Vacuum-ultraviolet photoabsorption imaging system for laser plasma plume diagnostics

We describe a recently designed and constructed system based on a 1 m normal incidence vacuum monochromator with corrected (toroidal) optics that produces a wavelength tuneable and collimated vacuum-ultraviolet (VUV) (λ=30–100 nm) beam. The VUV continuum source is a laser-generated gold plasma. The primary function of the system is the measurement of time resolved “images” or spatial distributions of photoabsorption/photoionization in expanding laser plasma plumes. This is achieved by passing the beam through the sample of interest (in our case a second synchronised plasma) and recording the “footprint” of the attenuated beam on a charge coupled device. Using this VUV photoabsorption imaging or “shadowgraphy” technique we track and extract column density distributions in expanding plasma plumes. We can also measure the plume front velocity. We have characterized the system, particularly in relation to spectral and spatial resolution and the experimental results meet very well the expectations from ray tracing done at the design phase. We present first photoabsorption images and column density distributions of laser produced Ca plumes from the system

State-resolved valence shell photoionization of Be-like ions: experiment and theory

High-resolution photoionization experiments were carried out using beams of Be-like C$^{2+}$, N$^{3+}$, and O$^{4+}$ ions with roughly equal populations of the $^1$S ground-state and the $^3$P$^o$ manifold of metastable components. The energy scales of the experiments are calibrated with uncertainties of 1 to 10 meV depending on photon energy. Resolving powers beyond 20,000 were reached allowing for the separation of contributions from the individual metastable $^3$P$^o_0$, $^3$P$^o_1$, and $^3$P$^o_2$ states. The measured data compare favourably with semi-relativistic Breit-Pauli R-matrixComment: 23 figures and 3 table

Trends in autoionization of Rydberg states converging to the 4s threshold in the Kr-Rb⁺-Sr²⁺ isoelectonic sequence: theory and experiment

We have measured the photoabsorption spectra of the Kr-like ions Rb+ and Sr2+ at photon energies corresponding to the excitation of 4s-np resonances using, the dual laser plasma photoabsorption technique. Dramatic changes in the line profiles, with increasing ionization and also proceeding along the Rydberg series of each ion, are observed and explained by the trends in 4s-transition amplitudes computed within a framework of configuration-interaction Pauli-Fock calculations. Total photoionization cross sections show very good agreement with relative absorption data extracted from the measured spectra

Absorption Spectra from 1s Inner Shell Electron of Ionized and Neutral Carbon

By using the technique of two laser-produced plasmas we have obtained the absorption spectra and the relative photoionization continua of ionized and neutral carbon in the soft X-ray region. In this spectral range (25-45 Å), the absorption is due to the 1s inner shell electron. The CV absorption spectrum has been completely analyzed in a previous work and is reported here only as a reference spectrum for the CIV and CIII spectra. A preliminary analysis of the CIV is reported. Finally the absorption spectra and the relative photoionization continua of neutral carbon are presented in three different states, solid, vapour (clusters) and CI atoms

Photoionization Measurements of Light Ions in the Soft X-Ray Spectral Range

Absorption spectra and photionization cross section measurements of light ions have been obtained, in the soft X\u2010ray spectral region, by using the two laser produced plasmas technique, one acting as background continuum source, the other as absorbing medium. The basic principles of the experiment are discussed and new results for the absorption due to the 1s inner shell electron of CII ion are reported. Also previous results for the isonuclear sequences of C ions from the 1s electron are reviewed. Measurements of photonization cross sections are derived by the known oscillators strengths of suitably chosen optically thin lines

Parameters of a laser produced plasma from XUV line profiles

The region of interaction between a focused 1 GW ruby laser beam and a plane beryllium target has been observed end-on with a high resolution stigmatic grazing incidence spectrograph. The profile of the Lyα line of BeIV is made up of two components : a broad asymmetric one, spatially corresponding to the radiation emitted by the high density ablation region and a narrow one superimposed, emitted by the expanding plasma. A model that solves the radiative transfer equation for a moving plasma with adjustable parameters reproduces very well the observed features. It is then possible to determine a unique and realistic set of plasma parameters in good agreement with previous independent observations.La région d'interaction entre le faisceau focalisé d'un laser à rubis de 1 GW et une cible plane de béryllium a été observée avec un spectrographe stigmatique à incidence rasante de haute résolution. Le profil de la raie Lyα de BeIV montre deux composantes : l'une asymétrique et large correspond spatialement à la radiation émise par la région à haute densité du plasma qui coincide avec le cratère et l'autre étroite est émise par le plasma en expansion. Un modèle qui résout l'équation de transfert radiatif pour un plasma qui se détend avec des paramètres ajustables reproduit très bien les formes observées. Il est donc possible de déterminer un ensemble unique et réaliste de paramètres du plasma, en bon accord avec des observations indépendantes précédentes

XUV Absorption Spectra of Carbon Ions

The photoabsorption spectra of the carbon ions have been obtained by using two laser-produced plasmas. The photoionization cross-section of the CV has been absolutely measured and the value at threshold, σ = (4.7±0.5) x 10-19 cm2, as well as its behaviour at higher energies agrees quite well with the theoretical calculations

Absorption Spectroscopy in the Extreme Ultraviolet of Light Ions

Absorption spectra and photoionization cross-section measurements of multiply charged ions have been performed by using two laser-produced plasmas, one acting as background continuum radiation source the other as absorbing medium. The basic principles of the experiment are discussed and results for Bell, III, IV ions, in the grazing incidence spectral region, are reported. Measurements of photoionization cross-section are derived by extrapolating to the continuum the known oscillator strength of suitably chosen optically thin lines