In this paper we describe several techniques for using lasers to study core-excited energy levels in the spectral region between 10 eV and 100 eV. We are particularly interested in levels that are metastable against autoionization and, in some cases, against both autoionization and radiation.

Laser techniques for extreme-ultraviolet spectroscopy

We describe the construction and operation of a 109-nm, photoionization-pumped, single-pass laser in Xe iii. The laser is pumped by soft x rays emitted from a laser-produced plasma in a traveling-wave geometry. Using a 3.5-J, 300-psec, 1064-nm laser pump pulse, we measure a small-signal gain coefficient of 4.4 cm−1 and a total small-signal gain of exp(40). The laser is fully saturated and produces an output energy of 20 μJ in a beam with 10-mrad divergence.

http://web.stanford.edu/~seharris/PublicationsLinksPDFs/1980/124.pdf

Saturation of the Xe III 109-nm laser using traveling-wave laser-produced-plasma excitation

Large population inversions on transitions ranging in wavelength from 109 to 748 nm have been produced in six elements pumped by photoionization through inner-shell and/or multielectron (shakeup) processes with the broadband soft-x-ray flux from laser-produced plasmas. Gain coefficients ranging from 0.25 to 11.5 cm−1 have been measured in He, Ar, Zn, Cd, In, and Xe using the soft-x-ray flux produced by 25–300-mJ, 70-psec-, and 10-nsec-duration pulses from a 1.06-μm laser. Extension of this scheme to Na suggests that a 10-mW average-power laser can be produced at 37.2 nm at a 10-Hz repetition rate by photoionizing Na vapor with the soft-x-ray flux produced with a 1-J, 50-psec-duration pulse from a 1.06-μm laser.

Photoionization lasers pumped by broadband soft-x-ray flux from laser-produced plasmas

A soft-x-ray-pumped inner-shell photoionization laser has been produced in Cd vapor at 4416 and 3250 A. A gain of 5.6 cm−1 has been measured at 4416 A, and a reasonably high-energy storage of 0.2 mJ/cm3 in the upper laser states has been obtained.

High-gain inner-shell photoionization laser in Cd vapor pumped by soft-x-ray radiation from a laser-produced plasma source.

Emission spectra of quasimetastable levels of alkali-metal atoms.

Soft x rays from a plasma generated by a neodymium: yttrium aluminum garnet laser beam (1.06 μm) are used to photoionize neutral Li vapor to produce Li+ metastables. Maximum metastable densities of Li+ (1s2s) 1S=6×1014 ions/cm3 and Li+ (1s2s) 3S=3×1015 ions/cm3 are obtained. The effective lifetimes of these levels are measured to be approximately 5 ns. At 50 mJ of incident laser energy the inferred conversion efficiency from 1.06‐μm radiation to soft x rays is ∼14%.

Soft x‐ray pumping of metastable levels of Li+

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.

/pdf/quasi-metastable-quartet-levels-in-alkalilike-atoms-and-ions-4e0hhwpcuq.pdf

Quasi-metastable quartet levels in alkalilike atoms and 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 ${\mathrm{Li}}^{+}(1s2s)$ 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 ${\mathrm{Li}}^{+}(1s2s)$ ions in excess of ${10}^{15}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ 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 106-\ensuremath{\mu}m radiation to blackbody radiation was estimated to be in excess of 5% A comparison has been made of the effect of using $Q$-switched ($\ensuremath{\tau}\ensuremath{\sim}10$ ns, $E\ensuremath{\sim}200$ mJ) and actively modelocked ($\ensuremath{\tau}\ensuremath{\sim}06$ ns, $E\ensuremath{\sim}50$ mJ) neodymium-doped yttrium aluminum garnet (Nd: YAG) lasers to make the soft-x-ray---emitting plasma ${\mathrm{Li}}^{+}(1s2s)$ population measurements have been made for the cases of Ta, Fe, Ni, and Li target materials, and the populations excited by photoelectrons into the $\mathrm{Li}(1{s}^{2}2p)$ and ${\mathrm{Li}}^{+}(1{s}^{2})$ levels have been measured The applications of these experiments to various proposed extreme-ultraviolet laser systems are discussed

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

A system is proposed in which Cs atoms photoionized by soft x rays from a laser-produced plasma undergo selective Auger decay, causing inversion and lasing at 63.8 nm in Cs iii. Rate-equation calculations show that lasing should occur when a small (1-J) 532-nm pump laser is used. A similar system in Rb iii is briefly discussed.

/pdf/proposal-for-an-extreme-ultraviolet-auger-laser-at-63-8-nm-1749xvsa60.pdf

Proposal for an extreme-ultraviolet Auger laser at 63.8 nm in Cs III

A detailed theoretical and experimental investigation is presented of the excited-level population distribution in a Li-Ne vapor under excitation by a photoionization electron source. This source produces a subnanosecond burst of hot (${T}_{e}$\ensuremath{\sim}20 eV) electrons with a peak density in excess of 5\ifmmode\times\else\texttimes\fi{}${10}^{15}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. The electrons are generated as a result of photoionization of Ne atoms by x rays emitted from a plasma produced by a 100-mJ laser pulse of 600 ps duration. Measurements of Li(1${s}^{2}$2p)-atom density, and of electron density, have been performed with subnanosecond time resolution under various conditions of excitation. A comprehensive rate-equation model is described and its predictions are seen to be in good agreement with experiment. It is shown that the population distribution in the Li-Ne vapor is strongly influenced by recombination. Under the conditions of these experiments, rates for this process as high as 3\ifmmode\times\else\texttimes\fi{}${10}^{9}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ were observed.

R. G. Caro

Papers

Soft x‐ray pumping of metastable levels of Li+

Quasi-metastable quartet levels in alkalilike atoms and ions.

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

Proposal for an extreme-ultraviolet Auger laser at 63.8 nm in Cs III

Population distributions in Li vapor excited by a photoionization electron source