Vacuum Ultraviolet Spectral Region Research Articles

Enhancement of Extreme Ultraviolet Emission Intensity from a Laser‐Produced Plasma Using an Alloy Target

SUMMARYWe observed that the emission intensity in the extreme and vacuum ultraviolet spectral regions using a laser‐produced copper–tungsten alloy plasma was 1.3 times higher than when using a pure tungsten plasma. The enhancement of the emission intensity was explained by the difference of the Rosseland mean opacity of each plasma.

Investigation of the characteristics of EUV backward transition radiation generated by 5.7 MeV electrons in mono- and multilayer targets

This paper presents experimental results of a study of radiation in the vacuum ultraviolet and extreme ultraviolet spectral region, generated by electrons with an energy of 5.7 MeV in a multilayer X-ray mirror. The angular distribution of the radiation generated in the rear hemisphere from the input surface of the target was measured. The contribution to the radiation yield caused by the periodic structure of the multilayer target was experimentally investigated.

Inter-pulse delay optimization in dual-pulse laser induced breakdown vacuum ultraviolet spectroscopy of a steel sample in ambient gases at low pressure

Time-integrated spatially-resolved Laser Induced Breakdown Spectroscopy (LIBS) has been used to investigate spectral emissions from laser-induced plasmas generated on steel targets. Instead of detecting spectral lines in the visible/near ultraviolet (UV), as investigated in conventional LIBS, this work explored the use of spectral lines emitted by ions in the shorter wavelength vacuum ultraviolet (VUV) spectral region. Single-pulse (SP) and dual-pulse LIBS (DP-LIBS) experiments were performed on standardized steel samples. In the case of the double-pulse scheme, two synchronized lasers were used, an ablation laser (200mJ/15ns), and a reheating laser (665mJ/6ns) in a collinear beam geometry. Spatially resolved and temporally integrated laser induced plasma VUV emission in the DP scheme and its dependence on inter-pulse delay time were studied. The VUV spectral line intensities were found to be enhanced in the DP mode and were significantly affected by the inter-pulse delay time. Additionally, the influence of ambient conditions was investigated by employing low pressure nitrogen, argon or helium as buffer gases in the ablation chamber. The results clearly demonstrate the existence of a sharp ubiquitous emission intensity peak at 100ns and a wider peak, in the multi-microsecond range of inter-pulse time delay, dependent on the ambient gas conditions.

Nonequilibrium radiation of the shock wave in air in the vacuum ultraviolet region

A numerical model of generation of the nonequilibrium molecular radiation in a vacuum ultraviolet spectral region (VUV radiation with a wavelength shorter than 200 nm) behind a shock wave in air in a range of varying its velocity of 4.5–9.5 km/s is developed. This numerical model is verified using the results of experiments in a shock tube for studying the photoionization effect before the shock-wave front in a wave-length range of 85–105 nm. Some features of nonequilibrium VUV radiation, particularly its generation a very thin high-temperature layer behind the shock-wave front, are established.

VUV Study of dissociative excitation of S-containing molecules following electron impact

Photoemission cross section data have been measured for the dissociative excitation of S2, S8 and CS2 by electrons in the impact energy range from threshold to 360 eV. Emissions in the vacuum ultraviolet spectral region (115-170 nm) have been observed and quantified, where possible, using emissions from H2 as a secondary standard. Numerous dissociation pathways have been identified. These data are important in a wide range of situations such as plasma physics and planetary atmospheres.

Determination of the coherence length in the vacuum-ultraviolet spectral region for the BPO4 crystal

An experimental system has been set up to determine the coherence lengths of nonlinear optical crystals in the vacuum-ultraviolet (VUV) spectral region by measuring the Maker fringes in the VUV region. Using this system, the Maker fringes generated by frequency conversion from 354.7 to 177.3 nm in the BPO4 crystal were obtained and the coherence length corresponding to the nonlinear optical (NLO) coefficient d36 of the BPO4 crystal was determined to be lc(d36) = (0.785 ± 0.002) μm. To the best of our knowledge, this is the first time that the coherence length of NLO crystals in VUV was determined by experiment, and the result could be an essential parameter for designing a quasi-phase-matched BPO4 device.

