Physica Scripta. Vol. 35, 541-546, 1987. Electronic Resonances in Rare-Gas Adsorbates Observed by Spin- Resolved Electron Spectroscopy* G. Schonhense, B. Kessler, N. Muller, B. Schmiedeskamp and U. Heinzmann Universitat Bielefeld, Fakultat fur Physik, D-4800 Bielefeld, F.R.G.; Fritz-Haber-Institut der MPG, Faradayweg 4 4 , D-1000 Berlin 33, F.R.G. Received August 8, 1986; accepted October 28, 1986 Abstract Recent experimental results of spin-polarized electron emission following photoexcitation of the valence orbitals of adsorbed rare-gas atoms are presented. As light source we employed the circularly polarized synchrotron radiation from the storage ring BESSY (Berlin). As detection technique served angle- and spin-resolved electron spectroscopy. The resolution of the experimental method is demonstrated by the example of photoemission from Ar on Pt(l1 I), where a doublet feature with a splitting of only 90 meV could be resolved, which is hardly discernible in intensity spectroscopy. Below the threshold for direct photoemission narrow electronic resonances have been observed, resulting from discrete np * n's excitations in the adsorbate. For the system Xe on Ir(l11) the dependence of the resonance features on the coverage from the sub-monolayer regime up to the 3D-crystal is discussed. 1. Introduction and experimental Energy-, angle-, and spin-resolved photoelectron emission from solids and adsorbates has proven to be a valuable tool in the investigation of the electronic structure (for a recent review, see Ref. [l]). One basic potential of the new method is that it allows to probe the symmetries of electronic states, especially for high-2 systems. For such elements the conven- tional technique of applying polarization selection rules for plane-polarized radiation fails because of strong mixing of orbital symmetries as a consequence of spin-orbit interac- tion. We have demonstrated the advantages of spin-redolved measurements for the determination of band symmetries by the examples of the Pt( 11 1) crystal and adsorbed Xe and Kr atoms. For platinum, a symmetry-resolved bandmapping of the A-direction was performed [2] and for the adsorbed rare- gas atoms an unambiguous quantum-number labelling of the Xe 5p and Kr4p hole states was given [3]. A very surprising experimental result was the discovery of sharp, photon-induced resonances in Kr and Xe adsorbates on Pt(ll1) and graphite(0001) [4]. Measurements of the electron-spin polarization (ESP) of the resonantly emitted electrons allowed to identify the resonances as discrete np + (n + 1)s excitations, exhibiting an additional line splitting due to the anisotropic bonding interactions on the surface. In the present article we present new experimental data obtained for argon monolayers on the platinum( 1 1 1) face and for xenon monolayers on the basal plane of natural single crystal graphite [5]. Furthermore, we demonstrate the sensitivity of the electronic resonances on the coverage by the example of xenon on the iridium(ll1) face from the sub- monolayer regime up to the three-dimensional xenon crystal. The method of photoelectron spin-polarization spectro- scopy and its experimental equipment has been recently described in some detail [6]. The method is based upon spin orientation by optical pumping with circularly polarized (a) light and photoelectron spin detection by a conventional high-energy Mott detector (operated at a scattering energy of 100 keV) preceded by a special angle-resolving photoelectron spectrometer. The experiment is set up at the storage ring BESSY (Berlin), utilizing the circularly polarized off-plane synchrotron radiation delivered by the 6.5 m normal-incidence monochromator [7]. Target preparation is made in a separate preparation chamber equipped with an ion gun, a scanning Auger system and LEED optics. The substrate crystal is mounted on a liquid-helium cooled target manipulator; the crystal temperature can be varied between 40 K and 1500K. During the measurements the base pressure in the photo- emission chamber was in the lo-" mbar range, allowing sampling times of about one hour without changing the adsorbate overlayer. 2. Spinresolved spectra of Ar monolayers Spinresolved photoemission from argon atoms in the adsorb- ate phase is of interest because for such light atoms (Z = 18), spin-orbit interaction, which is responsible for a photoelectron polarization, is weak. The fine-structure splitting of the final ionic states Ar+3pS(2P,,z, 2P,,2) in the gas phase is 180meV only, whereas those of Kr (2 = 36) and Xe (2 = 72) are 0.6 and 1.3 eV, respectively. Nonetheless, the photoelectron spin polarization in valence-shell photoemission from free Ar atoms has almost the same quantitative values as for free Kr and Xe atoms - provided the fine-structure splitting is experimentally resolved [8]. The gas-phase results show that the strength of spin-orbit interaction primarily determines the quantitative splitting of energy levels but not the degree of the photo ESP. For atoms on a surface we would expect in principle the same behavior but, on the other hand, there are two mech- anisms which change the situation. First, all photoemission lines are broadened due to lifetime and vibrational effects so that a small level splitting could possibly be washed out. Second, in addition to spin-orbit coupling the anisotropic bonding interaction comes into play which is induced by the presence of the substrate surface and by the neighboring adatoms. The latter leads to the well-known magnetic Imjl- sublevel splitting in adsorbed Xe and Kr layers [3, 9, 101. Figure 1 shows a spin-resolved photoelectron spectrum of Ar on Pt(ll1) at monolayer coverage. The spectrum has been taken with circularly polarized synchrotron radiation of - hv = 14eV at normal incidence and normal emission. The around 7.7 eV binding energy with respect to the Fermi energy of the platinum substrate. The photon bandwidth Ahv and * This paper was contributed to the 8th Vacuum Ultraviolet Radiation Physics International Conference, held in Lund, Sweden, 4-8 August, 1986, and will be included in part 11 of the Conference proceedings. (Editors: P. -0. Nilsson and J. Nordgren). total intensity (upper pane1) is essentially given by One peak Physica Scripta 35