Electric-field ionization of Rydberg states of ${\mathrm{H}}_3$

The stability of Rydberg states of triatomic hydrogen in an external electric field is investigated for principal quantum numbers ranging from n=50 to 110. When excited in the presence of an electric field, the states with an electric-field projection of the total angular momentum ${\mathit{M}}_{\mathit{N}}$=0 are found to be rapidly field ionized above the classical saddle-point energy. However, a significant proportion of the states for ${\mathit{M}}_{\mathit{N}}$=1 is observed to be stable above this threshold, with lifetimes exceeding ${10}^{\mathrm{-}6}$ s. A perturbative treatment of the molecular Stark effect indicates that different behavior of ${\mathit{M}}_{\mathit{N}}$=0 and 1 Stark states is a consequence of the stronger coupling among the states of the ${\mathit{M}}_{\mathit{N}}$=0 manifold. Following excitation under near-zero field conditions, a predominantly diabatic evolution of the Stark manifold of ${\mathrm{H}}_3$ is observed, for slew rates of a few times ${10}^7$ (V/cm)/s. This behavior results from the nondefinition of the projection of the total angular momentum of the molecule excited at near-zero field, where a broad manifold of l states is prepared within the bandwidth of the laser. For a comparison, similar experiments are carried out in atomic hydrogen and helium.