Laser Induced Breakdown Spectroscopy (LIBS) is an analytical technique, used to classify and potentially quantify elements in complex hosts (or matrices). Vacuum Ultraviolet Laser Induced breakdown Spectroscopy (VUV LIBS) can offer potential improvements in detection of light elements in bulk metals over traditional LIBS in the visible region. This is due to presence of an abundance of resonance transitions at shorter wavelengths. This extends the ability to discriminate between the emission from different elements, particularly light elements such as carbon, sulfur, lithium, beryllium etc. Additionally, the precision of LIBS is limited by the continuum emission at the early stage of the plasma lifetime. The performance of LIBS can be improved by using a time resolved detection system [5], reducing the contribution from the continuum. In this study, the detection of the carbon content in steel samples is performed by time- integrated and time-resolved VUV LIBS. The experimental setup consists of a dual pulse system with Nd:YAG laser (1064 nm, up to 450 mJ, pulse duration 6 ns) used to irradiate the samples, a vacuum system to prevent absorption of the VUV radiations and a VUV spectrometer to collect and measure the emission spectra. Samples of four different concentrations of carbon in steel are used for the study. The resultant time integrated LIBS limit of detection and signal to background ratio is compared with time resolved VUV measurements.
The photoabsorption spectra of Te I-Te IV have been recorded and analysed in the XUV spectral region using the dual laser produced plasma technique. Photoexcitation from the 4d subshell is the dominant process in the 35-150 eV energy region. For photon energies between 35-45 eV discrete structure corresponding to 4d-np (n>4) transitions were obtained. Above the 4d ionisation threshold the spectra of Te I-Te III were found to be dominated by a 4d-ef shape resonance, which peaks at ~88 eV in each case. A transfer of oscillator strength from the resonance to discrete 4d-nf (n>3) transitions with increasing ionisation is clearly evident, and the 4d-4f transitions are the strongest features in the Te IV spectrum. Hartee-Fock with configuration interaction and time dependent local density approximation calculations successfully account for this behaviour and permit identification of the discrete features. The use of a prepulse to maximise the brightness of a tungsten continuum emitting plasma was also investigated.
The search for a source of EUV radiation for photolithography in the 13.5 nm region has been narrowed down to laser produced or pulsed discharge plasmas containing either xenon or tin. Higher conversion efficiencies can however be obtained with tin based plasmas within this wavelength regime. It is known that EUV photoabsorption by the lower ion stages of xenon reduces the photon flux from a xenon source. This is due to shape resonances from 4d-epsilonf transitions within Xe I-IV. The corresponding resonances for Sn I-IV have been obtained by means of the dual laser plasma (DLP) technique. It is also found that above the 4d ionisation threshold the spectra of Sn I-IV are dominated by a 4d-epsilonf shape resonance which peaks at close to 65 eV in each case. A transfer of oscillator strength from the shape resonance to pseudo-discrete 4d→nf transitions with increasing ionisation is clearly evident. Hartree-Fock with configuration interaction and relativistic time dependent local density approximation calculations successfully account for this behaviour and also permit identification of the discrete features.
X-ray point projection absorption spectroscopy (X-PPAS) is a long established and extremely useful dynamic diagnostic of hydrodynamics, ionization balance, etc. in laser produced plasmas. It has however, to date, been exclusively the preserve of large-scale laser facilities and also used to probe plasmas formed by relatively short pulse (< 1 ns) lasers. We report here the design and selective instrumental performance measures of a new table-top scale X-ray spectrometer system which forms the core of the X-PPAS system. Our design goal is to make X-PPAS a routine diagnostic and also apply it to the measurement of deep inner-shell photoabsorption by plasma atoms and ions.
Time-integrated, spatially resolved emission spectroscopy, in the deep vacuum ultraviolet (VUV) region (40-160 nm), of laser-produced plasmas has been employed for the quantitative characterization of the carbon content in solid steel target materials; the samples under study contained carbon concentrations in the 0.001-1.32% range. Six prominent VUV carbon spectral lines, representing three different ionization stages, were selected and proved to be spectral-interference free. Several experimental parameters and conditions such as the focusing lens type, laser power density, background atmospheres and pressure were optimized, leading to an unprecedented lower limit of detection, for carbon in solid steel alloys, of 1.2 ppm (parts per million) obtained with the 97.70 nm CIII spectral line. Furthermore, the spectroscopic evaluation of the steel plasma physical parameters is briefly presented.
We report here some observations and preliminary findings from a study focussed on the vacuum-UV (λ, 40-60 nm) radiation emitted during the interaction of 150 ps laser pulses (100-400 mJ) with copper pre-plasmas formed by an electro-optically synchronised (0.1 - 0.8 J, 8 ns) long pulse laser. We have observed significant gains in VUV flux that scale with inter-laser delay. We also report preliminary observations on total X-ray emission from the interaction of a superintense 80 fs, 200 mJ laser pulse at the UK ASTRA laser facility with a similar pre-plasma at irradiances approaching 1019 W/cm2
We describe a new system based on a 1m normal incidence vacuum monochromator with corrected optics that produces a wavelength tunable and collimated vacuum-UV beam. The VUV source is a laser-generated plasma high-Z metal. The primary functin of the system is the measuremtn of time resolved 'images' or spatial distributions of photo-absorption/photoionization in expanding laser plasma plumes. This is achieved by passing the beam through the sample of interest and recording the 'footprint' of the attenuated beam on a CCD. Using the 'photoabsorption imaging' technique we track and extrat column density distriubtions in the expanding plasma plumes. We can also mesaure the plume front velocity. We have characterized the system, particulary in relation to spectral and spatial resolution and the experimental results meet very well expectations from the ray tracing done at the design phase. We present first photoabsorption images of laser produced Ca polumes from the system.
Eugene Kennedy, John Costello, Jean-Paul Mosnier, Attilio Cafolla, Martin Collins, Laurence Kiernan, Ulrich Koeble, Muhammad Sayyad, Matthew Shaw, Bernd Sonntag, Robert Barchewitz
A new multilaser multichannel spectrometer system has been developed that allows a wide range of investigations of the interaction of extreme-ultraviolet (EUV) photons with matter in the form of free atoms or ions, e.g., in gases or plasmas or bound as in solids. The EUV photons are generated by a laser-produced plasma. Applications to the study of photoabsorption by thin foils, gases, and ground- and excited-state atoms and ions are described. The design and performance of a collimated, quasi-monochromatic, intense source of EUV radiation based on the combination of a laser-produced plasma with a EUV multilayer mirror is also reported.
The measurement of photoionization cross-sections of free ions or refractory atoms is difficult as a suitable absorbing column must be produced and backlighted by a bright synchronized XUV-continuum emitting source. In the dual laser plasma (DLP) technique both the absorbing and backlighting plasmas are produced by the interaction of high-power laser beams with suitable solid targets. The authors describe some recent results obtained with the DLP technique. With a single-laser system and photographic detection, inner-shell excitations from both ground and excited states in Al and Si species up to the fourth stage of ionization have been recorded. New and unexpected results were obtained for La3+. By using a photoelectric-based multichannel detector and a synchronized multilaser system, the versatility and reliability of the DLP technique is greatly improved. Time-resolved spectra have been obtained for atomic Cr and Mn and their ions at photon energies between 40 and 70 eV where the dominant absorption mechanism is excitation of the 3p subshell. An intercomparison is made between results obtained with the DLP method and corresponding results recorded with alternative techniques. Preliminary results for atomic tungsten and platinum are shown.
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