Stimulated x-ray emission and inelastic x-ray scattering at XFELs
(Deutsche Elektronen-Synchrotron DESY,Universität Hamburg)
E1.173 (Main building XHQ E1.173)
Main building XHQ E1.173
With the invention of x-ray free-electron lasers (XFELs), studies of light-induced chemical reaction dynamics by time-resolved x-ray spectroscopy reach a new era. Despite the high brilliance of XFELs, it remains challenging to capture high-resolution x-ray emission spectra under exposure of a single XFEL pulse, which however is advantageous for comprehensive pump-probe studies with these highly fluctuating sources. In the soft x-ray range, coherent amplification of spontaneous x-ray emission  and stimulated resonant inelastic x-ray scattering (SRIXS)  have been demonstrated in atomic gases, with amplification levels of up to eight orders of magnitude. A next crucial step for advancing time resolved spectroscopy is the transfer of stimulated emission and SRIXS to the hard x-ray domain that would allow single-shot spectroscopy in chemically and biologically relevant samples in the liquid phase. Recently, amplified spontaneous K-Alpha emission was demonstrated in solid copper . Here, we present results of amplified spontaneous K-Alpha emission in Manganese salts in aqueous solutions  with highly focused XFEL beams. Coherent amplification of the Mn K-Alpha emission by four orders of magnitude and saturation of the signal has been demonstrated in MnCl2 one-molar solution. More excitingly, the chemical shifts of MnCl2 and KMnO4 aqueous solution is maintained in the strongly spectrally sharpened stimulated K-Alpha emission spectra and coherent amplification has been shown at lower concentration. A more comprehensive technique for the study of chemical structure is SRIXS. Although demonstrated in Neon , the realisation of SRIXS in molecular targets is more difficult to achieve, even in the soft x-ray range [5-7], due to a smaller stimulated gain-cross section, that is distributed over many electronic, vibrational and rotational channels. We present two experimental studies, based on different two-colour XFEL schemes, that were specifically developed to achieve vibrationally resolved SRIXS in CO. In a first attempt , the XFEL was operated in a two-colour self-amplified spontaneous emission (SASE) scheme, with one frequency band tuned to the Oxygen PI* resonance, and the other band overlapping with the Stokes-shifted emission frequencies. According to our theory , this setting should result in high-resolution SRIXS spectra by covariance analysis. Experimentally, the spectra are, however, contaminated by strong absorption features of molecular ions generated in competing processes. In a second experiment , the SASE pump pulse was replaced with a self-seeded, narrow-band pulse at considerably lower pulse energy that resulted in a significant decrease of the background. A comparison with theory shows that the experimental conditions were at the onset of an observable SRIXS signal, but so far, no statistical evidence is seen, to confidently report the demonstration of SXRIS in a molecular target. We however developed an experimental protocol that allows for the detection of relatively small SRIXS signals with highly fluctuating XFEL spectra. The challenges and the necessary experimental parameters to ultimately reach the conditions for stimulated x-ray emission spectroscopy in chemically relevant targets will be critically assessed. References  N. Rohringer et al., Nature 2012, 481, 488.  C. Weninger et al., Phys. Rev. Lett. 2013, 111, 233902.  H. Yoneda et al., Nature 2015, 446, 524.  T. Kroll, C. Weninger, F. Fuller, R. Alonso-Mori, A. Marinelly, A. Lutman, D. Sokaras, S. Boutet, A. Aquila, J. Koralek, D. DePonte, L. Mercadier,J. Kern, J. Yano, V. Yachandra, N. Rohringer and U. Bergmann, manuscript in preparation 2016.  N. Rohringer et al. Proc. of 14th Int. Conf. on X-ray Lasers, Springer Proc. in Physics. 2016, 169, 201.  V. Kimberg and N. Rohringer, Struct. Dyn. 2016, 3, 034101.  V. Kimberg, A. Sanchez-Gonzalez, L. Mercadier, C. Weninger, A. Lutman, D. Ratner, R. Coffee, M. Buchner, M. Mucke, M. Agåker, C. Såthe, C. Bostedt, J. Nordgren, J.-E. Rubensson and N. Rohringer, Faraday Discussions on “Ultrafast Imaging of Photochemical Dynamics, submitted 2016.