Rovibronic

Rovibronic refers to the combined rotational, vibrational, and electronic degrees of freedom of a molecule. In molecular spectroscopy, rovibronic states describe the quantum states that arise from the simultaneous coupling of a molecule’s electronic structure with its nuclear motion. Transitions between rovibronic levels produce spectra that span ultraviolet, visible, and infrared wavelengths, and are shaped by both vibrational overlaps and rotational selection rules.

The concept rests on the interplay of electronic and nuclear motion. While the Born-Oppenheimer approximation separates electronic and nuclear motion, real molecular systems exhibit non-adiabatic coupling that gives rise to rovibronic structure. A rovibronic wavefunction typically factors into electronic, vibrational, and rotational components, labeled by electronic term symbols (such as X, A, B), vibrational quantum number v, and rotational quantum number J (or related quantum numbers for polyatomic molecules). Energy levels depend on the electronic state and the rovibrational quantum numbers.

For transitions, electric dipole selection rules apply, leading to characteristic line patterns. Line intensities are governed by Franck-Condon factors (vibrational overlaps) and Hönl-London factors (rotational contributions). Diatomic systems often illustrate rovibronic structure with transitions such as the A^1Π–X^1Σ^+ or B^2Σ^+–X^2Σ^+ systems, where both vibrational and rotational progressions are observed.

Rovibronic analysis is essential for interpreting molecular spectra, determining molecular structure, and studying processes in atmospheric chemistry, astrophysics, combustion, and plasma physics.