Ultrafast X-ray spectroscopy as a probe of nonequilibrium dynamics in ruthenium complexes

Abstract

The ultrafast intersystem crossing in ruthenium complexes between the singlet and triplet metal-to-ligand charge-transfer states following photoexcitation is described. The absence of a clear decay mechanism between these states makes it difficult to explain this process within a conventional framework using rate equations based on Fermi’s golden rule. We show that the decay can be mediated by metal-centered (MC) triplet states leading to decay times of the order of several tens of femtoseconds. The calculated stable excited state probability is dominated by the 3MLCT configuration. The detailed nature of this process is clearly reflected in the calculated spectral lineshapes of the time-dependent nonequilibrium X-ray absorption spectroscopy that show a transient crystal-field collapse, dynamic broadenings, and changes in the branching ratio. We demonstrate that ultrafast X-ray spectroscopy is a suitable probe to deliver detailed new insights or discriminate between competing physical scenarios.