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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