Versatile high-repetition-rate phase-locked chopper system for fast timing experiments in the vacuum ultraviolet and x-ray spectral region

A novel light chopper system for fast timing experiments in the vacuum-ultraviolet (VUV) and x-ray spectral region has been developed. It can be phase-locked and synchronized with a synchrotron radiation storage ring, accommodating repetition rates in the range of ~8 to ~120 kHz by choosing different sets of apertures and subharmonics of the ring frequency (MHz range). Also the opening time of the system can be varied from some nanoseconds to several microseconds to meet the needs of a broad range of applications. Adjusting these parameters, the device can be used either for the generation of single light pulses or pulse packages from a microwave driven, continuous He gas discharge lamp or from storage rings which are otherwise often considered as quasi-continuous light sources. This chopper can be utilized for many different kinds of experiments enabling, for example, unambiguous time-of-flight (TOF) multi-electron coincidence studies of atoms and molecules excited by a single light pulse as well as time-resolved visible laser pump x-ray probe electron spectroscopy of condensed matter in the valence and core level region.

Enhancement of Extreme Ultraviolet Emission Intensity from a Laser-produced Plasma Using an Alloy Target

SUMMARY We observed that the emission intensity in the extreme and vacuum ultraviolet spectral regions using a laser-produced copper–tungsten alloy plasma was 1.3 times higher than when using a pure tungsten plasma. The enhancement of the emission intensity was explained by the difference of the Rosseland mean opacity of each plasma.

Electron-impact dissociative excitation of S2

Electron-impact dissociation of S2 has been studied by observation of the atomic emission features in the vacuum ultraviolet spectral region between 120 and 170 nm. The predominantly S2 targets were prepared by passing either S8 or CS2 through a microwave discharge. The excitation function of the S [3s23p4–3s23p3(2D°)4s, 3P-3D°] emission multiplet has been measured at 147.4 nm for electron-impact energies from below threshold to 370 eV. From appearance energy measurements in the near-threshold region, likely fragmentation channels are identified.

Optical spectroscopy of an atomic nucleus: Progress toward direct observation of the 229Th isomer transition

The nucleus of the thorium-229 isotope possesses a first excited nuclear state (229mTh) at an exceptionally low energy of 7.8±0.5eV above the nuclear ground state (229gTh), as determined by earlier indirect measurements. This is the only nuclear excited state known that is within the range of optical spectroscopy. This paper reports progress toward detecting the 229mTh state directly by luminescence spectroscopy in the vacuum ultraviolet spectral region. The estimated natural linewidth of the 229gTh↔229mTh isomer transition of 2π×0.1 to 2π×10mHz is expected to broaden to ∼10kHz for 229Th4+ doped into a suitable crystal. The factors governing the choice of crystal system and the substantial challenges in acquiring a sufficiently large quantity of 229Th are discussed. We show that the 229gTh↔229mTh transition energy can be identified to within 0.1nm by luminescence excitation and luminescence spectroscopy using the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory. This would open the door for subsequent laser-based measurements of the isomer transition and future applications of 229Th in nuclear clocks. We also show that 233U-doped materials should produce an intrinsic, continuous, and sufficiently high rate of 229mTh→229gTh luminescence and could be a useful aid in the initial direct search of the isomer transition.

Optical properties of the vacuum-ultraviolet nonlinear optical crystal—BPO_4

Studies on the linear and nonlinear optical properties of the BPO4 crystal have been carried out experimentally. The transmittance spectra in the vacuum-ultraviolet and near-infrared spectral regions were obtained, showing a transparency in the range from 134 to 4230 nm. The refractive indices were accurately measured at 12 wavelengths from 0.2537 to 1.530 μm and the Sellmeier equations constructed. Moreover, the nonlinear optical coefficients of the BPO4 crystal were measured with the Maker fringes technique, which gave a result of d36=0.76 pm/V.

