Vacuum Ultraviolet Spectral Region Research Articles

Vacuum-ultraviolet interconfigurational 4ƒ^3 → 4ƒ^25d absorption and emission studies of the Nd^3+ ion in KYF, YF, and YLF crystal hosts

The laser-induced vacuum-ultraviolet fluorescence spectra of KY3F10:Nd3+ (KYF:Nd) and YF3:Nd3+ (YF:Nd) single crystals pumped by a pulsed-discharge molecular F2 laser at 157 nm were obtained. A number of new fluorescence peaks were observed and were assigned to the 4f25d → 4f3 dipole-allowed transitions of the Nd3+ ion. The absorption spectra of LiYF4:Nd3+ (YLF:Nd), KYF:Nd, and YF:Nd crystal samples in the vacuum-ultraviolet spectral regions were also obtained. Finally, the splitting of the states of the 4f25d configuration of the Nd3+ ion, which is due to the crystal field, was observed in all the crystal samples.

Extending x-ray streak camera operation to vacuum ultraviolet wavelengths

A major limitation in the use of x-ray streak cameras at longer wavelengths in the vacuum ultraviolet spectral region is the strong absorption of the radiation in the conductive thin films that are used for transmission photocathode substrates. Reflection photocathodes require off-axis illumination and hence do not couple easily to instruments such as flat-field spectrographs. We are investigating the use and optimization of an open-geometry photocathode structure consisting of a fine-mesh wire screen coated with the photoemissive material. Photons impinge directly onto this coated screen. Photoelectrons emitted from the side surfaces of the wires are accelerated into the streak tube by the applied field penetrating into the plane of the wire screen. Coatings of CsI and CuI are under study. Laser-produced plasmas, as well as discharge sources, are being used for the testing and optimization. A flat-field grazing-incidence spectrograph and a 1 m, normal-incidence spectrograph are used to cover the spectral region from 3 to 200 nm.

Extension of the tuning range of a femtosecond Ti:sapphire laser amplifier through cascaded second-order nonlinear frequency conversion processes

The article presents single step and cascade methods for converting the femtosecond radiation from a 1 kHz repetition rate Ti:sapphire regenerative amplifier to the near-infrared (1.2–1.5 μm and 1.6–2.2 μm), visible (554–593 nm), and for the first time to our knowledge to the vacuum ultraviolet spectral region well below 190 nm. Using only solid state materials (nonlinear optical crystals) femtosecond pulses are generated through phase-matched mixing processes down to 172.7 nm. The developed scheme for femtosecond parametric conversion provides nearly bandwidth limited femtosecond pulses, whose duration is comparable or even shorter than that of the pump pulses. The temporal gain narrowing in the optical parametric amplifier is identified as a pulse compression mechanism on the femtosecond time scale leading to generation of infrared light pulses as short as ≊50 fs. As a whole our all-solid state laser complex provides independently tunable and synchronized sub-200 fs light pulses simultaneously at several wavelengths and practically without relative jitter.

Atomic branching ratio data for oxygen-like species

The branching ratio technique for radiometric calibrations in the vacuum ultraviolet spectral region is briefly reviewed. Lists of transitions suitable for use of the technique are given for oxygen-like species (O I and Ne III) along with pertinent data for their application.

VUV and UV fluorescence and absorption studies of Nd 3+ and Ho 3+ ions in LiYF 4 single crystals

The laser induced fluorescence spectra of LiYF 4:ND 3+ (YLF:Nd) and LiYF 4:Ho 3+ (YLF:Ho) single crystals, pumped by an F 2 pulsed discharge molecular laser at 157 nm were obtained in the vacuum ultraviolet (VUV) and ultraviolet (UV) regions of the spectrum. With this pumping arrangement a number of new fluorescence peaks were observed for the first time. They are assigned to the dipole allowed transitions 4f 25d→4f 3 and 4f 95d→4f 10 of the Nd 3+ and Ho 3+ ions, respectively. The absorption spectra of the same crystal samples in the VUV and UV spectral regions were taken as well.

