Extreme Ultraviolet Spectra Research Articles

Comparison of experimental and simulated extreme ultraviolet spectra of xenon and tin discharges

Xenon and tin both are working elements applied in discharge plasmas that are being developed for application in extreme ultraviolet (EUV) lithography. Their spectra in the 10-21-nm-wavelength range have been analyzed. A fully analytical collisional-radiative model, including departure from equilibrium due to a net ionization rate, was used to simulate the EUV spectra. Detailed Hartree-Fock calculations, using the COWAN package, were applied for determination of the energy levels and optical transition probabilities of the 8+ to 12+ ions of both elements. For the calculation of the radiation, the opacity of the plasma was taken into account. Time-resolved measurements of the spectra from ionizing phases of two different discharge plasmas were corrected for the wavelength-dependent sensitivity of the spectrometer, and compared to the results of the simulations. Fairly good agreement between the experiments and the model calculations has been found.

Characterization of extreme ultraviolet emission from laser-produced spherical tin plasma generated with multiple laser beams

Spherical solid tin targets were illuminated uniformly with twelve beams from the Gekko XII laser system to create spherical plasmas, and the extreme ultraviolet (EUV) emission spectra from the plasmas were measured. The highest conversion efficiency of 3% to 13.5nm EUV light in 2% bandwidth was attained for an irradiance of around 5×1010W∕cm2. The experimental results were reproduced fairly well using a theoretical model taking the power balance in the plasma into consideration.

Solar EUV Experiment (SEE): Mission overview and first results

The Solar EUV Experiment (SEE) is one of four scientific instruments on the NASA Thermosphere Ionosphere Mesosphere Energetics Dynamics (TIMED) spacecraft, which has been simultaneously observing the Sun and Earth's upper atmosphere since January 2002. The SEE instrument measures the irradiance of the highly variable, solar extreme ultraviolet (EUV) radiation, one of the major energy sources for the upper atmosphere. The primary SEE data product is the solar spectral irradiances from 0.1 to 194 nm in 1 nm intervals that are fundamental for the TIMED mission's investigation of the energetics in the tenuous, but highly variable, layers of the Earth's atmosphere above 60 km. The TIMED mission began normal operations on 22 January 2002, a time when the Sun displayed maximum levels of activity for solar cycle 23, and has provided daily measurements as solar activity has declined to moderate levels. Solar irradiance variability observed by SEE during the 2 years of the TIMED prime mission includes a variety of moderate and large flares over periods of seconds to hours and dozens of solar rotational cycles over a typical period of 27 days. The SEE flare measurements provide important, new results because of the simultaneous spectral coverage from 0.1 to 194 nm, albeit limited temporal coverage due to its 3% duty cycle. In addition, the SEE measurements reveal important, new results concerning phase shifts of 2–7 days in the intermediate‐term variations between different UV wavelengths that appear to be related to their different center‐to‐limb variations. The new solar EUV irradiance time series from SEE are also important in filling the “EUV Hole,” which is the gap in irradiance measurements in the EUV spectrum since the 1980s. The solar irradiances measured by SEE (Version 7, released July 2004) are compared with other measurements and predictions from models of the solar EUV irradiance. While the measurement comparisons show reasonable agreement, there are significant differences between SEE and some of the models in the EUV range. The data processing algorithms and calibrations are also discussed.

Extreme vacuum ultraviolet solar spectra obtained during the SPIRIT experiment aboard CORONAS-F: A catalog of lines in the range 280–330 Å

We present a catalog of 100 lines in the wavelength range 280–330 A detected by the RES-C spectroheliograph in solar active regions and flares during the SPIRIT experiment aboard the CORONAS-F orbital station. We identified 54 lines. The line intensities recorded during the X3.4 (GOES) solar flare of December 28, 2001, are given. The data reduction procedure is discussed.

