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

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 (MLCT)-M-3 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. (C) 2012 Elsevier B. V. All rights reserved.