This PDF file contains the front matter associated with SPIE Proceedings Volume 8077, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

This manuscript presents a first study of the contamination observed on some of the x-ray mirrors for the Linac Coherent Light Source (LCLS) free-electron laser, the implications to the mirror lifetime properties and an evaluation of candidate techniques towards successful recovery of these B₄C- and SiC-coated mirrors. Initial experimental results and plans for upcoming mirror recovery experiments are discussed. A summary of experimentally determined FEL damage thresholds of B4C and SiC materials is also given, and their wavelength dependence is discussed.

FERMI@Elettra is a new free-electron-laser (FEL) facility, presently under commissioning, able to generate subpicosecond photon pulses of high intensity in the far ultraviolet and soft X-ray range (λ=100-20 nm for the present FEL1 source, extended in future to 4 nm with the FEL2 source). Here we briefly describe the present status of the TIMEX end-station, devoted to perform experiments on condensed matter under extreme conditions. The layout of the end-station, presently in the final stages of construction, is reported showing the details of the optics and sample environment. The potential for transmission, reflection, scattering, as well as pump-and-probe experiments is discussed taking into account that FEL pulses can heat thin samples up to the warm dense matter (WDM) regime. The calculated deposited energy in selected elemental films, including saturation effects, shows that homogeneous heating up to very high temperatures (1-10 eV for the electrons) can be easily reached with a suitable tuning of the energy and focus of the soft x-ray pulses of FERMI@Elettra. The results of the first test of the TIMEX end-station are also reported.

Powerful free electron lasers (FELs) operating in the soft X-ray regime are offering new possibilities for creating and probing materials under extreme conditions. We describe here simulations to model the interaction of a focused FEL pulse with metallic solids (niobium, vanadium, and their deuterides) at 13.5 nm wavelength (92 eV) with peak intensities between 10¹⁵ to 10¹⁸ W/cm² and a fixed pulse length of 15 femtoseconds (full width at half maximum). The interaction of the pulse with the metallic solids was modeled with a non-local thermodynamic equilibrium code that included radiation transfer. The calculations also made use of a self-similar isothermal fluid model for plasma expansion into vacuum. We find that the time-evolution of the simulated critical charge density in the sample results in a critical depth that approaches the observed crater depths in an earlier experiment performed at the FLASH free electron laser in Hamburg. The results show saturation in the ablation process at intensities exceeding 10¹⁶ W/cm². Furthermore, protons and deuterons with kinetic energies of several keV have been measured, and these concur with predictions from the plasma expansion model. The results indicate that the temperature of the plasma reached almost 5 million K after the pulse has passed.

We discuss the dynamics of a silicon surface after incidence of a short, high energy pulse in the soft X-ray range. We focus on time-delays long enough after pulse incidence, so that the absorbed energy can be seen as a nonuniform time-dependent heat distribution in the solid. A model is developed using techniques of non-equilibrium hydro-thermodynamics, considering just the longitudinal and transverse acoustic phonon systems in the excited solid. The general theory leads to Maxwell-Cattaneo partial differential equations for the material medium n(r,t) and the energy h(r,t) volume densities; these reduce to the diffusion equation for the temperature T(r,t) and the usual thermo-mechanical elastic equation for the strain u(r,t) on further simplification. Here we solve the Maxwell-Cattaneo equation for T(r,t) and compare to previous results where the diffusion equation was used instead; the Maxwell- Cattaneo equation predicts faster cooling at short (dozens of fs, say) time delays. Previously obtained results for the strain field are briefly recalled.

Zone plates are circular diffraction gratings that can provide diffraction-limited nano-focusing of x-ray radiation. When designing zone plates for X-ray Free Electron Laser (XFEL) sources special attention has to be made concerning the high intensity of the sources. Absorption of x-rays in the zone material can lead to significant temperature increases in a single pulse and potentially destroy the zone plate. The zone plate might also be damaged as a result of temperature build up and/or temperature fluctuations on longer time scales. In this work we simulate the heat transfer in a zone plate on a substrate as it is exposed to XFEL radiation. This is done in a Finite Element Method model where each new x-ray pulse is treated as an instantaneous heat source and the temperature evolution between pulses is calculated by solving the heat equation. We use this model to simulate different zone plate and substrate designs and source parameters. Results for both the 8 keV source at LCLS and the 12.4 keV source at the European XFEL are presented. We simulate zone plates made of high Z metals such as gold, tungsten and iridium as well as zone plates made of low Z materials such as diamond. In the case of metal zone plates we investigate the influence of substrate material by comparing silicon and diamond substrates. We also study the effect of different cooling temperatures and cooling schemes. The results give valuable indications on the temperature behavior to expect and can serve as a basis for future experimental investigations of zone plates exposed to XFEL radiation.