Theoretical and experimental study of the extreme ultraviolet photoabsorption spectrum of triply ionized yttrium

The photoabsorption spectrum of Kr-like Y3+ ions in the very soft x-ray/extreme ultraviolet spectral range where single (4s–np) and double (4p4p–nℓn′ℓ′) electron excitations dominate has been measured using a dual laser plasma photoabsorption technique. The corresponding photoionization cross section has been computed within the framework of the configuration interaction Pauli–Fock approach. The calculations confirm that the 4s–5p resonance drops below the 4p ionization threshold so that the first member of the 4s–np photoionization resonance series becomes 4s–6p. Also, whereas the first such series member in Rb+ and Sr2+ appears as a window resonance, in Y3+ it switches to a normal, slightly asymmetric absorption peak. In addition, although the 4s–5p1/2 resonances in the 4p51/2 and 4p53/2 continua are mirror images in Rb+ and Sr2+ and hence cancel each other, they exhibit quite different shapes in Y3+ and hence we observe the residual signature of the 4s–6p1/2 resonance as a weak partner to the main 4s–6p3/2 resonance.

Simulation of the EUV Spectrum of Xe and Sn Plasmas

Complex spectra of Xe and Sn, and their feasibility for use in the lithographic extreme ultraviolet (EUV) source, are investigated. By combining calculations of atomic data using the HULLAC code and the Whiam collisional radiative model with a simple radiative transfer model, the EUV spectrum is shown to originate from a large number of fine structure transitions. Satellite lines of 4d-4f and 4d-5p transitions from near ten times ionized states are found to make a significant contribution to the emission from an optically thick plasma. The wavelengths and transition probabilities of emission lines, charge state distribution, and level population in the plasma, as well as radiation intensity from a laser-produced plasma source have been calculated. The effect of opacity is taken into account assuming a local thermodynamic equilibrium population in the plasma. The calculated result reproduces the observed spectra both for Xe and Sn, showing applicability of the present method to the spectroscopy of hot dense plasmas.

Focus on Spectroscopy

Focus on Spectroscopy

Properties of the Lower Transition Region: The Widths of Optically Allowed and Intersystem Spectral Lines

The widths of spectral lines in the ultraviolet (UV) and extreme ultraviolet (EUV) spectral regions that are formed in the solar transition region and corona are usually greater than the optically thin widths due to thermal Doppler broadening calculated under the assumption of ionization equilibrium. Although opacity can explain the widths of some lines, there are a host of optically thin lines for which the excess widths are attributed to nonthermal motions. Interest in these motions for coronal heating theories has led to the measurement and comparison of spectral line profiles/widths throughout the solar UV and EUV spectrum. We find that for the quiet Sun the widths of some optically allowed lower transition region lines, deduced from spectra obtained by the Solar Ultraviolet Measurements of Ultraviolet Radiation (SUMER) spectrometer on the Solar and Heliospheric Observatory (SOHO) spacecraft, are considerably larger than predicted from simply scaling previously measured wavelengths of other lines from the same ion. For example, the O III lines of the multiplet near 834 A are considerably wider than predicted from the previously measured (from Skylab) width of the optically thin O III 1666.15 A intersystem line. The excess widths are not due to nonthermal motions, as these are already included in the width of the 1666.15 A line. In this paper, we analyze the widths of some prominent optically allowed lines and discuss possible causes for discrepancies with previous measurements of intersystem lines.

Extreme ultraviolet emission spectra of highly ionized xenon and their comparison with model calculations

Xenon spectra involving emission from ion species of Xe7+ to Xe12+ were analyzed for a plasma focus discharge developed for extreme ultraviolet lithography. Low and higher resolution spectra were recorded in the 8–21 nm wavelength region for different operating conditions and different He–Xe gas mixtures. The spectra have been compared with Hartree–Fock calculations. The modeling included the distribution of the various xenon ion levels at a given electron equilibrium temperature and plasma opacity effects. Spectral analysis showed that the 4d–5p transition arrays are fairly well separated in wavelength for the ions Xe8+ to Xe11+. Good agreement between experiment and calculations was obtained for line positions and intensities, in particular, for the wavelength region at around 13.5 nm.

Time-resolved pinhole camera imaging and extreme ultraviolet spectrometry on a hollow cathode discharge in xenon.

