Probing the 3d spin momentum with X-ray emission spectroscopy: the case of molecular-spin transitions.

We report X-ray emission spectra of Fe(III), Fe(II), and Co(II) spin-crossover compounds in their high-spin and low-spin forms. It is shown that all X-ray emission features are sensitive to the spin state. Variations of the Kbeta and the Kalpha emission line shapes, which are in agreement with theory, can be used as quantitative probes of the spin state; it is suggested that with appropriate reference experiments one can extract the spin momentum for a general case. Resonant X-ray emission spectra unveil details of the redistribution of electrons on the 3d levels associated with the spin-state change by revealing features at the X-ray absorption preedge not accessible through standard absorption measurements.