Optogalvanic spectroscopic measurement of singlet helium Rydberg series

Atomic helium transitions from the metastable helium 2s1S to high Rydberg levels np1P, up to n = 46 have been measured in a low pressure low current d.c. discharge using optogalvanic spectroscopy. A frequency-doubled pulsed dye laser, pulse length 7 nsec, was used and the actual current perturbation or the voltage change of the discharge was measured accurately. The linewidth of the probing laser was 0.3 cm-1 permitting the linewidth of the singlet Rydberg series to be measured. The measurements were made just outside the cathode fall, in the negative glow and positive column region. The Stark broadening was found to be minimum in the negative glow region and maximum in the positive column. The linewidth variation of more than a factor of two was observed for the 291P transition. The series advance was observed to be proportional to the Stark broadening. These transitions permit the estimation of relatively low electric fields