X-ray resonant Raman scattering from noble gas atoms and beyond M. Z ˇ itnik * , M. Kavc ˇic ˇ, K. Buc ˇar, A. Mihelic ˇ J. Stefan Institute, Jamova 39, P.O. Box 3000, SI, 1000 Ljubljana, Slovenia article info Available online 3 November 2008 PACS: 32.30.Rj 32.70.Jz Keywords: High resolution X-ray spectroscopy Inelastic X-ray scattering Resonant X-ray scattering abstract We discuss general characteristics of the resonant X-ray scattering and briefly illustrate its applications in atomic, molecular and solid state physics by a few selected examples. The Ljubljana high resolution spec- trometer set-up is presented, which was recently employed at ELETTRA and ESRF synchrotron beamlines to study X-ray emission of inner-shell excited states of xenon and argon, and of simple sulfur containing molecules and solid compounds. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction Photoabsorption is one of the most useful spectroscopic tech- niques. Energy dependence of the ‘missing’ photon current after the light has passed through the target, displays characteristic peaks due to discrete excitations of an inner-shell target electrons passing smoothly into the continuum. Position and shape of the corresponding edge is related to the atomic valence (XANES: X-ray absorption near edge structure) and undulations above the threshold carry information about the kind and number of the nearest atomic neighbors (EXAFS: extended X-ray absorption fine structure). Superimposed are many weak structures, the finger- prints of electron multiple excitations and their respective thresh- olds (e.g. [1]). There is, however, an intrinsic limitation of this excellent technique: spectral features reflect the natural width of the inner hole created by the photon absorption. Such final state broadening is present also in photoelectron spectra (XPS: X-ray photoelectron spectroscopy) where the yield of ejected electrons is recorded as a function of their energy [2]. Complementary to these are the methods for observing chan- nels along which the excited states release the energy, mostly by Auger decay (AES: Auger electron spectroscopy) or by X-ray emis- sion (XES: X-ray emission spectroscopy). The available resolving power of spectrometers can be fully exploited in experiments where energy resolution is not limited by the lifetime of the hole – the resonant Raman effect. It follows from energy conservation that if for a given reaction, the energies of all particles involved in the production of initial hole and decay are measured and the total experimental energy uncertainty is smaller than the natural width of the hole, the corresponding spectral lines display a sub- stantial narrowing that reflects experimental rather than combined experimental plus inner hole broadening. Such a situation occurs most easily by the narrow band excitation of discrete states when observing Auger electrons (resonant Raman Auger effect) [3], or photons (RIXS: resonant inelastic X-ray spectroscopy) [4], emitted in the fast decay of an atomic inner hole. The energy positions, intensities, and line shapes in the RIXS spectrum depend on the precise energy and bandwidth of the incident X-ray beam in all the near ionization threshold region. After discussing a few basic aspects of RIXS, the selected results are presented, which were obtained by our high resolution X-ray spectrometer [5] at XAFS@Elettra and ID26@ESRF synchrotron beamlines. 2. Experimental set-up The horizontally polarized and energy selected photon beam at XAFS@Elettra with the cross section of 2 (vertical)  15 mm 2 (hor- izontal) and the flux of 10 10 photons/s emerged from the bending magnet and was energy selected by double crystal Si (1 1 1) mono- chromator and directed onto the target by a set of focusing mirrors. The stainless steel gas cell was filled with gas, typically at several hundred mbar pressure and provided b = 10 mm long interaction region (Fig. 1). This was separated from the spectrometer chamber by 12.5 lm thick kapton foil. The pressure inside the chamber with dimensions 1.6  1.3  0.4 m was kept below 10 5 mbar to mini- mize X-ray absorption. Scattered photons were observed in the horizontal plane at an angle 90° to the direction of incoming light. 0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2008.10.074 * Corresponding author. E-mail address: matjaz.zitnik@ijs.si (M. Z ˇ itnik). Nuclear Instruments and Methods in Physics Research B 267 (2009) 221–225 Contents lists available at ScienceDirect Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb