Knowledge Bank

One-photon and two-photon excitation from the metastable $N=0B¯2pA2-$ state of $H3$ are used to investigate the np, nd, and nf Rydberg series converging to the $N+=1$ and $N+=3$ states of $H3+$. The excitation event is detected by monitoring $H3+$ ions formed by field-ionization, forced autoionization, and rotational autoionization. The nd and nf Rydberg states are well described by Hund'case (d) since the quantum defects are quite small. On the other hand, the two $N=2$ members of the np Rydberg series show strong mixing [or intermediate coupling between Hund's cases (b) and (d) that is well described by MQDT using quantum defects $\mu 0=0.009$ and $\mu 1\simeq 0.4$. The np and nd Rydberg series both show ``giant'' windows (or unexpectedly reduced intensity extending over several n values) near $n=44$, and narrower windows at $n=61$. The intensities of the lines in these windows are very sensitive to external electric fields $(\sim 0.1V/cm)$ in the excitation region. Detailed studies now underway should allow the identification of the perturbing level and interaction mechanism responsible for these intensity variations.