A pinhole camera, an extreme ultraviolet (EUV) spectrometer, a fast gatable multichannel plate EUV detector, and a digital camera have been installed on the ASML EUV laboratory setup to perform time-resolved pinhole imaging and EUV spectroscopy on a copy of the Philips EUV hollow cathode discharge plasma source. The main properties of the setup have been characterized. Time-resolved measurements within the plasma pulse in the EUV have been performed on this source. Specific features of the plasma, such as a ring shape in the initiation phase and a propagating sphere during the pinch phase, have either been discovered or confirmed experimentally. Relative populations of various ionization stages in the pinch plasma have been estimated on the basis of line intensities and calculated transition probabilities. The changes in relative line intensities of a single ionization stage can be explained by a combination of temperature and excitation/deexcitation balance effects. Experiments with argon dilution on a newer version of the source show considerable effect on the shape of the xenon EUV spectrum.

Extreme ultraviolet spectra of highly charged Xe ions

Using the SuperEBIT electron-beam ion trap and a flat-field spectrometer, we observed extreme ultraviolet spectra of highly charged ions of Xe and measured the wavelengths of prominent lines from Li-, Be-, B-, Na-, and Mg-like ions. Our results for Li- and Na-like ions are as precise as the best available elsewhere. The results for Be-, B-, and Mg-like ions are much more precise than available data or extend those available from lower-$Z$ ions and reveal significant shortcomings of the various theoretical predictions.

Studies of high-repetition-rate laser plasma EUV sources from droplet targets

The water droplet laser plasma source has been shown to have many attractive features as a continuous, almost debris-free source for extreme ultraviolet (EUV) and X-ray applications. Through a dual experimental and theoretical study, we analyze the interaction physics between the laser light and the target. The hydrodynamic laser plasma simulation code, Medusa103 is used to model the electron density distribution for comparison to electron density distributions obtained through Abel inversion of plasma interferograms. In addition, flat field EUV spectra are compared to synthetic spectra calculated with the atomic physics code RATION.

Enhancement of the helium resonance lines in the solar atmosphere by suprathermal electron excitation - II. Non-Maxwellian electron distributions

In solar extreme ultraviolet spectra the He I and He II resonance lines show unusual behaviour and have anomalously high intensities compared with other transition region lines. The formation of the helium resonance lines is investigated through extensive non-local thermal equilibrium radiative transfer calculations. The model atmospheres of Vernazza, Avrett & Loeser are found to provide reasonable matches to the helium resonance line intensities but significantly overestimate the intensities of other transition region lines. New model atmospheres have been developed from emission measure distributions derived by Macpherson & Jordan, which are consistent with SOHO observations of transition region lines other than those of helium. These models fail to reproduce the observed helium resonance line intensities by significant factors. The possibility that non-Maxwellian electron distributions in the transition region might lead to increased collisional excitation rates in the helium lines is studied. Collisional excitation and ionization rates are recomputed for distribution functions with power-law suprathermal tails that may form by the transport of fast electrons from high-temperature regions. Enhancements of the helium resonance line intensities are found, but many of the predictions of the models regarding line ratios are inconsistent with observations. These results suggest that any such departures from Maxwellian electron distributions are not responsible for the helium resonance line intensities.

A new model of solar EUV irradiance variability 2. Comparisons with empirical models and observations and implications for space weather

Motivated by the need for reliable specification of the Sun's electromagnetic radiation in the extreme ultraviolet (EUV) spectrum, we have developed a new model of solar EUV irradiance variability at wavelengths from 50 to 1200 Å. Solar images are used to quantify changes in the sources of EUV irradiance during the solar cycle. Optically thin EUV emission line fluxes are estimated from differential emission measures (DEMs) that characterize the properties of the solar atmosphere in the source regions, while fluxes for optically thick lines are modeled directly by specifying the source region contrasts. We compare the new model, NRLEUV, with three different empirical models of solar EUV irradiance since 1975. For solar cycles 21 and 22, NRLEUV predicts overall lower EUV irradiances and smaller solar cycle variability than the empirical models. The average total EUV energy at wavelengths from 50 to 1050 Å is 2.9 mW m−2, smaller than the HFG, EUVAC, and SOLAR2000 models for which average energies are 3.7, 4.3, and 5.6 mW m−2, respectively. These differences have distinct wavelength dependencies. The solar cycle variation in total EUV energy is 1.9 for NRLEUV compared with 2.7, 2.9, and 2.3 for HFG, EUVAC, and SOLAR2000. Here, too, the differences are wavelength dependent. We compare both the NRLEUV and the empirically modeled EUV irradiances with selected wavelength bands and emission lines measured during 4 years in cycle 21 by Atmospheric Explorer‐E (AE‐E) and two broad bands at 170–200 and 260–340 Å measured in cycle 23 by the Solar X‐Ray Photometer (SXP) on the Student Nitric Oxide Experiment (SNOE) and the Solar EUV Monitor (SEM) on the Solar and Heliospheric Observatory (SOHO), respectively. The NRLEUV model reproduces the variations observed during solar rotation better than, or as well as, the empirical models. Comparisons of solar cycle variations are more ambiguous because undetected instrumental drifts can cause spurious trends in the observations over these longer timescales. Drifts in the AE‐E instruments may explain why the HFG and EUVAC models, which are based on parameterizations of these data, have larger solar cycle variations than NRLEUV. We assess the implications for space weather of the significant differences among the modeled EUV irradiances by using the Atmospheric Ultraviolet Radiance Integrated Code (AURIC) to quantify corresponding differences in upper atmosphere energy deposition and photoionization rates.