Communication: Vacuum ultraviolet laser photodissociation studies of small molecules by the vacuum ultraviolet laser photoionization time-sliced velocity-mapped ion imaging method

We demonstrate that the vacuum ultraviolet (VUV) photodissociation dynamics of N(2) and CO(2) can be studied using VUV photoionization with time-sliced velocity-mapped ion imaging (VUV-PI-VMI) detection. The VUV laser light is produced by resonant sum frequency mixing in Kr. N(2) is used to show that when the photon energy of the VUV laser is above the ionization energy of an allowed transition of one of the product atoms it can be detected and characterized as the wavelength is varied. In this case a β parameter = 0.57 for the N((2)D°) was measured after exciting N(2)(o(1)Π(u), v(') = 2, J(') = 2) ← N(2)(X(1)Σ(g) (+), v(") = 0, J(") = 1). Studies with CO(2) show that when there is no allowed transition, an autoionization resonance can be used for the detection of a product atom. In this case it is shown for the first time that the O((1)D) atom is produced with CO((1)Σ(+)) at 92.21 nm. These results indicate that the VUV laser photodissociation combined with the VUV-PI-VMI detection is a viable method for studying the one-photon photodissociation from the ground state of simple molecules in the extreme ultraviolet and VUV spectral regions.

Crystal growth and scintillation characteristics of the Nd 3+ doped LiLuF 4 single crystals

Sixty millimeter diameter single crystal of Nd 3+ doped LiLuF 4 was successfully grown by the Czochralski technique. No remarkable absorption due to unfavorable impurities was observed from optical absorption measurements in the vacuum ultra-violet spectral region. The high crystallinity and homogeneous luminescence characteristics were found from X-ray rocking curve and cathode-ray luminescence respectively. X-ray excited luminescence spectrum was measured and the significant 4f 25d–4f 3 luminescence at 182 nm was observed in the grown crystal. The pulse height spectrum was taken upon γ-ray irradiation. As a result, the grown crystals demonstrated sufficient response to the γ-ray showing the light yield of 420 ± 30 photons/MeV. The decay curve under α-ray irradiation was also investigated and described by two component decay kinetics which consists of the decay constants of 34 and 450 ns.

Study of two-photon resonant four-wave sum mixing in xenon in the spectral region of 105–110 nm

The two-photon resonant four-wave sum and difference mixing in Xe gaseous medium was studied by subjecting a variety of molecular samples, including acetone, furan, thiophene, triethylamine, and dimethylsilacyclobutane, to the coherent radiation sources produced from the four-wave mixing processes for ionization. Ionization of all samples studied showed similar behavior and is due to the laser radiation from four-wave sum mixing. It is demonstrated that in a vacuum ultraviolet (VUV) spectral region of 105–110 nm four-wave sum mixing in Xe can only occur in five discontinued regions, all of which are to the blue of a Xe atomic transition. The peak positions and widths of the five spectral regions were characterized. Study of the triethylamine sample with its ionization energy lower than the photon energy of the VUV radiation from the difference mixing illustrated the competition between the sum and difference mixing occurring simultaneously in the Xe gas cell.

Bilateral NIST–PTB comparison of spectral responsivity in the VUV

To compare the calibration capabilities for the spectral responsivity in the vacuum-ultraviolet spectral region between 135 nm and 250 nm, PTB and NIST agreed on a bilateral comparison. Calibrations of semiconductor photodiodes as transfer detectors were performed using monochromatized synchrotron radiation and cryogenic electrical substitution radiometers as primary detector standards. Great importance was attached to the selection of suitable transfer detector standards due to their critical issues in that wavelength regime. The uncertainty budgets were evaluated in detail. The comparison showed a reasonable agreement between the participants. However, it became obvious that the uncertainty level for this comparison cannot easily be further reduced due to the lack of sufficiently radiation-hard and long-term stable transfer standard detectors.Main text.To reach the main text of this paper, click on Final Report.The final report has been peer-reviewed and approved for publication by the CCPR-WGKC.

