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

A combined spectroscopic and theoretical investigation is devoted to the vi brational fine structure in the broad band spectrum at 4.2 K of the T-1(1g) and T-1(2g) states of Rh(III) d(6) doped in NaCl. The vibrational structur e is resolved with a noise reduction technique using quadrature mirror filt ers. A weak signal at 16805 cm-(1) is identified as the zero phonon line of the T-1(1g) <-- (1)A(1g) transition, 194 cm-(1) below a false origin of a progression in a single 264 cm-(1) mode. The observed spectrum can be repro duced quantitatively by ab initio calculations involving identification of the Jahn-Teller active distortions, derivation of the shapes of excited sta te potential energy surfaces, and calculation of the Franck-Condon factors. The dominant mode of the progression is identified as a combination of the a(1g) and e(g)(a) modes with a calculated frequency of 279 cm-(1). The cal culations further reveal the existence of a conical intersection between th e T-1(1g) and T-1(2g) states, situated in the optical window between the tw o absorption bands and expected to play an important role in the deactivati on of the T-1(2g) state. (C) 1999 American Institute of Physics.