Rein Ruus

Ti 2p and O 1s X-ray absorption of TiO2 polymorphs

Abstract We present and discuss the Ti 2p and O 1s X-ray absorption spectra (XAS) and resonant photoelectron emission spectra (RPES) of the various polymorphic forms (TiO2-II, rutile and anatase) of TiO2. The largest differences between the Ti 2p XAS of polymorphs of TiO2 are observed in the region of the double peak structure of the L3 edge. A strong delocalization of the excited 3d electron in this region is concluded from the RPES at the L3 threshold of Ti. The differences in XAS are attributed to increasing strength of the crystal field caused by delocalization of the excited 3d electron.

Multi-atom resonant photoemission in transition metal chlorides

Abstract Recently multi-atom resonances in photoemission of transition metal oxides were observed by Kay et al. [A. Kay, E. Arenholz, S. Mun, F.J. Garcia de Abajo, C.S. Fadley, R. Denecke, Z. Hussain, M.A. Van Hove, Science 281 (1998) 679]. We present the results of a systematic study of multi-atom resonant photoemission in transition metal chlorides. We measured the intensity of the chlorine 2p photoelectron line excited around the metal 2p photoabsorption edge of the transition metal compounds MnCl 2 , CrCl 2 , and VCl 2 as well as of the ionic solid KCl. Our experimental results show that the intensity of Cl 2p photoelectron lines enhance by about 29% for MnCl 2 , 21% for CrCl 2 , 17% for VCl 2 and 11% in the case of KCl at the metal 2p photoabsorption edge. We ascribe the systematic decrease of the magnitude of this effect, going from MnCl 2 to KCl, to the extension of the excited metal 3d wavefunction and, as a consequence, decrease of the overlap between the 3d and 2p atomic orbitals.

Resonant photoemission of CoCl2

Abstract We present the results of a study of the resonant photoemission from polycrystalline in situ evaporated thin film of CoCl2 excited at 2p photoabsorption edges of Co and Cl. At the Co 2p→3d resonance strong enhancement was observed for the Co 3p, valence band photoemission and for spectral regions of L23M23M23, L23M23M45 and L23M45M45 Auger transitions. The results of valence band resonant photoemission is discussed in terms of the available calculations. The valence band resonant spectra exhibit no enhancement in connection to the Cl 2p edge which shows that the Cl 2p absorption pre-edge structure is formed by d- and s-like final delocalized states.

Substrate-induced effects in the creation and decay of potassium 2p core excitations in ultrathin films of KCl on copper

Substrate-induced effects on the transport properties in thin KCl films on the Cu(100) surface have been studied using K + 2p photoelectron, photoabsorption and resonant Auger spectra. The measurements were performed at different KCl coverages ranging from a partial monolayer to a thick bulk-like film. The morphology and layer thickness were estimated from an analysis of the electron energy loss structure of the K + 2p and Cl − 2p photoelectron peaks, and from the variations in the photoelectron peak fine structure. The resonant Auger spectator decay spectra measured at the photoabsorption resonances show that the significant differences between the spectra of the solid and of thin layers are related to the charge delocalization from the K + 3d excited state into the metal substrate. The core-hole-clock approach yields an estimate of 1.5 fs for the corresponding charge transfer time at a single monolayer KCl coverage. (Some figures in this article are in colour only in the electronic version)

NaK photoabsorption and resonant KLL Auger spectra in naf and NaCl

The nature of Na Is photoabsorption in NaF and NaCl is investigated using the resonant Auger decay of Na Is core excitations. The appearance of new peaks in Auger spectra, when the energy of excitation coincides with the photoabsorption thresholds, shows that the core excitons are created in the threshold region of the Na 1s photoabsorption spectrum. The core excitons around 1074.2 eV originate from the dipole-forbidden 1s(-1)3s final state, the population of which is made possible by the symmetry disruption at the photoabsorption site. Another core exciton, excited at photon energies of 1076.8 eV in NaF and 1076.3 eV in NaCl, originates from 1s(-1)3p states similar to atoms of Ne and Na. Resonant Auger spectra show also that in NaCl the first sharp photoabsorption maximum at 1076.6 eV has only partially excitonic character.

Exciton lineshapes of LH2 photosynthetic antenna proteins

In photosynthetic membranes, antenna chlorophyll-protein complexes serve to carry out very efficient solar energy absorption and transfer into the photochemical reaction center where the free chemical energy is created. The exact nature of the antenna excitations has been studied for many years but remains a matter of considerable debate. Due to high density of chlorophyll molecules and resulting strong coupling between them, it seems natural that the excitations are excitons, delocalised over several antenna molecules. However, the experimental evidence has been limited as yet. One reason for this is the well-documented strong inhomogeneous broadening of in vivo chlorophyll spectra due to spectral disorder [1]. That obscures the possible exciton effects.

Auger decay of K+ L23 excitations in potassium halides

The L23M23M23 Auger spectra of K+ L23 excitation in potassium halides are measured using photon excitation with different energies in the region of the K+ L23 absorption edge (around 300 eV). The experimental electron spectra are compared with the configuration interaction HFP calculations. The new structures on the high-energy side of normal Auger lines we assigned to the transitions 2p53d → 3p43d. Only the low-energy component of the crystal-field-split K+ 3d orbital forms a genuine bound state with 2p holes, whereas its high-energy component lies in the conduction band continuum and behaves like a shape resonance.