Systematics of inner-shell photo-ionization X-ray lasers with constant pump rates

Population-rate equations are analytically solved for constant pump rates. Conditions for population inversions are developed for this simplified case. Numbers appropriate for inner-shell photo-ionization of magnesium and neon are used. These allow back-of-the envelope calculations for predicting lasing duration. Pump-rate thresholds are also given which permit lasing for the duration of the pumping. Blackbody-source temperatures associated with such pump rates are calculated. A method of using the solution of the small-signal population-rate equations to determine the saturated intensity when the lower lasing level has a decay channel is given. This method is applied to the case of constant pump rates. An analytic expression for the saturated intensity is developed for this case.     

Reduced absorption of neon-like bromine X-ray laser radiation in helium

We have measured the absorption of the 19.47-nm neon like bromine (J=2–1) X-ray laser line in low-pressure helium. The experiment was motivated by the coincidence of this line with the low-absorption wing of an autoionizing transition in helium. We observe that, with 1 mbar of helium, the continuum background and another bromine X-ray laser line at 19.82 nm are strongly reduced, enhancing the relative strength of the 19.47-nm laser line. Increasing the helium pressure to 1.5 mbar makes the continuum virtually disappear, resulting in an almost monochromatic emission of the X-ray laser line. An estimate of the absorption cross section for the 19.47-nm line is given as ≈3.9×10^-19 cm² and for the nearby continuum as 0.9–1.3×10^-18 cm². Received: 8 March 1999 / Revised version: 26 April 1999 / Published online: 11 August 1999     

A cadmium photoionization laser pumped by laser induced plasma radiation from a multi foci device

Soft x-rays from a laser-produced plasma were used to perform innershell photoionization of Cd atoms and to generate laser radiation at 442 nm. To achieve longer interaction zones between the Cd vapor and the soft x-ray flux, up to three plasma spots have been applied. In this way a maximum laser energy of 300 J with a 600 mJ Nd:YAG laser for the plasma production was achieved. Experimental investigations and corresponding rate-equation calculations indicate, that photoelectrons play an important role in the total laser kinetics.     

Conditions for XUV amplification considering recombination in clusters

3 cm^-l for a wavelength of λ_R≈200 Å in a time ≲1 ps can be expected. The measurable gain G_real depends on the cluster density N_cl. For N_cl≈10¹⁶ cm^-3 we expect G_real≳20 cm^-1. Received: 30 October 1997     

Effects of laser pulsewidth on higher-order harmonic generation in the barrier-suppression-ionization intensity regime

We studied the highest harmonic photon energies of high-order harmonic generation (HHG) for He, Ne, Ar, Kr, Xe, and N₂. This research employed 65-fs and 150-fs Ti:sapphire laser pulses, of which the peak intensities in a vacuum are higher than the barrier-suppression-ionization (BSI) intensities. We fitted two analytical formulae to the experimental results of HHG. One formula, which was obtained by fitting an analytical formula to the results by a quantum mechanical simulation of HHG, expresses the highest interaction intensity by a function of the BSI intensity and the Keldysh γ-parameter. The other formula is derived by introducing the saturation intensity of HHG, which was proposed by Chang et al. [Phys. Rev. Lett. 79, 2967 (1997)]. We discuss the highest interaction intensity for the condition when the peak laser intensities are higher than the BSI intensities. Received: 19 March 1999 / Revised version: 13 August 1999 / Published online: 27 October 1999     

Coherence properties of the radiation from X-ray free electron laser

We present a comprehensive analysis of coherence properties of the radiation from X-ray free electron laser (XFEL). We consider practically important case when XFEL is optimized for maximum gain. Such an optimization allows to reduce significantly parameter space. Application of similarity techniques to the results of numerical simulations allows to present all output characteristics of the optimized XFEL as functions of the only parameter, ratio of the emittance to the radiation wavelength, . Our studies show that optimum performance of the XFEL in terms of transverse coherence is achieved at the value of the parameter of about unity. At smaller values of the degree of transverse coherence is reduced due to strong influence of poor longitudinal coherence on a transverse one. At large values of the emittance the degree of transverse coherence degrades due to poor mode selection. Comparative analysis of existing XFEL projects, European XFEL, LCLS, and SCSS is presented as well.     

Gain-saturated Ni-like antimony laser at 11.4 nm in grazing-incidence pumping geometry

We report on gain-saturated operation of the 4d → 4p, J = 0-1, 11.4 nm soft-X-ray laser line in Ni-like antimony (Sb) at a pump energy of only 2.5 J. The driving laser used was a 1054 nm Nd:glass CPA laser system with a pulse duration of 7 ps (FWHM). The pump beam was focused with a tilted on-axis parabolic mirror in a grazing-incidence (GRIP) pumping configuration at an incidence angle of 45°. A fraction of 2.8% of the pump energy (∼70 mJ) was used for the prepulse, which was propagated along the same beam line as the main pulse and arrived at the target 4.4 ns before the main pulse.     

Absorption and ionization of molecular nitrogen by UV femtosecond laser pulses

Mechanisms of non-linear absorption and ionization of molecular nitrogen gas by UV femtosecond laser pulses were studied using photogalvanic and photoacoustic techniques. The effect of the intermediate Rydberg resonance, its dynamic Stark perturbation and ponderomotive upshift of the first ionization potential of nitrogen molecules by the intense laser pulses has been revealed by observing an increase of a power slope of ion yield from three to four at increasing laser intensity.     

