Role of the oxidation state in the ultrafast transient dynamics of molecular scale neutral vanadium oxide clusters

The ultrafast dynamics of neutral vanadium oxide clusters are measured using femtosecond pump-probe spectroscopy following ultraviolet excitation. Clusters with similar stoichiometry exhibit nearly identical excited state lifetimes, thereby demonstrating the prominent role of the metal center's oxidation state in relaxation behavior. More specifically, the measured excited state lifetimes decrease linearly with oxidation on the sub-picosecond timescale, extrapolating to the timescale of a single vibrational period of a V-O bond for oxygen rich clusters. The propensity for achieving long-lived states through relaxation also decreases linearly with oxidation. Our results suggest that fragmentation is evident under multiphoton ionization, even with femtosecond laser pulses, albeit to a lesser extent than with nanosecond laser pulses. No clusters requiring metal centers with oxidation states beyond +5 are recorded in the mass spectra. The simple picture of sequential oxidation of vanadium oxide clusters highlights the influence of oxidation on systematic differences in relaxation behavior, and by extension, allows for a deeper understanding of charge-carrier dynamics in bulk-scale vanadium oxide materials.