Infrared (IR) radiometry and time resolved reflectivity (TRR) methods can be used for investigation of laser pulse effects on materials in nanosecond time scale. The methods in combination are capable to quantify object temperature and detect phase transformations in the solid state, melting and plasma formation from vapour. Measurements with different laser pulse energy densities provide threshold of the transformation. The melt duration can be also determined. The experimental system is described. It contains KrF excimer laser with homogenizer and variable attenuator, fast IR detector for radiometry, continuous probing laser with Si photodiode for reflectivity measurement and UV detector for pump laser pulse reflection measurement. The system was applied to investigation of responses to laser light of silicon and different pure metals and alloys. The range of energy densities used was 1-5500 mJ.cm^-2 and measurements were done with temporal resolution of 6 ns for radiometry and 1 ns for reflectivity.

The next SOLO (SOLar Orbiter) mission will carry onboard the METIS (Multi Element Telescope for Imaging and Spectroscopy) instrument which will perform broad-band and polarized imaging of the visible K-corona and narrow-band imaging of the UV (HI Ly α, 121.6 nm) and EUV (He II Ly α, 30.4 nm) corona as well as in the visible spectral range. Several multilayer optics with high reflectivity in the all ranges of interest have been studied. Since SOLO will fly at the short distance from the Sun of 0.23 AU at its perihelion, a careful determination of the heat load and the solar wind effect on the multilayers must be carried in order to check if degradation occurs. To test thermal stability, a thermal analysis experiment has been conceived: the proposed multilayer structures, which are based on different pairs of materials and different capping layers design, must be subjected both to heating and cooling, reproducing the temperatures experienced in orbit. Reflectance in the EUV range of interest has been measured before and after each treatment to verify possible degradation.

We have investigated responses of PDMS, PMMA and acrylic block copolymers (BCP) to EUV light from laserproduced plasma beyond ablation thresholds and micromachining. We generated wide band EUV light around 100 eV by irradiation of Ta targets with Nd:YAG laser light. In addition, narrow band EUV light at 11 and 13 nm were generated by irradiation of solid Xe and Sn targets, respectively, with pulsed CO₂ laser light. The generated EUV light was condensed onto samples, using an ellipsoidal mirror. The EUV light was incident through windows of contact masks on the samples. We found that through-holes with a diameter of 1 μm can be fabricated in PDMS sheets with thicknesses of 10 μm. PDMS sheets are ablated if they are irradiated with EUV light beyond a threshold power density, while PDMS surfaces were modified by irradiation with the narrow band EUV light at lower power densities. Effective ablation of PMMA sheets can be applied to a LIGA-like process for fabricating micro-structures of metals using the practical apparatus. Furthermore, BCP sheets were ablated to have micro-structures. Thus, we have developed a practical technique for microma chining of PMMA, PDMS and BCP sheets in a micrometer scale.

In this work results of investigations concerning ablation and surface modification of polymers and some other solids using a laser-plasma EUV source are presented. The plasma radiation was produced using a gas puff target and was focused with a gold-plated grazing incidence ellipsoidal collector. The ablation process was investigated using a scanning electron microscope (SEM) and a quadrupole mass spectrometer (QMS). The chemical changes were investigated by X-ray photoelectron spectroscopy (XPS). Different kinds of micro- and nanostructures created in nearsurface layers of the materials were obtained. Forms of the structures depend on a particular material and the EUV exposure. In case of some polymers even a single shot was sufficient for creation of the visible changes in surface morphology. In case of inorganic solids visible changes required usually the exposure with tens or hundreds of EUV pulses. XPS investigations revealed chemical changes in near surface layers of polymers. Significant differences were revealed in the XPS spectra acquired for irradiated and not-irradiated polymers. Significant decrease of functional groups containing oxygen was indicated. Analysis of QMS spectra indicate emission of different kinds of fragments of the polymer chains including the repeating structural units. In case of some polymers only fragments of the repeating unit were detected.

