Measurement of the hyperfine structure in low-l, high-n Rydberg states of ortho H-2 by millimeter wave spectroscopy

Millimeter wave spectroscopy has been used to record high-resolution spectr a of high-n (n=51-64), low-l (l=1-3) Rydberg states of ortho H-2 located be low the N+=1 rotational level of the X (2)Sigma (+)(g)(upsilon (+)=0) groun d vibronic state of H-2(+). The spectral resolution of better than 1 MHz en ables 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 H-2. The Stark effect is used to distinguish experimentally between p an d f Rydberg states. In the weakly penetrating nf series, the hyperfine inte raction dominates and the observed hyperfine components are of mixed single t (S=0) and triplet (S=1) character. In the penetrating np series, the domi nant interactions are between the electron orbital and spin angular momenta and the molecular rotation and the observed hyperfine components are chara cterized by a well-defined total electron spin. The nd Rydberg states show a behavior intermediate between these two limiting cases. The observed leve ls are of mixed singlet (S=0) and triplet (S=1) character but the main ener gy separation departs from the energy separation between the G(c)=1/2 and G (c)=3/2 levels of the H-2(+) ion. (C) 2000 American Institute of Physics. [ S0021-9606(00)01042-4].