High-sensitivity detection of selenium and arsenic by laser-excited atomic fluorescence spectrometry using electrothermal atomization

High-sensitivity detection of the trace elements selenium and arsenic is reported. The method applied is laser-excited atomic fluorescence spectrometry using electrothermal atomization within a graphite furnace atomizer. For the production of tunable laser radiation in the vacuum ultraviolet (VUV) spectral region a laser system was developed that consists of two laboratory-built dye lasers pumped by a Nd:YAG laser. The laser radiations are subsequently frequency doubled and sum frequency mixed by non-linear optical KDP or BBO crystals, respectively. The system works with a repetition rate of 20 Hz and provides output energies of up to 100 µJ in the VUV at a pulse duration of 5 ns. The investigations focused on the detection of selenium and arsenic in aqueous solutions and in samples of human whole blood. From measurements on aqueous standards, detection limits of 1.5 ng l–1 for selenium and 5.4 ng l–1 for arsenic were obtained, with corresponding absolute detected masses of only 15 or 54 fg, respectively. The linear dynamic ranges spanned six orders of magnitude and good precision was achieved. In the case of human whole blood samples, the recovery was found to be within the range 96–104%. The determination of the selenium content yielded medians of 119.5 ± 17.3 µg l–1 for 200 frozen blood samples taken in 1988 and 109.1 ± 15.6 µg l–1 for 103 fresh blood samples.

Direct Detection of Vacuum Ultraviolet Radiation for Nonmetal Determinations with a Helium Microwave-Induced Plasma

A study of the determination of metals, nonmetals, and metalloids (F, Cl, Br, I, S, P, Se, As, Sb, and Pb) in the vacuum-ultraviolet spectral region with a kilowatt-plus helium microwave-induced plasma is presented. A spectrometer system similar to that used for far-ultraviolet optical sampling with an inductively coupled plasma was designed and implemented. This arrangement was produced by using an optical sampler to couple the plasma discharge with a helium-purged monochromator. Monitoring was performed at wavelengths less than 200 nm with no N2 or O2 in the optical path. Helium was chosen as the purge gas because it is transparent in the wavelength region of interest (>58 nm). Sample introduction was accomplished with ultrasonic nebulization. Limits of detection, linear ranges, and matrix effects caused by group IA and IIA elements were studied. Analytical calibration plots were linear over a range of 2–3 orders of magnitude. Detection limits ranged from 0.06 μg/mL for sulfur to 5 μg/mL for selenium.

Atomic branching ratio data for carbon-like ions

The branching-ratio technique for radiometric calibrations in the vacuum ultraviolet spectral region is briefly reviewed. A list of transitions suitable for use of the technique is given for carbon-like ions along with pertinent data for their application.

Third-Order Nonlinear Susceptibility of Cadmium for the Generation of Continuously Tunable Vacuum Ultraviolet Radiation

Third-order nonlinear susceptibility χ3 for the two-photon resonant sum-frequency mixing in Cd vapor is calculated in the vacuum ultraviolet (VUV) spectral region from 110 nm to 138 nm, where the four levels, 6 1S0, 5 1D2, 7 1S0, 6 1D2, are used for the two-photon resonance. The results show that χ3 for each resonance has a specific wavelength dependence, and the 5 1D2 resonance is especially advantageous for the generation of broadly- and continuously-tunable VUV radiation in the spectral region concerned. The calculated results are compared with the wavelength dependences of the experimentally observed VUV power.

Efficient frequency converter of ultrashort pulses to different spectral regions based on atomic barium vapor in a leucosapphire cell

A frequency converter for ultrashort pulses, based on a leucosapphire cell in which atomic barium vapor, is formed was realized for the first time. This converter makes it possible to generate picosecond and femtosecond pulses in the infrared, visible, ultraviolet, and vacuum ultraviolet spectral regions with a high efficiency. A method is given for determining the gain G for stimulated Raman scattering in a single laser shot. In the case of stimulated electronic Raman scattering in barium vapor with a 30 cm−1 detuning, at a pressure of 0.1 Torr, and for a pump pulse duration of ~ 20 ps, a value of G = (0.2)Ip cm/MW was obtained (where Ip is the pump intensity) while the threshold pump energy was below 1 μJ. Other possible applications of a leucosapphire cell containing atomic barium vapor are also described.

Anti-Stokes Raman scattering to the vacuum ultraviolet spectral region in rare-gas discharge plasmas

An investigation was made of anti-Stokes Raman scattering of YAG:Nd3+ laser radiation to the vacuum ultraviolet spectral region from the 2p53p neon levels excited in a hollow cathode discharge. Radiation was generated at wavelengths 62.3–63.0 nm in lines having spectral widths of 1 cm−1 within which tuning was possible. The wavelengths of the lines were determined with an accuracy of better than 10−3 nm. When the initial level was pumped from a metastable level using a second laser, the intensity of the anti-Stokes Raman scattering vacuum ultraviolet line almost doubled and at a wavelength of 62.434 nm, for example, the spectral brightness was 1015 photons/(scm2srcm−1). This is comparable with the spectral brightness of the best synchrotron radiation sources.

Radiative lifetimes in Si I from laser-induced fluorescence in the visible, ultraviolet, and vacuum ultraviolet.

Radiative lifetimes for 47 levels in neutral silicon are reported. Lifetimes are measured using time-resolved laser-induced fluorescence (LIF) on a beam of silicon atoms. The transitions studied span the spectral range 160--410 nm. The experimental methods used are broadly applicable for measuring lifetimes in almost any neutral or singly ionized element using LIF in the visible, ultraviolet, and vacuum-ultraviolet spectral regions. Lifetimes are combined with previously reported branching fractions or branching ratios to generate 36 absolute transition probabilities, most with total uncertainty of 5--10 %.

Optical Properties and Structure of Defects in Silica Glass

This paper reviews what is known about the structures and optical properties of point defects in silica glass and other amorphous forms of SiO2. The available structural information derives mainly from the results of electron spin resonance (ESR) spectrometry-a method which is directly applicable only to the subclass of defects which are paramagnetic. The intrinsic paramagnetic centers so far identified by ESR include E' centers (≡Si·), nonbridging-oxygen hole centers (≡Si-O·), peroxy radicals (≡Si-O-O·), and self-trapped holes. Examples of intrinsic diamagnetic defects believed to occur in silica in include neutral oxygen vacancies (≡Si-Si≡), two-coordinated silicone (O-Si-O-), and peroxy linkages (≡Si-O-O-Si≡). The most common extrinsic defects are associated with hydroxyl and chloride impurities. Radiolytically induced H0 and H2 play important roles in the post-irradiation growth and decay kinetics of defects in glasses containing_??_100ppm OH. A compendium of ESR/optical correlations taken from the literature points to the most likely origins of many defect-related optical absorption bands in the visible, ultraviolet and vacuumultraviolet spectral regions. Others of these bands have been ascribed to interstitial O2, O3 and Cl2 molecules on the basis of similarities of their spectral positions and widths to those of the corresponding species in the gas phase.

On-line tangential soft x-ray/VUV tomography on COMPASS-C

A tangentially viewing pinhole camera has been used to image the COMPASS-C plasma in the vacuum ultraviolet and soft x-ray spectral regions with framing rates up to 330 Hz. The PC-based system acquires up to 256 128×128 pixel images per shot and incorporates an array processor to allow on-line tomographic reconstructions. Data compression is used to reduce the data load by a factor of 4.

On polythiophene film as a photocathode

The photoelectron emission from thin films of lightly doped polythiophene was investigated in the ultraviolet and vacuum ultraviolet spectral region. Absolute quantum yields rising from 4*10-6 at 5 eV to 3*10-3 at 10.5 eV were measured. The photoemission threshold was determined to be Eth=5.06+or-0.05 eV. The films were produced electrochemically with perchlorate ions as the dopant.