GLF - A simulation code for X-ray lasers

This paper describes the multi-dimensional Non-Local Thermodynamic Equilibrium (NLTE) simulation code GLF. GLF simulates those physical processes needed for modeling X-ray lasers: atomic kinetics, radiation transport, hydrodynamics and basic laser-plasma interactions. GLF is constructed to be modular, portable and efficient. This paper concentrates on the physical formulations and numerical methods used in GLF.     

X-ray lasers as sources for resonance-fluorescence experiments

We point out that existing Ne-like X-ray lasers have sufficient brightness to be used as pump sources in resonant pump-fluorescence experiments, and we discuss some of the potential benefits and limitations of such experiments. In preliminary experiments, we have measured the line overlap of the Ne-like yttrium X-ray laser near 155 with 4f–3d transitions in H-like Na and He-like Mg. These experiments used a high-resolution grating spectrometer, and calibrated the wavelength of the yttrium laser by comparison against lines of known wavelength. The results indicate that the wavelength of the yttrium laser is 154.985 ± 0.025 , a factor of 2–4 improvement in precision over previous calibrations. We find that the yttrium laser is 100 m to the long-wavelength side of both the He-like Mg line and the H-like Na line, so that neither resonance is perfect; however, Stark broadening could improve the overlap in the latter scheme, and bulk Doppler shifting could improve both resonances. We also find a good resonance between the yttrium laser and an un-identified transition which we tentatively attribute to copper.     

Above-barrier ionization and quantum interference in strong laser fields

Classical predictions of field ionization of hydrogen and of Coulomb scattering in strong laser fields are compared with corresponding exact numerical solutions of the time-dependent Schr?dinger equation. In the quasi-static approximation, ionization rates are significantly lower than the quasi-classical tunneling rates in the regime of above-barrier ionization. This reduction is consistent with a simple classical barrier-suppression model. In time-periodic fields, the major differences between the classical and quantum-mechanical treatments arise from quantum-mechanical interference oscillations. These are typical for tunneling barriers in an applied field and for scattering by an oscillating potential. Received: 3 November 1999 / Published online: 30 June 2000     

Relativistic and correlation effects in the anisotropy of near-threshold Kr 3d and Xe 4d photoionization

Angular distributions of the spin–orbit split components and their branching ratios have been studied experimentally and theoretically for the 3d photoelectrons of Kr and 4d photoelectrons of Xe. The focus was on the electron dynamics near the ionization threshold of each spin–orbit split component and its behaviour as a function of Z   in passing from Kr to Xe. The experimental spectra were measured with high photon and electron energy resolutions with photon energies at about 3–12 eV above the 3d_3/2$3{\text{d}}_{3/2}$ and 3d_5/2$3{\text{d}}_{5/2}$ thresholds for Kr and at about 5–12 eV above the Xe 4d_3/2$4{\text{d}}_{3/2}$ and 4d_5/2$4{\text{d}}_{5/2}$ thresholds. Experimental results for the angular distribution parameters have been compared with theoretical values obtained with relativistic Dirac–Fock method and results from independent particle approximation with a modified Hartree potential [A. Derevianko, W. Johnson, K. Cheng, Atom. Data Nucl. Data Tables 73 (1999) 153]. The branching ratios were compared with theoretical predictions from Dirac–Fock and relativistic random-phase approximation [K. Cheng, W. Johnson, Phys. Rev. A 28 (1983) 2820].     

Polarization effects in resonance-enhanced frequency tripling and multiphoton ionization in xenon under excitation by crossed laser beams

Generation of resonance-enhanced third harmonic and multiphoton ionization in xenon have been studied for different polarizations of crossed laser beams. Polarization control over the harmonic generation and resonant ionization processes in crossed beams is analyzed and discussed.     

Multiphoton-induced X-ray emission and amplification from clusters

The development of a unified picture of short-pulse high-intensity multiphoton processes, embracing atoms, molecules, and solids, appears possible through the study of clusters. Of particular significance are possible intra-cluster processes that can influence the mechanism of ionization and lead to the production of inner-shell vacancies. Inner-shell excitation leading to prompt X-ray emission is specifically considered and the treatment leads to the definition of a critical cluster size n _c representing the achievement of maximal X-ray emission from the ensemble. These results suggest the possibility of designing a new class of molecular materials optimized for the efficient production and amplification of X-rays.     

Verification of X-ray line coincidences by high-resolution spectroscopy

Photo-resonant pumping of X-ray lasers requires close coincidence of a strong emission line with a suitable absorbing transition. We use a high-resolution crystal spectrometer to verify five coincidences, one of which has apparently not yet appeared in published work. All but one of the coincidences are found to lie within one linewidth. With respect to X-ray laser photo-pumping three of the five coincidences seem to be promising candidates.     

Ionization of fast rydberg atoms in longitudinal and transverse electric fields

The main properties of longitudinal and transverse electric field ionizers for fast Rydberg atoms n=21–40 have been investigated. The dispersion and the background due to collisional processes between fast atoms and residual gas molecules have been measured and calculated. The kinetic energy spread of ions formed by field ionization of Rydberg atoms and their trajectories have been calculated. The potassium beam energy was 3.9 keV.     

Rescattering effect in above-threshold ionization processes

The Keldysh-Faisal-Reiss (KFR) theory for Above-Threshold Ionization (ATI) phenomenon is generalized to include the effect that ionized electrons return to the vicinity of the ion core and rescatter with it. The theoretical calculation of such rescattering effects for the ground state of hydrogen under linearly polarized laser light yields good agreement with recent findings in ATI experiments.     

Lasers excited by photodissociation: The thalliumiodide laser

This review is concerned with lasers excited by photodissociation of molecules. The principle of photolytic excitation is described and a number of existing lasers is presented. As an example the properties of the thallium laser such as spectral and temporal emission, achievable gain and efficiency are discussed in detail.