Observation of Seeded Mn Kβ Stimulated X-Ray Emission Using Two-Color X-Ray Free-Electron Laser Pulses

K beta x-ray emission spectroscopy is a powerful probe for electronic structure analysis of 3d transition metal systems and their ultrafast dynamics. Selectively enhancing specific spectral regions would increase this sensitivity and provide fundamentally new insights. Recently we reported the observation and analysis of K alpha amplified spontaneous x-ray emission from Mn solutions using an x-ray free-electron laser to create the is core-hole population inversion [Kroll et al., Phys. Rev. Lett. 120, 133203 (2018)]. To apply this new approach to the chemically more sensitive but much weaker K beta x-ray emission lines requires a mechanism to outcompete the dominant amplification of the K alpha emission. Here we report the observation of seeded amplified K beta x-ray emission from a NaMnO4 solution using two colors of x-ray free-electron laser pulses, one to create the is core-hole population inversion and the other to seed the amplified K beta emission. Comparing the observed seeded amplified K beta emission signal with that from conventional K beta emission into the same solid angle, we obtain a signal enhancement of more than 10(5). Our findings are the first important step of enhancing and controlling the emission of selected final states of the K beta spectrum with applications in chemical and materials science.