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Electron emission in low-energy grazing collisions of C+ ions with W(110) partially covered by alkali atoms
A563 Surface Science 291 (1993) 67-77 North-Holland
The electronic structure of CsI adlayers on W(110) A. Hitzke, S. Piilm, H. Miiller, R. Hausmann, J. Giinster, S. Dieckhoff, W. Maus-Friedrichs and V. Kempter Physikalisches Institut der TU Clausthal, D-3392 Clausthal-Zellerfeld, Germany Received 10 October 1992; accepted for publication 15 March 1993 The electronic structure of thin CsI adlayers deposited on W(110) at room temperature is studied by ion impact electron spectroscopy (lIES) with He + and He 2+ ions, metastable impact electron spectroscopy (MIES) with He* atoms, ultraviolet photoelectron spectroscopy (UPS), work function measurements, and to some extent with electron energy loss spectroscopy (EELS). The transition from the electronic structure of single adsorbed molecules to that of the bulk structure could be studied by following the emergence of bulk properties, in particular the bandgap, as a function of the CsI exposure. The experimental results are consistent with the following conclusions: The first step in the layer growth is molecular adsorption of CsI perpendicular to the surface with the iodine end of the molecule pointing toward the substrate accompanied by decrease of the surface work function. The next step, starting with the occurrence of a pronounced work function minimum, is the formation of two-dimensional islands formed by CsI molecules. The results are consistent with lateral growth of the islands until the substrate is largely covered by CsI. The electronic structure of the adlayer cannot be distinguished anymore from that of CsI bulk material by means of electron spectroscopic techniques at exposures which are more than three times that of the occurrence of the work function minimum. The production of vacancies in the I - (5p) and Cs + (5p) orbitals under the influence of the ionizing radiation is studied as a function of the CsI exposure. Their relevance for the desorption processes induced by electronic transitions (DIET) is discussed.
Surface Science 291 (1993)78-86 North-Holland
Electron emission in low-energy grazing collisions of C ÷ ions with W(ll0) partially covered by alkali atoms H. Miiller, R. Hausmann, H. Brenten and V. Kempter Physikalisches Institut der Technischen Universitiit Clausthal, Leibnizstrasse 4, D-3392 Clausthal-Zellerfeld, Germany Received 28 January 1993; accepted for publication 17 March 1993 Electron energy spectra of slow (50 eV) C + ions colliding under grazing incidence with clean and partially alkalated (Li, Na, K, Cs) W(ll0) surfaces are reported. The spectra can be interpreted as due to various inter- and intra-atomic Auger processes occurring in the vicinity of the surface: for clean W(ll0) hardly any electrons are observed. For small alkali coverages the emission is due to Auger capture involving two electrons from the surface. For sufficiently large alkali coverages the resonant capture of one or two electrons from the surface leads to the formation of core-excited states of the neutralized projectile (beyond 0.3 monolayer alkali coverage in terms of a completed monolayer, typically) and Feshbach resonances of the configuration C-*(2s 2 2p3/3/') (beyond 0.5 monolayer K or Cs coverage). These states manifest themselves by their decay via Auger deexcitation and autodetachment, respectively. The availability of three multiplet states of the C(2s 2 2p 2) configuration into which Auger processes can take place produces richly structured spectra. In addition we report the ion scattering spectra (ISS) of the C - ions scattered into 90 ° as a function of the coverage of W(ll0) by alkali atoms.
Surface Science 291 (1993) 87-92 North-Holland
Structural and vibrational properties of alkali-metal overlayers on metals H. Ishida and Y. Morikawa Institute for Solid State Physics, University of Tokyo, Roppongi, Minato-ku, Tokyo 106, Japan Received 19 January 1993; accepted for publication 9 March 1993
The electronic structure of CsI adlayers on W(110) A. Hitzke, S. Piilm, H. Miiller, R. Hausmann, J. Giinster, S. Dieckhoff, W. Maus-Friedrichs and V. Kempter Physikalisches Institut der TU Clausthal, D-3392 Clausthal-Zellerfeld, Germany Received 10 October 1992; accepted for publication 15 March 1993 The electronic structure of thin CsI adlayers deposited on W(110) at room temperature is studied by ion impact electron spectroscopy (lIES) with He + and He 2+ ions, metastable impact electron spectroscopy (MIES) with He* atoms, ultraviolet photoelectron spectroscopy (UPS), work function measurements, and to some extent with electron energy loss spectroscopy (EELS). The transition from the electronic structure of single adsorbed molecules to that of the bulk structure could be studied by following the emergence of bulk properties, in particular the bandgap, as a function of the CsI exposure. The experimental results are consistent with the following conclusions: The first step in the layer growth is molecular adsorption of CsI perpendicular to the surface with the iodine end of the molecule pointing toward the substrate accompanied by decrease of the surface work function. The next step, starting with the occurrence of a pronounced work function minimum, is the formation of two-dimensional islands formed by CsI molecules. The results are consistent with lateral growth of the islands until the substrate is largely covered by CsI. The electronic structure of the adlayer cannot be distinguished anymore from that of CsI bulk material by means of electron spectroscopic techniques at exposures which are more than three times that of the occurrence of the work function minimum. The production of vacancies in the I - (5p) and Cs + (5p) orbitals under the influence of the ionizing radiation is studied as a function of the CsI exposure. Their relevance for the desorption processes induced by electronic transitions (DIET) is discussed.
Surface Science 291 (1993)78-86 North-Holland
Electron emission in low-energy grazing collisions of C ÷ ions with W(ll0) partially covered by alkali atoms H. Miiller, R. Hausmann, H. Brenten and V. Kempter Physikalisches Institut der Technischen Universitiit Clausthal, Leibnizstrasse 4, D-3392 Clausthal-Zellerfeld, Germany Received 28 January 1993; accepted for publication 17 March 1993 Electron energy spectra of slow (50 eV) C + ions colliding under grazing incidence with clean and partially alkalated (Li, Na, K, Cs) W(ll0) surfaces are reported. The spectra can be interpreted as due to various inter- and intra-atomic Auger processes occurring in the vicinity of the surface: for clean W(ll0) hardly any electrons are observed. For small alkali coverages the emission is due to Auger capture involving two electrons from the surface. For sufficiently large alkali coverages the resonant capture of one or two electrons from the surface leads to the formation of core-excited states of the neutralized projectile (beyond 0.3 monolayer alkali coverage in terms of a completed monolayer, typically) and Feshbach resonances of the configuration C-*(2s 2 2p3/3/') (beyond 0.5 monolayer K or Cs coverage). These states manifest themselves by their decay via Auger deexcitation and autodetachment, respectively. The availability of three multiplet states of the C(2s 2 2p 2) configuration into which Auger processes can take place produces richly structured spectra. In addition we report the ion scattering spectra (ISS) of the C - ions scattered into 90 ° as a function of the coverage of W(ll0) by alkali atoms.
Surface Science 291 (1993) 87-92 North-Holland
Structural and vibrational properties of alkali-metal overlayers on metals H. Ishida and Y. Morikawa Institute for Solid State Physics, University of Tokyo, Roppongi, Minato-ku, Tokyo 106, Japan Received 19 January 1993; accepted for publication 9 March 1993