Electric-field ionization of Rydberg states of H3.

The stability of Rydberg states of triatomic hydrogen in an external electric field is investigated for principal quantum numbers ranging from [ital 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 [ital M][sub [ital N]]=0 are found to be rapidly field ionized above the classical saddle-point energy. However, a significant proportion of the states for [ital M][sub [ital N]]=1 is observed to be stable above this threshold, with lifetimes exceeding 10[sup [minus]6] s. A perturbative treatment of the molecular Stark effect indicates that different behavior of [ital M][sub [ital N]]=0 and 1 Stark states is a consequence of the stronger coupling among the states of the [ital M][sub [ital N]]=0 manifold. Following excitation under near-zero field conditions, a predominantly diabatic evolution of the Stark manifold of H[sub 3] is observed, for slew rates of a few times 10[sup 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 [ital l] states is prepared within the bandwidth of the laser. For a comparison, similar experiments are carried out inmore » atomic hydrogen and helium.« less