Direct IR Spectroscopic Detection of a Low-Lying Electronic State in a Metal Carbide Cluster

The electronic structure of metal clusters is notoriously difficult to detect spectroscopically, due to rapid relaxation into the ground state following excitation. We have used IR multiple photon excitation to identify a low-lying electronic state in a tantalum carbide cluster. The electronic excitation is found at 458 cm^-1 , and is confirmed by experiments on isotopically labeled clusters. Time-dependent density functional theory (TD-DFT) calculations confirm the current assignment, but a second predicted electronic state was not observed.