Measurement of the hyperfine structure in low-l, high-n Rydberg states of ortho H2 by millimeter wave spectroscopy

Abstract

Millimeter wave spectroscopy has been used to record high-resolution spectra of high-n (n=51–64), low-l (l=1–3) Rydberg states of ortho H2 located below the N+=1 rotational level of the X 2Σg+(v+=0) ground vibronic state of H2+. The spectral resolution of better than 1 MHz enables the observation of the hyperfine structure in these spectra. A simple procedure, based on the determination of combination differences, is used to reconstruct the energy level structure in np, nd, and nf Rydberg states of H2. The Stark effect is used to distinguish experimentally between p and f Rydberg states. In the weakly penetrating nf series, the hyperfine interaction dominates and the observed hyperfine components are of mixed singlet (S=0) and triplet (S=1) character. In the penetrating np series, the dominant interactions are between the electron orbital and spin angular momenta and the molecular rotation and the observed hyperfine components are characterized by a well-defined total electron spin. The nd Rydberg states show a behavior intermediate between these two limiting cases. The observed levels are of mixed singlet (S=0) and triplet (S=1) character but the main energy separation departs from the energy separation between the Gc=1/2 and Gc=3/2 levels of the H2+ ion.