Extreme ultraviolet (EUV) light at 13.5 nm is a candidate for advanced optical lithography where sub-32 nm features will spawn the next generation of microchips. Plasma-based Sn sources have been used to generate EUV light, but debris contamination and damage limits the life expectancy of plasma-facing collector optics. Although mitigation systems reduce the amount of vapor, thermal and energetic particles still reach mirror surfaces. Three Rh mirror samples were exposed to thermal Sn (14.7-nA) and energetic 1 keV Xe+ (12-nA, 120-nA, and 1.2-uA) particles for 36 minutes and three Ru samples were exposed to thermal Sn (14.7-nA) and energetic Xe+ (100eV, 500eV, and 1000eV). In-situ measurements of EUV reflectivity and surface composition were correlated with ex-situ morphology investigations. For the Rh mirrors, a combination of 14.7-nA thermal Sn and 1.2-uA energetic Xe⁺ reduced mirror reflectivity by only 18.7%, as opposed to the 48.5% and 41.7% decreases observed with 120-nA and 12-nA energetic Xe⁺, respectively. For the Ru mirrors, the higher 1000eV Xe⁺ irradiation lowered the reflectivity by 41.6%, while the 500eV and 100eV energy decreased the reflectivity by 44.9% and 51.4% respectively. In-situ low-energy ion scattering spectroscopy (LEISS) monitored the Sn surface fraction. For Rh samples irradiated with 120-nA and 12-nA energetic Xe⁺, the fraction was 1 throughout the experiment. The third sample (1.2-uA) initially had a 0.71 surface fraction, which increased towards 1, dropped to 0.48 after 18 minutes, and then continued rising to 0.8 after 36 minutes. The drop suggests a threshold, where perhaps surface structure abruptly changed, that was too high for the other samples to cross. The results imply that at certain fluences, debris bombardment may not be as detrimental to collector mirror EUV reflectivity as at others. For the Ru samples, the Sn surface fraction jumped to 100% for the sample irradiated at 100eV Xe⁺, to 85% for the 500eV case, and 75% for the 1keV case. The reflectivity was worse with the 100eV sample, with a drop of 51.4%, while the 500eV and 1keV had drops of 44.9% and 41.6%, respectively. Morphology analyses with an atomic force microscope at Purdue University's Radiation Surface and Interface Science Laboratory revealed that the morphology of the mirror surface plays a key role in the reflectivity performance of the Rh and Ru mirrors.

Intense soft X-ray illumination near the carbon core edge induces significant structural damage in tetrahedral amorphous carbon films and less significant but detectable defect formation in single-walled carbon nanotubes (SWNTs). The efficiency spectra of the photo-induced damage commonly show a resonant peak between the π* and σ* peaks of X-ray absorption spectra in addition to a non-resonant background. The resonant damage cannot be explained by the coreexciton mechanism but can be attributed to a recoil damage accompanying the photo-desorption of heavy chemisorbates that is resonantly decomposed by the spectator Auger mechanism. The cause of the non-resonant damage in SWNTs may be attributed to anomalous radiation damage by low-energy electrons generated by secondary effects of soft X-ray illumination, which is supported by the recent finding by authors that hot electron injection from probe tips of scanning tunneling microscopes generates defects in the SWNT samples.

Scanning photoemission spectromicroscopy is a powerful tool for the chemical characterization of materials based on third generation synchrotron light sources. Due to the X-ray focalization properties it represents also an extremely bright source of radiation ideal for the investigation of radiation effects on matter. Despite the experiments involving inorganic materials, such as oxides, lead in the last decade have already shed light on the potentials of this techniques to perform lateral resolved chemical analysis, still there is a lack of information concerning the possibilities to measure organic compounds. This paper reports on the investigation of inorganic (Rh oxide thin films) and organic (polymers) samples performed with the scanning photoemission microscope hosted at the Elettra Synchrotron Light Laboratory. Results show that the typical photon densities in the focused spot of the microscope allow the investigation of the oxide films but produce an immediate damaging of the polymeric film.

