Soft X-ray Spectral Region Research Articles

Performance and Properties of Ultra-Thin Silicon Nitride X-ray Windows

The spectral transmittance of a new generation of SiN based X-ray windows is characterized. The windows are strengthened by low aspect-ratio support grid. As expected for this unprecedented thin window material, the transmittance in the soft X-ray spectral region outperforms the present technologies. A detailed study of the various performance properties of the fabricated SiN X-ray windows is presented. Besides their high transmittance, the windows also have high uniformity, high mechanical strength and good leak tightness. The windows can withstand temperatures from cryogenic range to approximately 250°C. SiN foils are the first real nanotechnology-based choice for the practical realization of X-ray windows and bring the performance to a level that only nanotechnology can offer.

Electron impact excitation of Mg VIII

Context. Mg viii emission lines are observed in a range of astronomical objects such as the Sun, other cool stars and in the coronal line region of Seyfert galaxies. Under coronal conditions Mg viii emits strongly in the extreme ultraviolet (EUV) and soft X-ray spectral regions which makes it an ideal ion for plasma diagnostics. Aims. Two theoretical atomic models, consisting of 125 fine structure levels, are developed for the Mg viii ion. The 125 levels arise from the 2s 2 2p, 2s2p 2 , 2p 3 , 2s 2 3s, 2s 2 3p, 2s 2 3d, 2s2p3s, 2s2p3p, 2s2p3d, 2p 2 3s, 2p 2 3p and 2p 2 3d configurations. Electron impact excitation collision strengths and radiative transition probabilities are calculated for both Mg viii models, compared with existing data, and the best model selected to generate a set of theoretical emission line intensities. The EUV lines, covering 312‐790 A, are compared with existing solar spectra (SERTS‐89 and SUMER), while the soft X-ray transitions (69‐97 A) are examined for potential density diagnostic line ratios and also compared with the limited available solar and stellar observational data. Methods. The R-matrix codes Breit-Pauli RMATRXI and RMATRXII are utilised, along with the PSTGF code, to calculate the collision strengths for two Mg viii models. Collision strengths are averaged over a Maxwellian distribution to produce the corresponding e ective collision strengths for use in astrophysical applications. Transition probabilities are also calculated using the CIV3 atomic structure code. The best data are then incorporated into the modelling code CLOUDY and line intensities generated for a range of electron temperatures and densities appropriate to solar and stellar coronal plasmas. Results. The present e ective collision strengths are compared with two previous calculations. Good levels of agreement are found with the most recent, but there are large di erences with the other for forbidden transitions. The resulting line intensities compare favourably with the observed values from the SERTS-89 and SUMER spectra. Theoretical soft X-ray emission lines are presented and several density diagnostic line ratios examined, which are in reasonable agreement with the limited observational data available.

New technique for aberration diagnostics and alignment of an extreme ultraviolet Schwarzschild objective

Schwarzschild objectives are widely used in the extreme ultraviolet (EUV)/soft X-ray spectral region both as reduction and magnification optics, e.g. for small-field projection lithography and microscopy, respectively. When using a Schwarzschild objective as a micro-exposure tool (MET) at high spatial resolution (half-pitch≤0.1μm), in addition to the tight requirements on the design and surface figure for the single optics, also an accurate alignment between the two mirrors is needed to reach the planned spatial imaging detail. Ideally, at-wavelength alignment should be done in order to overcome limitations due to diffractive effects. While this can be easily performed on synchrotron beam lines, it becomes time expensive (and components consuming) on low-power laboratory plasma sources. In this work we propose and test a new technique to align a EUV Schwarzschild objective by means of ultraviolet light. The aligned objective allowed the attainment of lithographic patterning with edge response of 90nm, as part of the laboratory-scale MET for EUV projection lithography realized at the ENEA Frascati Research Centre.

