The optical absorption of iridium hexafluoride

Abstract

The optical absorption of IrF 6 in the visible and near-infrared region consists of five different electronci transitions connecting the 4S G 3 2g ground state, X, with higher states of the (5 d f 2 g ) 3 configuration. This absorption has been measured for the vapor state and for helium-cooled polycrystalline samples. Vibrational structure making up each of the five band systems has the following general characteristics: (i) the Franck-Condon shift is small, as expected for transitions of a nonbonding electron; (ii) the magnetic dipole component, including the origin band, is weak, with intensity variations from one system to another that confirm the electronic assignments; and (iii) an electric-dipole component introduced by vibronic coupling with higher, odd-parity electronic states is responsible for most of the intensity observed. Short progressions (one to four intervals) are present not only for ν 1, the a 1 g mode, but also for ν 2 and ν 5, the e g and f 2 g modes, respectively. In the magneticdipole component of these spectra, the progressions in these nontotally symmetric modes are seen only in transitions to upper electronic states of symmetry G 3 2g and provide definite physical evidence of Jahn-Teller coupling in those states. (The ground state with its nearly pure spin-degeneracy has effectively no Jahn-Teller coupling.) The magnitude of the corresponding coupling parameters for these excited states varies from D 2 = 0.1 in the b ← X system to D 5 = 0.7 in the a ← X system, corresponding to weak coupling of the electronic and vibrational motion. For the electric-dipole components, similar nontotally symmetric progressions are observed in each of the five band systems, including c ← X and d ← X where the final states are Kramers doublet states, E 1 2g and E 5 2g , respectively. Because Kramers degeneracy is unaffected by the electrostatic Jahn-Teller forces, the ν 2 or ν 5 progressions must here be due wholly to Jahn-Teller anharmonicity in the odd-parity donor electronic states from which the observed transitions derive their electric-dipole intensity. This analysis suggests further that for the remaining transitions to excited G 3 2g states, in which a certain amount of Jahn-Teller coupling is known from the magnetic-dipole spectra to be present, the observed nontotally symmetric progressions in the electric-dipole spectra must be attributed in part to Jahn-Teller coupling in the odd-parity donor electronic states, and in part to such coupling in the original even-parity excited electronic states.