Spectroscopy, resolution, and ab initio calculation of vibronic fine structure in the optical absorption of Rh(III) doped in NaCl

Abstract

A combined spectroscopic and theoretical investigation is devoted to the vibrational fine structure in the broad band spectrum at 4.2 K of the 1T1g and 1T2g states of Rh(III) d6 doped in NaCl. The vibrational structure is resolved with a noise reduction technique using quadrature mirror filters. A weak signal at 16805 cm−1 is identified as the zero phonon line of the 1T1g←1A1g transition, 194 cm−1 below a false origin of a progression in a single 264 cm−1 mode. The observed spectrum can be reproduced quantitatively by ab initio calculations involving identification of the Jahn-Teller active distortions, derivation of the shapes of excited state potential energy surfaces, and calculation of the Franck–Condon factors. The dominant mode of the progression is identified as a combination of the a1g and eg(a) modes with a calculated frequency of 279 cm−1. The calculations further reveal the existence of a conical intersection between the 1T1g and 1T2g states, situated in the optical window between the two absorption bands and expected to play an important role in the deactivation of the 1T2g state.