Influence of the metallic contact in extreme-ultraviolet and soft x-ray diamond based Schottky photodiodes

X-ray and UV photovoltaic Schottky photodiodes based on single crystal diamond were recently developed at Rome “Tor Vergata” University laboratories. In this work, different rectifying metallic contact materials were thermally evaporated on the oxidized surface of intrinsic single crystal diamond grown by chemical vapor deposition. Their impact on the detection performance in the extreme UV and soft x-ray spectral regions was studied. The electrical characterization of the metal/diamond Schottky junctions was performed at room temperature by measuring the capacitance–voltage characteristics. The diamond photodiodes were then tested both over the extreme UV spectral region from 10 to 60 eV by using He-Ne DC gas discharge as a radiation source and toroidal vacuum monochromator, and in the soft x-ray range from 6 to 20 keV at the Diamond Light Source synchrotron x-ray beam-line in Harwell (UK). In both experimental setups, time response and spectral responsivity were analyzed for all the investigated Schottky contact materials. A good agreement between the experimental data and theoretical results from Monte Carlo simulations is found

Multilayer X-ray mirrors based on La/B4C and La/B9C

Multilayer X-ray mirrors that are based on La/B4C and La/B9C and intended for the reflection of X-ray radiation in the spectral region near λ = 6.7 nm are prepared and studied. Reflection coefficients at a level of 40–60% are achieved for mirrors with various periods. The difference in the interlayer roughnesses reconstructed from the data measured in the hard and soft X-ray spectral regions is explained using a structural model with an asymmetric permittivity profile in a mirror period. A proximate technique is developed to estimate the permittivity profile in a multilayer-structure period using reflectometry data. The effect of antidiffusion Sn, Cr, and Mo barriers on the reflection coefficient of multilayer La/B4C structures is studied experimentally

Multilayer thin-film filters of extreme ultraviolet and soft X-ray spectral regions

Multilayer thin-film filters from different materi al pairs with spectral windows within the wavelength range 7n < 60 nm were developed. Optical characteristics of samples in the extreme ultraviolet, visible, and IR spectrum ranges were studied. Ultrathin freestanding Zr/Si large-aperture filters with transparencies up to 76% at 7n = 13 nm were manufactured for projection lithography test benches; variations in their properties during long-term thermal loads were studied.

Nanofabrication of diffractive elements for soft x-ray and extreme ultraviolet applications using ion beam lithography

In the soft x-ray (SXR) and extreme ultraviolet (EUV) spectral region, optical elements are mainly based on diffractive structures. We report on experiments showing the fabrication of such elements using ion beam lithography. This allows single-step milling of freestanding transmission as well as reflection gratings. Minimum structure sizes of 70 nm were achieved. The optical elements were tested in slit grating spectrographs at an ethanol-jet laser-induced plasma source. Relative spectral resolutions of λ/Δλ≈100 were observed in the SXR- and EUV-region.

Adaptive spatially resolving detector for the extreme ultraviolet with absolute measuring capability

A spatially resolving detector for the extreme ultraviolet (XUV) and soft x-ray spectral region is presented. Principle of operation is conversion of XUV radiation to visible light by a scintillator crystal. Luminescence is detected using charge coupled device camera and imaging optics. Single layer and multilayer coatings are applied to match the system to different spectral regions of interest. Field of view and spatial resolution can be adapted to the application. Calibration of the system enables to absolutely measure in-band radiation flux on the scintillator. The setup is designed for the characterization and optimization of XUV sources and XUV optical systems. Measurements, carried out to characterize the focus in a soft x-ray microscope, are presented as an application example.

Dissociation of the benzene molecule by ultraviolet and soft X-rays in circumstellar environment

Benzene molecules, present in the proto-planetary nebula CRL 618, are ionized and dissociated by ultraviolet (UV) and X-ray photons originated from the hot central star and by its fast wind. Ionic species and free radicals produced by these processes can lead to the formation of new organic molecules. The aim of this work is to study the photoionization and photodissociation processes of the benzene molecule, using synchrotron radiation and time-of-flight mass spectrometry. Mass spectra were recorded at different energies corresponding to the vacuum UV (21.21 eV) and soft X-ray (282–310 eV) spectral regions. The production of ions from the benzene dissociative photoionization is here quantified, indicating that C6H6 is more efficiently fragmented by soft X-ray than UV radiation, where 50 per cent of the ionized benzene molecules survive to UV dissociation while only about 4 per cent resist to X-rays. Partial ion yields of H+ and small hydrocarbons, such as C2H+2, C3H+3, C4H+2, are determined as a function of photon energy. Absolute photoionization and dissociative photoionization cross-sections have also been determined. From these values, half-life of benzene molecule due to UV and X-ray photon fluxes in CRL 618 was obtained.