Vacuum-ultraviolet ellipsometry spectra and structural properties of Pb(Zr,Ti)O3 films

Optical properties of PbZrxTi1−xO3 material have been actively studied in the visible and near band-gap region, but data in the vacuum-ultraviolet spectral region is rather scarce. In this work we focus on well known interband transitions for the perovskite materials, 2p→dγ, located in VUV spectral region. Dielectric functions of chemical solution deposited and sputtered PZT were obtained in the spectral range 1–8.8eV. Differences between the absorption maxima for chemical solution deposited and sputtered PbZrxTi1−xO3 samples near Kahn–Leyendecker 2p→dγ interband transition were found and are interpreted by change of interatomic distances. This is confirmed by different lattice constants. In the case of PbZrxTi1−xO3 film with microcracks, the void fraction was estimated from the effective medium approximation. Direct band-gap energies appear to be nearly the same for all reactive sputtered samples weakly depending of Zr/Ti concentration.

Electronic excitations in B12As2 and their temperature dependence by vacuum ultraviolet ellipsometry

The dielectric response function of epitaxialB12As2 films on 4H-SiC was determined at room temperature and at 10 K in the spectral region of3.6–9.8 eV, i.e., in the vacuum ultraviolet (VUV) spectral region, by synchrotronellipsometry. The experimental dielectric function was simulated with the criticalpoint parabolic band model. The parameters of the dispersive structures werederived by numerical fitting of the experimental data to the proposed model. Newhigh energy optical transitions are resolved at 5.95, 7.8 and 8.82 eV and theirlineshape and origin are discussed. The temperature dependence of the criticalpoint energies and transition strengths was determined, and the excitonic effect isconsidered.

Crystal growth and scintillation characteristics of the Nd 3+ doped LaF 3 single crystal

Two-inch diameter Nd 3+ doped LaF 3 single crystals were successfully grown by the Czochralski technique. No remarkable absorption due to unfavorable impurities or color centers was observed from optical absorption measurements in the vacuum ultra-violet spectral region. The high crystallinity and homogeneous Nd 3+ distribution were found from X-ray rocking curve and chemical composition analysis, respectively. X-ray excited luminescence spectrum was measured and the significant 4f 25d–4f 3 luminescence at 173 nm was observed in the grown crystal. The pulse height spectrum and scintillation decay curves were taken upon γ-ray irradiation. As a result, the grown crystals demonstrated sufficient response to the γ-ray, the light yield of 140 ± 30 photons/MeV, and fast scintillation decay of 7.6 ns.

Dissociative excitation of H2S by electron impact studied in the VUV

Absolute photoemission cross section data have been measured for the dissociative excitation of H2S by electrons in the impact energy range from threshold to 300 eV. Emissions in the vacuum ultraviolet spectral region have been observed and quantified using emission from H2 as a secondary standard. Numerous dissociation pathways have been identified. H2S data are important in a wide range of situations such as plasma physics, interstellar space and blood flow in humans.

Intense heavy ion beams as a pumping source for short wavelength lasers

AbstractThe high energy loss of heavy ions in matter as well as the small angular scattering makes heavy ion beams an excellent tool to produce almost cylindrical and homogeneously excited volumes in matter. This aspect can be used to pump short wavelength lasers. For the first time, a beam of heavy ions was used to pump a short wavelength gas laser in an experiment performed at the GSI ion accelerator facility in December 2005. In this experiment, the well-known KrF* excimer laser was pumped with an intense uranium beam. Pulses of an uranium beam compressed down to 110 ns (full width at half maximum) with initial particle energy of 250 MeV per nucleon were stopped inside a gas laser cell. A mixture of an excimer laser premix gas (95.5%Kr + 0.5%F2) and a buffer gas (Ar) in different proportions was used as the laser gas. The maximum beam intensity reached in the experiment was 2.5 × 109particles per pulse, which resulted in 34 J/g specific energy deposited in the laser gas. The laser effect on the transition at λ = 248 nm has been successfully demonstrated by various independent methods. There, the laser threshold was reached with a beam intensity of 1.2 × 109particles per pulse, and the energy of the laser pulse of about 2 mJ was measured for an ion beam intensity of 2 × 109particles per pulse. As a next step, it is planned to reduce the laser wavelength down to the vacuum ultraviolet spectral region, and to proceed to the excimer lasers of the pure rare gases. The perspectives for such experiments are discussed and the detailed estimations for Xe and Kr cases are given. We believe that the use of heavy ion beams as a pumping source may lead to new pumping schemes on the higher lying level transitions and considerably shorter wavelengths, which rely on the high cross sections for multiple ionization of the target species.