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Using a pulsed-hollow-cathode discharge, we have observed the emission spectra of core-excited levels of Na I and Mg II. Line identifications and implications for extreme-ultraviolet lasers are discussed.

A system is proposed whereby Zn atoms that are photoionized by soft x rays from a laser-produced plasma undergo selective super-Coster-Kronig decay leading to inversion and lasing on several XUV Zn III transitions. Calculations indicate that lasing will occur when a moderate-sized ('10-J) 1.06-am pump laser is used.It was recently demonstrated that soft x rays from laser plasmas produced by relatively small (<1-J) to the 3d 8 4s 2 configuration of Zn iii, which is thereby inverted with respect to the 3d 9 4s and 3d 9 4p configurations.The use of Auger processes to create gain in the XUV region was first suggested by McGuire,5 who proposed that (KLL) transitions in Na II would selectively populate levels in Na V, leading to gain at 410 nm. The required pump power for lasing for this system is large: 300 J in 1 nsec. Bokor et al. 6 have demonstrated a visible laser in Ba, which is pumped by a selective autoionization process. Recently, Krolik and Shapiro 7 proposed a scheme similar to McGuire's starting with 0 IV and using KLL Auger transitions to produce inversions in 0 VI with gain at 103.5 nm. They predict inversions of about 10%. The Zn system reported here has the following advantages: (1) The use of superCoster-Kronig transitions leads to large inversions in the lasing species, (2) the initial species is neutral Zi vapor rather than a multiply charged ion, and (3) the laser power required for superfluorescent laser action in the XUV is moderate-about 10 J in 1 nsec. In the remainder of this Letter we discuss in detail the mechanism and necessary conditions for creating the population inversion.Figures 1 and 2 show the levels and transitions relevant to the proposed scheme. The level positions are taken from the results of Dick. 8 The proposed experimental geometry is that used by Caro and Wang in Ref.1. An intense 1.06-,um laser is focused through Zn vapor maintained in a heat-pipe oven onto a solid Ta target. The resultant plasma radiates soft x rays with an approximately blackbody distribution and photoionizes the surrounding vapor.The key concept in this proposal is that 3p vacancies created by photoionization undergo rapid super-Coste'r-Kronig decay into the Zn III 3d84s 2 configuration, where they preferentially populate the 3d84s 2 1 G 4 level.9 - 12Nonsuper-Coster-Kronig transitions (i.e., decays to the 3d 9 4s and 3d' 0 configurations in Zn III) occur with only about 10% probability. Hence the super-Coster-Kronig process tends to leave the IG 4 level inverted with respect to levels in the 3d 9 4s and 3d 9 4pconfigurations of Zn III. Population of the lower Zn III configurations by other processes (i.e., electron ionization and excitation) will be insignificant if the electron density is made sufficiently small by increasing the distance to the target of the lasing volume or if the pulse is made sufficiently short.In Fig. 2

We describe the properties of a subclass of quartet levels of alkalilike atoms and ions that often retain metastability against autoionization and may have large radiative yields. Gain cross sections for XUV lasers with wavelengths between 20 and 100 nm are given.It is well known that, in many cases, the quartet level of highest J of a given configuration of an alkalilike atom or ion is metastable against both autoionization and against radiation in the extreme ultraviolet (XUV).This occurs sincu, irrespective of the extent to which LS coupling holds, there is no doublet level in the configuration to which the quartet level may couple. 1 In this Letter we note the existence of a subclass of quartet levels that, even in heavier elements, often retain relative metastability against autoionization and are radiatively allowed. In elements such as Cs and Ba these levels are candidates for the upper level of XUV lasers. In lighter alkalilike atoms and ions they provide storage levels for store and transfer lasers, which require orders-of-magnitude less transfer laser power than an earlier proposed Li system. 2 The distinguishing property of, these quasi-metastable levels is that the selection rules on the spin-orbit matrix elements allow nonzero matrix elements only to doublet basis levels, which are themselves prohibited from autoionizing. In second order, through the diagonalization, these levels do develop components of autoionizing doublet levels and therefore do autoionize, but often sufficiently slowly that the branching ratio for XUV radiation remains large.To illustrate this idea, consider the 2 p 5 3 s3p configuration of Na. First, the pure doublet levels that may The pertinent selection rule on the spin-orbit element is AL -0, + 1. Therefore, of the possible quartet levels, those that couple only to the nonautoionizing 2PI/2,3/ 2 levels are 4S3/2 and 4DI/ 2 . We term these levels quasi-metastable. Table 1 lists the quartet levels of the 2p 5 3s3p configuration and their autoionizing rates as predicted by the atomic-physics code RCN/RCG, 3 with only this configuration included. Single and double stars in the left-hand column denote predicted quasi-metastability and metastability, respectively. The nonzero autoionizing rates of the quasi-metastable levels result from small components of autoionizing doublet levels, which are acquired in second order through the diagonalization. The code expansion of the quasi-metastable 2p 5 3s3p 4S3/2 level of Na isThe ( 2 P 31 2 at 41.5 nm. The quasi-metastable 4D 1 12 level lies higher in the configuration and couples more closely to nearby doublet levels. In second order it acquires a (-0.014)2 component of ( 1 P) 2 8 11 2 , which in turn leads to its autoionizing rate of 2 X 109 sec-'. Table 2 tabulates the quasi-metastable quartet levels of all sp,d configurations of the alkalilike atoms and ions. The requirement for quasi-metastability is summarized by I J-L I = 3/2; parity and angular momentum must be both even or both odd. Table 3 tabulates the calculated single confi...

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