Absorption and Raman spectroscopy of mass-selected tantalum tetramers in argon matrices

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Abstract
We have examined both the absorption and resonance Raman spectra of mass‐selected Ta tetramers. The tetramers are produced in a sputtering source and mass filtered with a Wien filter, then neutralized and deposited in an Ar matrix at low temperatures. The absorptionspectrum indicates two broad transitions, one in the red at 768 nm and another to the blue at 532 nm. Raman resonances could be excited in both regions giving three distinct fundamental frequencies at 270.2(1) cm−1, 185.1(1) cm−1, and 130.6(2) cm−1. The lowest vibration shows a long progression (up to seven overtones) with alternating separations of 135 cm−1 and 126 cm−1, indicative of a weak Jahn–Teller effect in the tetramer ground state. This, along with the observation that the fundamental frequency ratios are close to 2:√2:1 indicate that the molecule has a tetrahedral ground state geometry with an electronic E state symmetry. The appearance of three fundamentals in the resonance Raman spectrum indicates that both excited states corresponding to the observed absorption bands arise from severely distorted T electronic states.

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