A Search for Helium in Ultramassive White Dwarfs

We have obtained extreme ultraviolet (EUV) spectra of eight hot, ultramassive white dwarfs (M = 0.9-1.3 M☉) with the Extreme Ultraviolet Explorer. The stars are, on average, twice as massive as normal white dwarfs. The relatively high turnout of ultramassive white dwarfs in EUV surveys is difficult to explain in the context of single-star evolution, and it has been suggested that ultramassive white dwarfs may result from the merger of two normal white dwarfs. The final configuration of merged white dwarfs is uncertain, but the resulting stars may have peculiar atmospheric compositions similar to the ultramassive DAO white dwarf GD 50. Our analysis of the EUV spectra of massive white dwarfs shows that all objects, with the possible exception of the magnetic white dwarf PG 1658+441, have EUV continuum emission characteristic of pure hydrogen atmospheres and that the peculiar atmospheric composition of the white dwarf GD 50 remains a unique phenomenon.

Electron-beam ion-trap spectra of tungsten in the EUV

At the Livermore electron-beam ion-trap facility, extreme-ultraviolet spectra of tungsten have been recorded in the wavelength range 40–85 Å. The electron-beam energy was varied systematically to identify the individual spectra of Rb-like W37+ to Cu-like W45+. About 60 spectral features have been identified. PACS Nos.: 32.30Rj, 39.30+w, 31.50+w

Quiescent and Flaring Structure in RS Canum Venaticorum Stars

Four of the most active RS CVn stars (V711 Tau, II Peg, σ Gem, and UX Ari) have been observed for a total of 3 Ms with the Extreme Ultraviolet Explorer satellite (EUVE) between 1992 and 2000 January. Flaring and quiescent states of extreme ultraviolet spectra (λλ70-740) and light curves (λλ75-175) have been analyzed to provide emission measure distributions (EMD) for these systems in the range log Te(K) ~ 5.6-7.4, based principally on iron lines. Flux measurements obtained with IUE and the Orbiting and Retrievable Far and Extreme Ultraviolet Spectrometer (ORFEUS) complete the EMD in the lower temperature range [log Te(K) ~ 4.0-5.6]. Frequent flaring activity has been found in the systems, including an increase during the rise phase by a factor of ~9 in the flux of σ Gem, the largest flare enhancement observed with EUVE. Analyses of the EUVE emission in the active single star AB Dor and the low-rotation giant star β Cet are also included. The EMDs are remarkably similar among all the stars, showing a narrow enhancement or bump around log Te(K) ~ 6.9. These narrow bumps are apparently unrelated to rotation rate, spectral type, binarity, or evolutionary stage. Significant material is found at log Te(K) 7.0 for the most active stars. Modulation of the EUV flux outside of flaring occurs in four of the stars (σ Gem, V711 Tau, UX Ari, AB Dor). The electron density ranges between Ne ~ 1012 and ~1013 cm-3, measured at log Te(K) ~ 7.0, and may reach higher values during flares. These densities and EMD values imply small scale sizes for emitting regions.

