cesiated surface after large exposure to O 2 or CO 2 (> 2 L), As 2 and Cs desorb simultaneously at ~ 450°C. The desorption peaks do not appear at this temperature for a clean GaAs surface or a pure Cs adsorbed surface. Simultaneous desorption of Cs and As 2 indicates the existence of A s - O - C s bonds. In contrast, Cs and As 2 desorb at different temperatures from the surface exposed to CO. From these observations and the fact that CO does not affect NEA, we conclude that A s - O - C s bonds in the surface layer increase the surface electron affinity, i.e., works against creating an NEA surface.
Surface Science 283 (1993) 221-225 North-Holland
Direct measurement of nonequilibrium electron-energy distributions in sub-picosecond laser-heated gold films W.S. Fann a,., R. Storz b, H.W.K. T o m c and J. Bokor d a Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan, ROC b A T& T Bell Laboratories, Holmdel, NJ 07733, USA c Department of Physics, University of California, Riverside, CA 92521, USA a Department of Electrical Engineering, University of California, Berkeley, CA 94720, USA Received 22 April 1992; accepted for publication 27 April 1992 The electron-energy distribution in a gold film was measured with ~ 700 fs time-resolved photoemission spectroscopy following laser heating by a 400 fs visible laser pulse. The measured distribution can be fit by the Fermi-Dirac function at an eleCated temperature except within 800 fs of the heating pulse (time-resolution limited), when a reproducible departure is observed. As a result, the relaxiation of nonequilibrium electrons was found to be inadequately described by the standard electron-phonon coupling model.
An unusual adsorption state of hydrogen on the Pd( 100)-p(2 × 2)-p4g-A1 bimetallic surface Hiroshi Onishi, Tetsuya Aruga and Yasuhiro Iwasawa * Department of Chemistry, Faculty of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan Received 23 April 1992; accepted for publication 11 May 1992 A Pd(100) surface modified by aluminum was investigated with AES, LEED and H2-TPD. The AI/Pd(100) surface gives an ordered surface alloy with p(2 × 2)-p4g symmetry upon annealing at 700 K. Molecular hydrogen dissociates spontaneously on the p(2× 2)-p4g surface alloy, whereas the AI monolayer deposited at 300 K prevents the adsorption of H 2. A recombinative desorption (6-H 2) process starts on the alloy at 360 K with first-order kinetics. The activation energy and the pre-exponential factor for the 6 desorption state are evaluated to be 144 +_ 10 kJ/mol and 4 x 1020 ±1 s - t , respectively. These unusually large values are ascribed to bimetallic ensembles on the ordered surface alloy.
Surface Science 283 (1993) 217-220 North-Holland
Bonding of 0 2, CO 2, and CO on a Cs/p-GaAs(100) surface and its relation to negative electron affinity J. Sakai, G. Mizutani and S. Ushioda Research Institute of Electrical Communication, Tohoku University, Sendai 980, Japan Received 21 April 1992; accepted for publication 15 May 1992 The electron affinity of the negative electron affinity (NEA) surface of p-GaAs, created by coadsorption of Cs and 0 2, increases (becomes less negative), when it is exposed to CO2 or excess 0 2, while CO has no effect. To find the cause for this difference, we have investigated the bonding structure of these molecules (or atoms) at the Cs/p-GaAs surface by thermal desorption spectroscopy (TDS). The TDS data show a common feature that is shared by the (Cs + O 2) and (Cs + CO 2) coadsorbed surfaces but not by the (Cs + CO) coadsorbed surface. From the cesiated surface after large exposure to O 2 or CO 2 (> 2 L), As 2 and Cs desorb simultaneously at ~ 450°C. The desorption peaks do not appear at this temperature for a clean GaAs surface or a pure Cs adsorbed surface. Simultaneous desorption of Cs and As 2 indicates the existence of A s - O - C s bonds. In contrast, Cs and As 2 desorb at different temperatures from the surface exposed to CO. From these observations and the fact that CO does not affect NEA, we conclude that A s - O - C s bonds in the surface layer increase the surface electron affinity, i.e., works against creating an NEA surface.
Surface Science 283 (1993) 221-225 North-Holland
Direct measurement of nonequilibrium electron-energy distributions in sub-picosecond laser-heated gold films W.S. Fann a,., R. Storz b, H.W.K. T o m c and J. Bokor d a Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan, ROC b A T& T Bell Laboratories, Holmdel, NJ 07733, USA c Department of Physics, University of California, Riverside, CA 92521, USA a Department of Electrical Engineering, University of California, Berkeley, CA 94720, USA Received 22 April 1992; accepted for publication 27 April 1992 The electron-energy distribution in a gold film was measured with ~ 700 fs time-resolved photoemission spectroscopy following laser heating by a 400 fs visible laser pulse. The measured distribution can be fit by the Fermi-Dirac function at an eleCated temperature except within 800 fs of the heating pulse (time-resolution limited), when a reproducible departure is observed. As a result, the relaxiation of nonequilibrium electrons was found to be inadequately described by the standard electron-phonon coupling model.