Single-color photoionization optogalvanic spectroscopy in U--Xe and U--Ne hollow-cathode discharges

Resonance ionization spectroscopy is a powerful tool used to study and identify highly excited states of atoms and molecules, but usually requires complex experimental setups. Recently, we described a new method for this spectroscopy based on previous work on the rapid optogalvanic effect. The optogalvanic photoionization spectroscopy method is particularly attractive as a replacement for the atomic beam when refractory elements are considered. As an illustration of the method, we measured the rapid optogalvanic spectra in U--Ne and U--Xe hollow-cathode discharges over the 16500--20500 cm-1 range, where we have identified about 900 lines. Although showing a larger density of lines, the rapid optogalvanic spectrum of UI reproduces the main characteristics of the resonant ionization spectra obtained in field-free low-pressure atomic uranium beams. Over 600 of the observed lines are interpreted as two or three photon photoionization transitions, from which we propose nearly 400 new highly excited levels of UI. Electric field and collision ionization of Rydberg states is also considered but is shown to play only a minor role.PACS Nos.: 32.30, 31.50, 32.80, and 52.70.