Atomic hydrogen under strong soft X-ray pulses

We discuss theoretically the behavior of atomic Hydrogen under irradiation with strong light pulses in the soft X-ray spectral region. The method consists in the direct numerical solution of the time dependent Schrodinger equation. We find ranges of (high) peak incident intensity I0, where 2-photon absorption becomes more probable than 1-photon absorption. At very high intensity, the total ionization probability goes very close to 1 and then decreases as I0 is further increased.

Optical polarizer integrated with suppression of higher harmonics in the vacuum ultraviolet and soft x-ray spectral regions

Based on a concept of Au–Si–Au triple reflection, we present a low pass polarizer that suppresses higher-order harmonics from vacuum ultraviolet and soft x-ray monochromators. By appropriate optimization of the material and the angle of incidence, the polarizer enables the possibility to tune its cutoff edge of photon energy, and therefore to suppress higher-order diffraction light. Vacuum ultraviolet ellipsometry experiments with BESSY II synchrotron radiation in 9–23eV have demonstrated the suppression of higher-order harmonics.

Excited Ions in Intense Femtosecond Laser Pulses: Laser-Induced Recombination

Electron-ion recombination in a laser-induced electron recollision is of fundamental importance as the underlying mechanism responsible for the generation of high-harmonic radiation and hence for the production of attosecond pulse trains in the extreme ultraviolet and soft x-ray spectral regions. By using an ion beam target, remotely prepared to be partially in long-lived excited states, the recombination process has for the first time been directly observed and studied.

An investigation of Fe XVI emission lines in solar and stellar extreme-ultraviolet and soft X-ray spectra

New fully relativistic calculations of radiative rates and electron impact excitation cross-sections for Fe XVΙ are used to determine theoretical emission-line ratios applicable to the 251-361 and 32-77 A portions of the extreme-ultraviolet (EUV) and soft X-ray spectral regions, respectively. A comparison of the EUV results with observations from the Solar Extreme-Ultraviolet Research Telescope and Spectrograph (SERTS) reveals excellent agreement between theory and experiment. However, for emission lines in the 32A 9 A portion of the soft X-ray spectral region, there are large discrepancies between theory and measurement for both a solar flare spectrum obtained with the X-Ray Spectrometer/Spectrograph Telescope (XSST) and for observations of Capella from the Low-Energy Transmission Grating Spectrometer (LETGS) on the Chandra X-ray Observatory. These are probably due to blending in the solar flare and Capella data from both first-order lines and from shorter wavelength transitions detected in second and third order. By contrast, there is very good agreement between our theoretical results and the XSST and LETGS observations in the 50-77 A wavelength range, contrary to previous results. In particular, there is no evidence that the Fe XVI emission from the XSST flare arises from plasma at a much higher temperature than that expected for Fe xvi in ionization equilibrium, as suggested by earlier work.

Design and performance of a tunable polarizer for VUV and soft x-ray spectral regions

A low pass polarizer is presented for suppressing higher order diffraction light from vacuum ultraviolet and soft x-ray monochromators. By optimization of materials combination and angle of incidence, the polarizer demonstrates the capability to match to the desired photon energy cut-off. With the proposed hybrid polarizer, a higher order suppression unit, such as a thin film filter, gas filter or double reflecting mirror, is no longer required. The filtering effect of higher harmonics is integrated within the proposed polarizer.

Tunable thin film polarizer for the vacuum ultraviolet and soft x-ray spectral regions

A low pass polarizer that suppresses higher-order diffraction light from vacuum ultraviolet and soft x-ray monochromators is presented in this paper. This vacuum ultraviolet and soft x-ray polarizer is based on a concept of sandwiched metal-dielectric-metal triple reflection configuration. By appropriate optimization of material and angle of incidence, the proposed Au–SiC–Au polarizer demonstrates the capability of matching to desired cutoff edge of photon energy. Furthermore, the optimized soft x-ray polarizer shows the possibility to tune cutoff photon energy in a broadband spectral region ranging from 80 down to down to 20eV.

Characteristics of focused soft X-ray free-electron laser beam determined by ablation of organic molecular solids

A linear accelerator based source of coherent radiation, FLASH (Free-electron LASer in Hamburg) provides ultra-intense femtosecond radiation pulses at wavelengths from the extreme ultraviolet (XUV; lambda<100nm) to the soft X-ray (SXR; lambda<30nm) spectral regions. 25-fs pulses of 32-nm FLASH radiation were used to determine the ablation parameters of PMMA - poly (methyl methacrylate). Under these irradiation conditions the attenuation length and ablation threshold were found to be (56.9+/-7.5) nm and approximately 2 mJ*cm(-2), respectively. For a second wavelength of 21.7 nm, the PMMA ablation was utilized to image the transverse intensity distribution within the focused beam at mum resolution by a method developed here.

