D. J. Walker

Bio: D. J. Walker is an academic researcher from Stanford University. The author has contributed to research in topics: Laser pumping & Extreme ultraviolet. The author has an hindex of 6, co-authored 8 publications receiving 130 citations.

Laser spectroscopy of Na i quartets

Abstract: By using a pulsed hollow-cathode discharge, metastable atoms are produced in the (2${p}^{5}$3s3p)${}^{4}$${D}_{7/2}$ and (2${p}^{5}$3s3p)${}^{4}$${S}_{3/2}$ levels of Na i. A tunable laser is used to excite these atoms to levels in the 2${p}^{5}$3s3d and 2${p}^{5}$3s4s configurations, and thereby to establish a partial Grotrian diagram for the quartet system. Several possible Na i xuv laser systems are described.

Quasi-metastable quartet levels in alkalilike atoms and ions.

Abstract: 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.

Low-energy pumping of a 108.9-nm xenon Auger laser.

Abstract: We report extensive measurements of gain in the Xe III system initially observed by Kapteyn et al. [Phys. Rev. Lett. 57, 2939 (1986)]. The dependence of this gain on pressure, pumping-pulse length, and pump energy is presented. By optimizing these parameters we have achieved a gain of exp(3.2) by using only 0.56 J of 1064-nm energy on target, representing an efficiency improvement of nearly 100. Total gains as high as exp(6.6) have been measured when using higher energies. Our data indicate that effective laser-produced plasmas can be created with applied power densities as low as 5 x 10(10) W cm(-2).

Observation of super Coster-Kronig-pumped gain in Zn III.

Abstract: We report the observation of laser gain in the vacuum ultraviolet pumped by super Coster-Kronig decay. Using a 5-J, 300-psec pump pulse of 1064-nm radiation, we have observed gain on transitions in Zn III at 127.0, 130.6, and 131.9 nm with total gains of exp(2.4), exp(5.1), and exp(3.2), respectively. The large branching ratios of the rapid super Coster-Kronig decay into a small number of final levels makes high-efficiency operation possible.

Measurement of lithium and sodium metastable quartet atoms in a hollow-cathode discharge.

Abstract: We report the measurement, in a pulsed hollow-cathode discharge, of metastable quartet atoms of Li and Na. By using a tunable probe laser, population densities of 3 × 1010 atoms cm−3 and 1011 atoms cm−3 were measured in the Li(1s2s2p)4P° and Na(2p53s3p)4D7/2 levels, respectively. These levels are candidates for energy storage for extreme-ultraviolet lasers.

Wavelengths, Transition Probabilities, and Energy Levels for the Spectra of Sodium (NaI–NaXI)

Abstract: Energy levels, with classifications and uncertainties, have been compiled for the spectra of the neutral atom and all positive ions of sodium (Z=11). Wavelengths with classifications, intensities, and transition probabilities are also tabulated. In addition, ground states and ionization energies are listed. Where available, the hyperfine structure constants and the percentages of the leading components of the energy levels are included. For all ionization stages of sodium, at least some experimental data are available; however, for those for which only a few transitions have been measured, theoretical calculations or values obtained by isoelectronic fitting are reported. Similarly, theoretical or isoelectronically determined ionization energies are given when they are thought to be more accurate than the available experimental data would produce.

Photoionization-pumped x-ray lasers using ultrashort-pulse excitation

Abstract: Recent advances in the production of ultrashort x-ray pulses by using femtosecond laser-produced plasmas coupled with the development of terawatt ultrashort-pulse lasers may make possible ultrashortpulse photoexcited x-ray lasers. I examine the creation of a population inversion on the K-alpha transition of neon at 1.5 nm by using the photoionization scheme first suggested by Duguay and Rentzepis in 1967. It is shown that this laser can be produced by using a pump laser of ~ 10 J in 50 fs, provided that a sufficiently bright laser-produced plasma x-ray source can be created. Recent experimental and theoretical results are discussed that verify the potential feasibility of this scheme.

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

Abstract: 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.

Optical studies of multiply excited states

Abstract: Optical studies of multiply-excited states are reviewed with emphasis on emission spectroscopy. From optical measurements, properties such as excitation energies, lifetimes and autoionization widths can be determined with high accuracy, which constitutes a challenge for modern computational methods. This article mainly covers work on two-, three- and four-electron systems, but also sodium-like quartet systems. Furthermore, some comments are given on bound multiply-excited states in negative ions. Fine structure effects on transition wavelengths and lifetimes (autoionization) are discussed. In particular, the most recent experimental and theoretical studies of multiply-excited states are covered. Some remaining problems, which require further attention, are discussed in more detail.

Basic Mechanisms Contributing to the Hollow Cathode Effect

Abstract: If a single plane cathode in a glow discharge is replaced by a cathode with some hollow structure such as a cylindrical or slit shaped hole, then, in a specific range of operating conditions the negative glow is found to be inside the hollow structure of the cathode. Under such conditions at a constant current the voltage is found to be lower and, at a constant voltage, the current is found to be orders of magnitude larger than for the plane cathode. This effect is called the hollow cathode effect (Pashen, 1916).