Proposal for a 207-A laser in lithium.

At constant oscillator strength, the radiative lifetime varies as the square of the transition wavelength and for a strong transition near 200 A is about 10-11 sec. To overcome the difficulty of obtaining sufficient gain in this short a time, it was suggested several years agol'2 that atoms first be stored in a metastable level and then, by use of an intense tunable laser, rapidly transferred to the upper level of the lasing transition. These proposals were based on single-electron-excited states and thereby included the difficult requirement that the population of the metastable level be inverted with respect to the ground state of the species. This Letter describes how, by permitting the participation of both innerand outer-shell electrons, the properties of the autoionizing levels of Li I may be utilized to construct a two-step laser that has the following properties: (1) the storage level is metastable against both autoionization and radiation, (2) the upper level of the lasing transition (the target level) is nmetastable against autoionization but is strongly radiatively allowed, and (3) the terminal level of the lasing transition is other than ground and is emptied both radiatively and by photoionization by the incident pump laser. An energy-level diagram of the proposed system is shown in Fig. 1. Figure 2 shows the changes in electron configuration and spin that are involved. Before proceeding, I note that other authors 3 -5 have noted how, in different ways, the properties of long-lived autoionizing states may bear on the soft x-ray laser problem. By use of electron excitation, energy is first stored in the 1As2sp 4P0 level. This level is the lowest of the quartet series and, as described in classic papers by Feldman and Novick,6 is coulombically stable (spin forbidden) against both autoionization and radiation. Its fine-structure components, J = '4, 3, %, are separated by about 1 cm-' and have experimentally measured lifetimes of 0.14, 0.46, and 5.8 gsec, respectively. 7 The level may be populated not only by direct electron excitation but also by radiative cascade from all higher levels in the quartet series8' 9 and by recombination. The cross section or direct excitation by 60-eV electrons is about 1019 cm2 .6 ,7 In the second step of excitation, we use an intense pump laser at Xp = 2949 A to transfer the (previously unmoved) 2s electron to a 2p orbit and also to flip its spin, thereby rapidly populating the radiatively allowed 1s2p2 2p level (Figs. 1 and 2). The oscillator strength of this intercombination line (ls2s2p 4P0 -1s2p2 2 P) is not known and is key to this proposal. As a guide, I take it equal to that of the comparable 23S-21P of Li+, which involves the same orbit and spin change, and assume that / = 2.83 X 10-7.10 It is particularly important that the target level