Technical Report: Metrology and Test Beamline at SOLEIL

We propose to build at SOLEIL a facility for the accurate measurement of the properties of optical elements and detectors from the extreme ultraviolet (EUV), soft X-ray spectral regions up to the hard X-rays. The beamline will serve as a standard for such measurements and hence we call it a calibration and standards beamline, or alternatively, a metrology beamline. This beamline will also be valuable as a general-purpose beamline to prepare, test and set up a wide range of experiments. A complementary important aspect of this installation is the realization of a primary standard (as in BESSY II) [1]: in this case, the metrology beamline of SOLEIL could become the national primary standard source in collaboration with the Laboratoire National d'Essais (LNE) partner of this project. In another field, this installation will be used in the design and characterization of several diagnostics for the Megajoule Laser in Bordeaux in collaboration with the CEA DIF, also a partner of this project.

Soft X‐Ray Emission Lines of Fexvin Solar Flare Observations and theChandraSpectrum of Capella

Recent calculations of atomic data for Fe XV have been used to generate theoretical line ratios involving n = 3-4 transitions in the soft X-ray spectral region (~52-83 A), for a wide range of electron temperatures and densities applicable to solar and stellar coronal plasmas. A comparison of these with solar flare observations from a rocket-borne spectrograph (X-Ray Spectrometer/Spectrograph Telescope [XSST]) reveals generally good agreement between theory and experiment. In particular, the 82.76 A emission line in the XSST spectrum is identified, for the first time to our knowledge in an astrophysical source, as the 3s 3d 3D3-3s 4p 3P2 transition of Fe XV. Most of the Fe XV transitions that are blended have had the species responsible clearly identified, although there remain a few instances in which this has not been possible. The line ratio calculations are also compared with a co-added spectrum of Capella obtained with the Chandra satellite, which is probably the highest signal-to-noise ratio observation achieved for a stellar source in the ~25-175 A soft X-ray region. Good agreement is found between theory and experiment, indicating that the Fe XV lines are reliably detected in Chandra spectra and hence may be employed as diagnostics to determine the temperature and/or density of the emitting plasma. However, the line blending in the Chandra data is such that individual emission lines are difficult to measure accurately, and fluxes may only be reliably determined via detailed profile fitting of the observations. The co-added Capella spectrum is made available to hopefully encourage further exploration of the soft X-ray region in astronomical sources.

Recombination and population inversion in plasmas generated by tunneling ionization

Above-threshold ionization (ATI) ionization by linearly polarized light has been proposed by several authors as a means of driving recombination lasers in the soft x-ray spectral region. The pump radiation generates a cold electron plasma with ions in a single ionization stage, which is an ideal starting condition for strong recombination. Population inversions form during the recombination cascade to the ground state of the next ionization stage. In the absence of any relaxation the electron distribution is strongly peaked near zero energy. However, a number of different processes all heat the cold electrons towards Maxwellian, and may thereby reduce the recombination rate in the higher levels. Using numerical models we investigate these relaxation processes and their effect on recombination. We show that the recombination can be well described by the standard cascade model, provided an appropriate temperature is used. We examine two cases in detail, hydrogen-like lithium where the inversion is with respect to the ground state, and lithium-like nitrogen where it is with the first excited state. The two cases differ markedly in the degree of relaxation achieved, and in the duration of the population inversion.

Deep-Ultraviolet Quantum Interference Metrology with Ultrashort Laser Pulses

Precision spectroscopy at ultraviolet and shorter wavelengths has been hindered by the poor access of narrow-band lasers to that spectral region. We demonstrate high-accuracy quantum interference metrology on atomic transitions with the use of an amplified train of phase-controlled pulses from a femtosecond frequency comb laser. The peak power of these pulses allows for efficient harmonic upconversion, paving the way for extension of frequency comb metrology in atoms and ions to the extreme ultraviolet and soft x-ray spectral regions. A proof-of-principle experiment was performed on a deep-ultraviolet (2 x 212.55 nanometers) two-photon transition in krypton; relative to measurement with single nanosecond laser pulses, the accuracy of the absolute transition frequency and isotope shifts was improved by more than an order of magnitude.