Optogalvanic single-color multiphoton ionization spectroscopy of uranium in a hollow cathode discharge

Single color multiphoton ionization spectra of atoms can be obtained using the fast pulsed ({approximately} 10{sup {minus}9} s) optogalvanic signal generated by photoionization in the dark space of a hollow cathode discharge. Such spectra have been measured for uranium in the spectral range of rhodamine 6G with a high laser bandwidth excitation (0.3 cm{sup {minus}1}). In the process of ascribing an ionization scheme to each line through the known levels of uranium, many possibilities appear. In order to help the authors select the correct scheme, they perturbed the atomic level population distribution by optical pumping of the 16900.37 cm{sup {minus}1} level ({sup 7}M{sub 7}). The spectrum obtained this way show many new strong lines. They try to explain these lines by a three photon resonant or quasi-resonant scheme, using known levels of uranium, or by a two photon scheme from the 16900.37 cm{sup {minus}1} level. In both perturbed and unperturbed spectra, many ionization schemes are still possible for each line. For this reason the authors have performed higher resolution scans of selected lines (in particular 5915.4{angstrom}) in order to confirm the existence of more than one scheme for many lines.