Please cite this article in press as: I. Kuusik, et al., J. Electron Spectrosc. Relat. Phenom. (2011), doi:10.1016/j.elspec.2011.02.002 ARTICLE IN PRESS G Model ELSPEC-45917; No. of Pages 5 Journal of Electron Spectroscopy and Related Phenomena xxx (2011) xxx–xxx Contents lists available at ScienceDirect Journal of Electron Spectroscopy and Related Phenomena journal homepage: www.elsevier.com/locate/elspec The sub-bandgap energy loss satellites in the RIXS spectra of beryllium compounds I. Kuusik a,∗ , T. Käämbre a , K. Kooser a,b , V. Pustovarov c , V. Ivanov c , E. Kukk b , A. Kikas a a Institute of Physics, University of Tartu, Riia 142, 51014 Tartu, Estonia b Department of Physics and Astronomy, University of Turku, Turku, Finland c Ural State Technical University-UPI, Yekaterinburg, Russian Federation article info Article history: Received 26 August 2010 Received in revised form 30 January 2011 Accepted 6 February 2011 Available online xxx Keywords: RIXS Core exciton Hot luminescence abstract Resonant X-ray inelastic scattering spectra have been measured in BeO, phenakite (Be 2 SiO 4 ) and chrysoberyl (BeAl 2 O 4 ) with the excitation energy near the beryllium K edge. The RIXS spectra excited in the vicinity of the Be 1s core resonance show two principal features: the scattering on a valence excitation (which at higher excitation energies verges into the characteristic K  emission), and a remarkably strong energy loss sideband to the elastic scattering peak. The energy loss shoulder appears to result from lattice relaxation in the absorption site. The comparison of the RIXS spectra of phenakite, chrysoberyl and BeO shows that the strength of the low energy sideband differs greatly; it is strongest in BeO and weakest in phenakite. The Si 2p RIXS spectra of phenakite also display a similar strong sub-bandgap energy loss tail. To gain further insight to this process, transitions in a system with a single vibrational mode have been modelled. The phonon relaxation has been simulated empirically by “smearing” the photoabsortion- populated vibrational levels with lower levels. This simple model is able to qualitatively explain this wide energy loss shoulder. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Resonant X-ray inelastic scattering (RIXS) has in recent years attracted attention as a tool capable of providing a unique variety of information about the electronic structure of solids. In this work we present results, which show that information about the lattice vibrations is also accessible through RIXS. Ma et al. [1] first observed a long energy loss tail to the elastic scattering peak in graphite and diamond, while in the absorption spectrum only a relatively sharp core exciton peak was observed. The authors suggested that strong vibronic coupling leads to a local lattice distortion, and to the corresponding shifts of the potential energy in the excited electron state. The appearance of the energy loss sideband near the electroni- cally elastic part of the RIXS spectra indicates that some distortion must occur during the lifetime of the inner-shell hole. A similar phenomenon is typical of valence excitons in alkali halides and in oxides, where self-trapping of valence excitons occurs in the course of lattice relaxation, and the radiation emitted during the lattice relaxation process is known as “hot luminescence” [2]. The same term (hot luminescence) has also been used to describe the pro- ∗ Corresponding author. Tel.: +372 7374779. E-mail address: ivar@fi.tartu.ee (I. Kuusik). cesses related to the appearance of the energy loss sideband of the elastic peak in RIXS spectra by Harada et al. [3], who studied the inelastic scattering in graphite. They emphasized, that the process is similar to the hot luminescence as known in optical spectroscopy. We observed in BeO a wide and very strong energy loss shoulder to the elastic peak [4] for excitations near the Be 1s threshold, which was also attributed to phonon losses. The origin of the energy loss feature can be a mixture of inelastic scattering (a coherent process) and hot luminescence (an incoherent process). The aim of the present work is to study whether similar features also exist in the Be 1s RIXS spectra of phenakite and chrysoberyl. X-ray spectroscopic studies of phenakite have been done earlier [5], but to our knowledge RIXS spectra of phenakite have not been measured previously. Phenakite and BeO have a hexagonal structure [6], while chrysoberyl has an orthorhombic structure [7]. In all the three compounds the beryllium atoms are tetrahedrally coordinated by oxygen and so are the silicon atoms in phenakite. 2. Experimental The measurements were performed on beamline I511-3 in Max Lab, Sweden. The crystallographic c-axis was parallel to the probed sample surface, and the crystal was oriented so that the c-axis was per- 0368-2048/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2011.02.002