Tabletop M-edge XANES reveals hidden states in molecular transition metal photocatalysts

X-ray absorption near edge spectroscopy (XANES) is a powerful technique for electronic structure determination. Recent developments in extreme ultraviolet (XUV) light sources using the laser-based technique of high-harmonic generation have enabled core-level spectroscopy to be performed on femtosecond to attosecond timescales. We have extended the scope of tabletop XUV spectroscopy and demonstrated that M_2,3-edge XANES, corresponding to 3p→3d transitions, can reliably measure the electronic structure of first-row molecular transition metal complexes with femtosecond time resolution. In the same 40-100 eV energy range, the 5p→5d and 4f→5d transitions give similar information about short-lived states in third-row metal complexes. We use this ability to track the excited-state relaxation pathways of photocatalysts and chromophores, uncovering hidden loss mechanisms and providing new design principles for transition metal photochemistry.