ABSORPTION AND RAMAN-SPECTROSCOPY OF MASS-SELECTED TANTALUM TETRAMERSIN ARGON MATRICES

We have examined both the absorption and resonance Raman spectra of ma ss-selected Ta tetramers. The tetramers are produced in a sputting sou rce and mass filtered with a Wien filter, then neutralized and deposit ed in an Ar matrix at low temperatures. The absorption spectrum indica tes 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 giv ing 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 progr ession: (up to seven overtones) with alternating separations of 135 cm (-1) and 126 cm(-1), indicative of a weak Jahn-Teller effect in the te tramer ground state. This, along with the observation that the fundame ntal frequency ratios are close to 2:root 2:1 indicate that the molecu le 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 obse rved absorption bands arise from severely distorted T electronic state s. (C) 1995 American Institute of Physics.