Stimulated x-ray emission spectroscopy in transition metal complexes

Author(s): Kroll, T; Weninger, C; Alonso-Mori, R; Sokaras, D; Zhu, D; Mercadier, L; Majety, VP; Marinelli, A; Lutman, A; Guetg, MW; Decker, FJ; Boutet, S; Aquila, A; Koglin, J; Koralek, J; Deponte, DP; Kern, J; Fuller, FD; Pastor, E; Fransson, T; Zhang, Y; Yano, J; Yachandra, VK; Rohringer, N; Bergmann, U | Abstract: © 2018 American Physical Society. We report the observation and analysis of the gain curve of amplified Kα x-ray emission from solutions of Mn(II) and Mn(VII) complexes using an x-ray free electron laser to create the 1s core-hole population inversion. We find spectra at amplification levels extending over 4 orders of magnitude until saturation. We observe bandwidths below the Mn 1s core-hole lifetime broadening in the onset of the stimulated emission. In the exponential amplification regime the resolution corrected spectral width of ∼1.7 eV FWHM is constant over 3 orders of magnitude, pointing to the buildup of transform limited pulses of ∼1 fs duration. Driving the amplification into saturation leads to broadening and a shift of the line. Importantly, the chemical sensitivity of the stimulated x-ray emission to the Mn oxidation state is preserved at power densities of ∼1020 W/cm2 for the incoming x-ray pulses. Differences in signal sensitivity and spectral information compared to conventional (spontaneous) x-ray emission spectroscopy are discussed. Our findings build a baseline for nonlinear x-ray spectroscopy for a wide range of transition metal complexes in inorganic chemistry, catalysis, and materials science.