X-Ray Resonant Raman Scattering: Observation of Characteristic Radiation Narrower than the Lifetime Width

Abstract

Intense tuneable synchrotron radiation was used to perform a high-resolution study of the resonant scattering response of Cu metal in the x-ray regime. One finds in the transition regime from fluorescence to resonant scattering that the linewidth of the emitted radiation becomes narrower than the lifetime-limited width of the fluorescent radiation.Recently there has been considerable interest in the phenomena of resonant x-ray Raman scattering. 1 "* 3 Experiments and theory have to date focused on the gross features of the phenomena. The experiments have all used low-resolution (~ 200 eV) detection systems while the theory has been one-electron in character and has not included the effects of the lifetime of the final state.In this work we report very-high-resolution studies of resonant x-ray Raman scattering of copper in the resonant regime where lifetime considerations become very important. We find for the first time in the field of resonant Raman scattering, either in the visible or x-ray regime, the remarkable though understandable result that at resonance the energy width of the scattered radiation is -30% less than the lifetime-determined width which is observed above resonance in the fluorescence regime. In addition we will show that one can uniquely identify the energy of excitation to the Fermi surface through this technique.The experimental configuration is the same as described previously in Ref. 3, hereafter referred to as I, except that the solid-state detector was replaced by a double-silicon-(lll)-crystal Bragg spectrometer which had an essentially Gaussian resolution function with a full width at half-maximum (FWHM) of 0 o 8 eV. The input radiation provided by the Stanford University storage ring SPEAR was monochromatized and made continuously tunable by a slit-channel-cut silicon-(220) spectrometer which had an essentially Lorentzian resolution function with a FWHM of 0.9 eV. This system provided about 10 10 photons per second in the 0.9 eV bandwidth.As in I the experiment was performed by first tuning the incident energy to a value in the vicin-ity of the K absorption edge of Cu (£2^ +E F =8980 eV). One then determines the spectrum of radiation scattered at 90° in the energy region around the Ka x fluorescence energy of Cu {Q, K -£l L3 / 2 = 8048 eV) by scanning the double-crystal Bragg spectrometer. The specific energy levels of copper that are involved are schematically indicated in Fig. 1. The signals were so weak (2 counts/ sec at the peak of Ka x at resonance) that we decided to focus our attention on the Kot 1 region which has twice the signal of the Ka 2 region because the P 3/2 states have twice the degeneracy of the P 1/2 states.The results for the dispersion of the peak (i.e., energy in versus energy out) and the linewidth J K I FIG. 1. Schematic of the energy levels of Cu involved in this study.623

A von Hamos x-ray spectrometer based on a segmented-type diffraction crystal for single-shot x-ray emission spectroscopy and time-resolved resonant inelastic x-ray scattering studies

“…At synchrotron sources, hard x-ray emission spectroscopy is used for resonant inelastic x-ray scattering studies (RIXS), 7 high energy resolution fluorescence detected XAS spectroscopy (HERFD-XAS or alternatively called partial fluorescence yield XAS) (Refs. [8][9][10][11] and to study core-to-core and valence-to-core transitions. [12][13][14] The high energy resolution XAS/RIXS techniques require the experimental resolution to be lower than the natural lifetime broadening of the excited state.…”

“…15 If the experimental resolution is below the K-or L-lifetime broadening, an enhanced sensitivity on the measured XAS/RIXS spectral shape can be obtained. 8,9 High energy resolution XAS is used to follow the dynamic changes of the chemical state, electronic and geometric structure of matter under reaction conditions. 16 In contrast, RIXS spectroscopy is usually performed under steadystate conditions, due to its relatively large acquisition time ranging from several minutes to a few hours.…”

Stimulated Electronic X-Ray Raman Scattering

“…Tuning the photon energy across an isolated resonance of energy E 2 À E 1 results in emission of an inelastic scattered photon of energy E 2 À E 3 þ Á where E 2 À E 3 denotes the energy difference between the intermediate and final state and Á is the detuning from the resonance. The scattered light follows the linear energy dispersion of the resonance scattering process [11,33,34]. Now consider actual SASE pulses, which have a spiky spectrum within a broad envelope [35], as shown in Fig.…”

Stimulated resonant x-ray Raman scattering with incoherent radiation

“…X-ray Raman scattering with a narrowband source (less than the lifetime width of the core-excited state) shows a linear dispersion between the incoming ω in and outgoing radiation [11,12,36]. The energy of the outgoing radiation is…”