Spectral emission of fractional quantum energy levels of atomic hydrogen from a helium–hydrogen plasma and the implications for dark matter

From a solution of a Schrödinger-type wave equation with a nonradiative boundary condition based on Maxwell's equations, Mills predicts that atomic hydrogen may undergo a catalytic reaction with certain atomized elements and ions which singly or multiply ionize at integer multiples of the potential energy of atomic hydrogen, m27.2 eV wherein m is an integer. The reaction involves a nonradiative energy transfer to form a hydrogen atom that is lower in energy than unreacted atomic hydrogen that corresponds to a fractional principal quantum number ( n=1/ p=1/integer replaces the well-known parameter n=integer in the Rydberg equation for hydrogen excited states). One such atomic catalytic system involves helium ions. The second ionization energy of helium is 54.4 eV ; thus, the ionization reaction of He + to He 2+ has a net enthalpy of reaction of 54.4 eV which is equivalent to 2×27.2 eV . Since the products of the catalysis reaction have binding energies of m27.2 eV , they may further serve as catalysts. Extreme ultraviolet (EUV) spectroscopy was recorded on microwave and glow discharges of helium with 2% hydrogen. Novel emission lines were observed with energies of q13.6 eV where q=1,2,3,4,6,7,8,9, or 11 or these lines inelastically scattered by helium atoms wherein 21.2 eV was absorbed in the excitation of He (1 s 2) to He (1 s 12 p 1) . These lines were identified as hydrogen transitions to electronic energy levels below the “ground” state corresponding to fractional quantum numbers. Furthermore, astrophysical data was reviewed and such transitions were found to match certain spectral lines of the extreme ultraviolet background of interstellar space. They may resolve the paradox of the identity of dark matter and account for many celestial observations such as: diffuse H α emission is ubiquitous throughout the Galaxy and widespread sources of flux shortward of 912 A ̊ are required. Fractional hydrogen transitions were also assigned to unidentified lines in the solar EUV spectrum which may resolve the solar neutrino problem, the mystery of the cause of sunspots and other solar activity, and why the Sun emits X-rays.

Extreme ultraviolet emission from Xe clusters excited by high-intensity lasers

The property of extreme ultraviolet (EUV) generation from Xe clusters irradiated with intense lasers was studied. The Xe cluster jet was well characterized by the interferometric method. In order to obtain the adequate irradiation condition for strong EUV generation, EUV spectra were taken with various laser systems. Then, the wavelength, the pulse width, and the pump energy were widely varied. Through this survey, even with the comparatively low-density Xe jet of ⩽5×1018 cm−3 average atomic density, the highest conversion efficiency of over 10% from laser energy to EUV (5–18 nm) was obtained with a subpicosecond KrF laser pulse, where a 4π source was assumed. This EUV source is considered to be attractive as an EUV lithography light source because of its low average atomic density and small Xe cluster.

The Rest‐Frame Extreme‐Ultraviolet Spectral Properties of Quasi‐stellar Objects

We use a sample of 332 Hubble Space Telescope spectra of 184 QSOs with z > 0.33 to study the typical ultraviolet spectral properties of QSOs, with emphasis on the ionizing continuum. Our sample is nearly twice as large as that of Zheng et al. (1997) and provides much better spectral coverage in the extreme ultraviolet (EUV). The overall composite continuum can be described by a power law with index alpha_EUV = -1.76 +/- 0.12 (f_nu ~ nu^alpha) between 500 and 1200 Angstroms. The corresponding results for subsamples of radio-quiet and radio-loud QSOs are alpha_EUV = -1.57 +/- 0.17 and alpha_EUV = -1.96 +/- 0.12, respectively. We also derive alpha_EUV for as many individual objects in our sample as possible, totaling 39 radio-quiet and 40 radio-loud QSOs. The typical individually measured values of alpha_EUV are in good agreement with the composites. We find no evidence for evolution of alpha_EUV with redshift for either radio-loud or radio-quiet QSOs. However, we do find marginal evidence for a trend towards harder EUV spectra with increasing luminosity for radio-loud objects. An extrapolation of our radio-quiet QSO spectrum is consistent with existing X-ray data, suggesting that the ionizing continuum may be represented by a single power law. The resulting spectrum is roughly in agreement with models of the intergalactic medium photoionized by the integrated radiation from QSOs.