Stark effect in triatomic hydrogen Rydberg states

The influence of an external electric field on the Rydberg states of triatomic hydrogen has been studied from the viewpoint of the structure of the spectra and the dynamics of the excited states. Rydberg states with principal quantum numbers ranging from 30 to 125 were observed, and low-field as well as very-strong-field regimes investigated. A perturbative treatment taking into account the coupling between the core rotation and the Rydberg-electron motion was developed. The good knowledge of the low-l Rydberg series in ${\mathrm{H}}_3$ (ns, np, nd, and nf) allowed us to achieve this treatment without adjustable parameters. The agreement between calculation and experimental results is excellent and reveals features resulting from predissociation of selected members of the Stark manifold. Accidental predissociations due to vibrationally excited interlopers are clearly distinguished from the systematic predissociation induced by the mixing with the ns Rydberg series. This is a complete perturbative treatment of the Stark effect in molecular Rydberg states.