AVS2001 Session SS1-MoP: Electronic Structure Poster Session

Monday, October 29, 2001 5:30 PM in Room 134/135
Monday Afternoon

Time Period MoP Sessions | Topic SS Sessions | Time Periods | Topics | AVS2001 Schedule

SS1-MoP-1 The Surface Sensitivity of the Unoccupied Bands of Mo(112)
H.-K. Jeong, T. Komesu (University of Nebraska-Lincoln); I.N. Yakovkin (National Academy of Sciences of Ukraine); P.A. Dowben (University of Nebraska-Lincoln)
We present results of unoccupied states of Inverse photoemission on Mo (112). The data are compared with the calculated band structure. The comparison between the calculated and experimental bands along the of the surface Brillouin zone is key to the elucidation of the important features of the electronic structure of Mo (112) surface. Because the Brillouin zone does not change along for these two surfaces, the angle -resolved inverse photoemission spectra can be compared to determine surface sensitivity all along this symmetry direction from zone center to zone edge. Enhanced surface sensitivity of inverse photoemission from the comparison of spectra for clean and the p(1x2) oxygen covered surfaces has been investigated. In particular, different sensitivity of the surface states 0.5 eV and 2.2 eV to oxygen adsorption can be attributed to different symmetry of the states at the surface Brillouin zone center. Thus, the 0.5 eV state found to have a1 symmetry in theory, while the 2.2 eV band is found to be odd with respect to the mirror plane in theory.
SS1-MoP-3 High Spatial Resolution Soft X-ray Photoemission Study of WO3 Thin Films
L. Lozzi, M. Passacantando, S. Santucci (University of L'Aquila, Italy); S. La Rosa, N. Yu Svetchnikov (Sincrotrone Trieste SCpa, Italy)
Tungsten trioxide (WO3) is a wide gap n-type semiconductor and it is the subject of an intense both theoretical and experimental studies because of its interesting applications, such as gas sensors towards different gases, like NO2 and H2S, and as electrochromic film. Many of these possible applications are mainly due to the oxygen vacancies. For example, in the sensing mechanism, the gas species are adsorbed on the surface changing the concentration of the free electrons on the surface. These electrons are present on the surface because of the oxygen vacancies. The variation of this concentration modifies the electrical conductivity of the film. Similarly, the presence of substoichiometric WO3-x compounds, determines the optical properties of these films. An important parameter in the preparation of WO3 thin films is the thermal treatment following the sample growth. For example it has been shown that the gas sensitivity and response time are strongly influenced by the annealing procedure, because of the phase transitions induced by the annealing. In this work the WO3 surface chemical composition has been studied by means of high resolution soft X-ray photoemission spectroscopy. We have studied the surface properties of both as deposited samples and samples after annealing in air at high temperatures. Valence band and W 4f core levels have been analysed on different samples positions and high resolution maps have been acquired. The valence band spectra have shown W 5d density of state at the Fermi level, indicating the presence of metallic tungsten on the surface. This has been confirmed by the W 4f signal, which present both metallic and oxidized phases. The high resolution maps, obtained following both valence and core states, have clearly evidenced the presence non stoichiometric areas and of some metallic islands.
SS1-MoP-4 Femtosecond Dynamics of Electrons in Image-potential States on Stepped Cu(001)
M. Roth, M. Pickel, J. Wang, M. Weinelt, Th. Fauster (Universität Erlangen-Nürnberg, Germany)
The Rydberg-like series of image-potential states (|n>, n = 1, 2, ...) is a prototype system for loosely bound electrons at a metal surface. The electronic structure and the femtosecond dynamic of these states is studied by energy- and time-resolved two-photon photoemission spectroscopy. The electron trapped in the image-potential moves virtually free lateral to the surface exhibiting a parabolic dispersion with effective mass close to the free-electron mass. In this intermediate state the electron is subject to inelastic and quasielastic scattering processes which cause decay of the population and phase relaxation. The influence of surface corrugation on these processes has been investigated for stepped Cu(117) and Cu(119) surfaces which are vicinal to Cu(001). The dynamics depend on both the distance of the electron in front of the surface (~ n2) and the parallel momentum perpendicular to the step edge. Only for the lowest image-potential state |1> we can identify umklapp processes. An electron in this state which moves upstairs apparently has a longer lifetime than an electron moving downstairs. In the second image-potential state |2> the decay rate decreases linearly with kinetic energy but independent of direction. While quasielastic scattering with small momentum transfer is mainly observed for the first image-potential state, the population of state |2> decays in addition via resonant scattering from the band bottom to states of band |1> with finite momentum. Compared to the Cu(001) surface the latter process is considerably enhanced on the stepped surfaces.
Time Period MoP Sessions | Topic SS Sessions | Time Periods | Topics | AVS2001 Schedule