Use of laser-produced soft x rays for the production of excited metastable ions

A detailed investigation is presented of the use of soft x rays, emitted from a laser-produced plasma, for the excitation of highly energetic ions. The emphasis is on the excitation of ${\mathrm{Li}}^{+}\left(1s2s\right)$ metastable levels with energies of approximately 60 eV. The experiments were made possible by the use of a geometry in which the laser beam was focused through the Li vapor onto a massive target placed inside the vapor. Populations of ${\mathrm{Li}}^{+}\left(1s2s\right)$ ions in excess of ${10}^{15}$ ${\mathrm{cm}}^{-3\mathrm{}}$ have been measured and the effect of varying parameters such as laser energy, distance from the target, Li density, and time delay after the initial excitation has been examined. In this work the plasma emission can be considered as radiation from a blackbody with a temperature of 10-100 eV, and the conversion efficiency from 1.06-μm radiation to blackbody radiation was estimated to be in excess of 5%. A comparison has been made of the effect of using $Q$-switched ($\tau \sim 10$ ns, $E\sim 200$ mJ) and actively modelocked ($\tau \sim 0.6$ ns, $E\sim 50$ mJ) neodymium-doped yttrium aluminum garnet (Nd: YAG) lasers to make the soft-x-ray—emitting plasma. ${\mathrm{Li}}^{+}\left(1s2s\right)$ population measurements have been made for the cases of Ta, Fe, Ni, and Li target materials, and the populations excited by photoelectrons into the $\mathrm{Li}\left(1\mathrm{}{s}^{2}2p\right)$ and ${\mathrm{Li}}^{+}\left(1\mathrm{}{s}^2\right)$ levels have been measured. The applications of these experiments to various proposed extreme-ultraviolet laser systems are discussed.