An improved window for ring imaging Cerenkov detectors

Calcium fluoride crystal is used as a window in RICH (ring imaging Cerenkov) detectors because of its vacuum ultraviolet transmission down to 125.7 nm. This permits triethylamine (TEA) photosensitive gas to be used in the detector. The crystal would be more useful if larger, single crystals could be achieved. A crystal growing furnace was designed, based upon the concepts of a vertical slab geometry, to produce large, single crystals. These concepts are embodied in a furnace which produces calcium fluoride crystals 35 cm square by 7.5 cm thick. These ingots are an optimal geometry for the fabrication of RICH detector windows. Surface strengthening experiments indicate that large-area calcium fluoride windows can be made much more reliable than historic, nominal stress-at-failure values would predict. It is noted that maximum transmission of Cerenkov radiation in the vacuum ultraviolet spectral region can be ensured with special purification technology. Very pure calcium fluoride crystals show increased resistance to forming color centers after exposure to several megarads of ionizing radiation.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Semiconductor photodiodes in the VUV: Determination of layer thicknesses and design criteria for improved devices

Abstract Semiconductor photodiodes are shown to be suited as detectors in the vacuum ultraviolet (VUV) spectral region. The spectral responsivity can be described with a simple model, which requires only the knowledge of the thicknesses of the dead layer and of the sensitive volume. These parameters are determined by measuring the angular dependence of the photocurrents. The spectral responsivities calculated with these parameters are in agreement with radiometric measurements of the spectral responsivity. Design criteria for semiconductor detectors to be used in various parts of the VUV are derived from the measurements.

Porous plastic membranes used as extreme and far ultraviolet radiation diffraction filters

The optical properties of porous Lavsan films used as diffraction filters in the vacuum ultraviolet and ultrasoft X-ray spectral region are considered. Samples with thickness L of 2.5, 5 and 10 μm, having cylindrical pores with diameters lying in the micron and submicron range, have been investigated. For the filters with various thicknesses, the dependence of the filter transmittance on the pore diameter D has been experimentally measured in the 115–125 nm spectral band. The transmittance T 1 of a single opening in the Lavsan film is obtained. It is found that the value of T 1 can change by more than 10 orders when the diameter changes from 0.1 to 1.0 μm.

Projected performance of rf-linac-driven free-electron lasers in the extreme-ultraviolet spectral region

A free-electron laser user facility for scientific experimentation in the vacuum-ultraviolet and soft X-ray spectral regions (together termed the XUV) is being developed at Los Alamos. The design includes a series of laser oscillators and amplifiers, driven by a single, rf-linear accelerator, that will generate broadly tunable, picosecond-pulse, coherent radiation from 1 to 400 nm. Below 300 nm, the peak- and average-power output of these FEL devices should surpass the capabilities of any existing, continuously tunable photon sources by many orders-of-magnitude. The design and output parameters of this facility are described, including comparison with synchrotron radiation sources and recent progress in developing the three primary components (electron beam, magnetic undulator and resonator mirrors) is cited.

Simple nitrogen-purged optical system for atmospheric pressure helium microwave induced plasma atomic emission spectrometry in the vacuum ultraviolet spectral region.

A simple nitrogen-purged optical system has been developed to measure spectral lines in the vacuum ultraviolet (VUV) region by microwave induced plasma atomic emission spectrometry (MIP-AES). For this purpose, both a monochromator and an optical path between an MIP source and an entrance slit of a spectometer are purged with nitrogen to reduce light absorption by oxygen. This system allows measurement of the emission lines in a VUV spectral region between 170 and 190nm. This system has been successfully coupled with a continuous-flow iodine generation technique as a means of sample introduction into an at-mospheric pressure He-MIP for the determination of iodine by AES in the VUV region of the spectrum.