Extreme-ultraviolet emission spectra of core-excited levels in sodium and magnesium

As noted in an early paper by McGuire,' certain of the energy levels of Na I and Mg II that lie above a lower continu- um and that have reasonably large radiative yields are of potential interest for the construction of extreme-ultraviolet (XUV) lasers. Depending on the speed of the excitation source relative to the Auger transition rate of the level, these levels may be excited either directly2 or by store and trans- fer 3 from a metastable level. During the past several years we have developed a pulsed- hollow-cathode discharge4 that operates at a ground-level atom density of several Torr and that produces large densi- ties of core-excited metastable atoms. Using this discharge, Holmgren has measured populations of 1011 atoms/cm3 and 3 X 1010 atoms/cm 3 in the 2p 53s3p 4D7/2 and 2p 53s3p 4S3/2 levels of Na (Ref. 5) and in turn has used laser-induced fluorescence from these levels to develop a Grotrian dia- gram6 for the three lowest configurations of the Na quartet system. In this paper we give the results of a series of experiments in which a similar hollow-cathode discharge is used to take the XUV emission spectra of Na and Mg II. The results complement the Holmgren research and also help to define XUV laser systems in these elements. Section 2 describes the experimental apparatus; Section 3 gives the emission spectra; line identifications are discussed in Section 4; and the implications for laser systems are discussed in Section 5.