<title>Open-shell octahedral molecules: a first insight into the full rovibronic problem</title>

Abstract

Despite its great successes for the standard rovibrational problem, there is still a large domain in which high-resolution molecular spectroscopy lacks both extensive experimental data and a complete and convincing theory: the full rovibronic problem. This is the case of a molecule in a degenerate electronic state, <i>i.e</i>. when the Born-Oppenheimer approximation breaks down. It means that the electronic, vibrational and rotational degrees of freedom must be treated simultaneously. Complex (ro)vibronic couplings (Jahn-Teller “effect”, <i>etc</i>) are then involved. Some octahedral transition metal hexafluorides and hexacarbonyls are known since many years to be stable open-shell systems with degenerate electronic ground states leading to very unusual absorption spectra. However, the extreme complexity of this kind of problem (from both experimental and theoretical sides) prevented to go any further in this direction until very recently. In this paper, we will give an overview of the tensorial rovibronic model that we have developed in the Dijon group for such systems. This model enables the systematic development of the rovibronic Hamiltonian and transition moments (dipole moment and polarizability) in a given degenerate electronic state. We will then review the current status of rovibronic spectroscopy of octahedral molecules. This includes the recent data obtained using supersonic expansion jet infrared spectroscopy V(CO)₆ and ReF₆, for which we will present detailed analyses.