Micro-Focused MHz Pink Beam for Time-Resolved X-Ray Emission Spectroscopy

X-ray emission spectra (XES) in the valence-to-core (vtc) region offer direct information on occupied valence orbitals. They emerge as a powerful tool for the ligand identification, bond length, and structural characterization. However, the vtc feature is typically two orders of magnitude weaker than K$\alpha$ emission lines, making it hard to collect, especially for transient species. To overcome the difficulty, pink beam excitation capability was demonstrated recently at Sector 7 of the Advanced Photon Source. A water-cooled at mirror rejects higher harmonics, and beryllium compound refractive lenses (CRLs) focus the reflected fundamental beam (pink beam) to a 40$\mu m$ x 12$\mu m$ elliptical spot at sample target that matches the laser spot size used for photoexcitation. With an X-ray flux of $10^{15}$ photons per second, non-resonant XES spectra were taken on iron(II) hexacyanide and on photoexcited iron(II) tris(2, 2’-bipyridine). We could reproduce previous measurements with only a fraction of the acquisition time, demonstrating the ability to measure high quality spectra of low concentration species.