The recent development of intense sources in the XUV range (10-100 nm), such as X-ray laser, Free Electron Laser and High order Harmonics (HoH), allows the study of high flux processes and ultra-fast dynamics in various domains. At the SLIC facility of CEA-Saclay, we have built a gas-harmonic beamline to investigate the interaction of intense XUV pulse with solids. High Harmonics of an IR laser (Ti:Sa at 800 nm, 35 fs, 13 mJ/pulse, 1 kHz) are generated in a rare gas cell (Xe). The useful XUV range (40-60 nm) is selected with metallic filters. The harmonic beam is focused with a parabolic mirror to a 10 μm focal spot on sample, leading to a fluence per shot of up to 1 mJ/cm² (within a typical 10 fs pulse duration). Studies aimed at understanding the damaging mechanisms caused by XUV irradiation on surface of various samples by systematically varying of fluence and exposure time. For PMMA irradiated in the desorption regime (fluence/shot ≤ 0.2 mJ/cm2), the surface presents craters whose profile depends on the dose (Grey [Gy] = 1 J/kg). The crater evolution proceeds from the competition between two main degradation processes, that is chain scission and cross linking. Namely, at low dose (≤ 1 GGy) polymer chain scission is followed by the blow up of the volatile, molecular fragments, forming the crater. At high dose (> 10 GGy) the broken chain-ends, in the near-surface layer of the remaining material, recombine by cross-linking, opposing desorption by surface hardening. In a recent experiment at LCLS FEL facility, PMMA was irradiated at high fluence; the cross-linking signature was identified from Raman spectroscopy. A kinetic model could be adapted for interpreting these original and very promising results.

The degradation mechanisms are a critical issue if multilayers are used as monochromators for white beam synchrotron applications. To quantify the radiation impact x-ray reflectivity measurements before, during, and after white beam exposure were performed. For the in-situ irradiation study a versatile vacuum chamber was developed and tested using a high power undulator source. The device is equipped with a cooling system for the multilayer samples to distinguish thermal effects from pure radiation induced ones. The x-ray reflectivity was measured at fixed angle of incidence in an energy dispersive mode and as a function of time. The energy dispersive detection allows for the simultaneous observation of the multilayer reflectivity spectrum over a wide range. The white beam study includes various long-term exposures with an incoming load up to 250 W. Ex-situ x-ray reflectivity measurements and beam imaging were carried out with monochromatic radiation at 8 keV before and after the white beam exposure. TEM analysis provides complementary information on the layer structure in the stack. Depending on the material system, the total radiation dose, and the sample environment, different degrees of modifications in the multilayer structure were observed.

The monochromatic zone plate focused soft X-rays of scanning transmission X-ray microscopes (STXM) can be used to directly write patterns in common photoresists, analogous to lithography with a focused electron or ion beam. A radiation damage spreading phenomenon when patterning with high doses was recently determined to be due to the point spread function of the optical system (Leontowich et al., Applied Physics A: Materials Science and Processing 103, 1 (2011)). We have used this phenomenon to measure the point spread function of three different STXMs by making a series of single pixel exposures in a photoresist at focus over a controlled dose range. Our results suggest this measurement is sensitive to zone plate aberrations; thus, it could be valuable feedback for optimizing zone plate fabrication schemes and STXM performance.

In solids under irradiation with femtosecond laser pulses, photoabsorption produces a strongly nonequilibrium highly energetic electrons gas. We study theoretically the ionization of the electronic subsystem of either a semiconductor (silicon) or a metal (aluminum) target, exposed to an ultra-short laser pulse (pulse duration ~10 fs) of VUV-XUV photons. We developed a numerical simulation technique, based on the classical Monte-Carlo method, to obtain transient distributions of electrons within conduction band. We extend the Monte-Carlo method in order to take into account quantum effects such as the electronic band structure, Pauli's exclusion principle for electrons in the conduction band and for holes within the valence band (for semiconductors), and free-free electron scattering (for metals). In the presented work, the temporal distribution of the energy density of excited and ionized electrons were calculated. The transient dynamics of electrons is discussed regarding the differences between semiconductors and metals. It is demonstrated that for the case of semiconductors, since a part of the energy is spent to overcome ionization potentials, the final kinetic energy of free electrons at the end of the laser pulse is much less than the total energy provided by the laser pulse. In contrast, for metals all the energy is present as kinetic energy in the electronic subsystem, unless the photon energy is greater that an ionization potential of a deep atomic shell. In the latter case, a part of the energy is shortly kept by deep-shell holes, and is released back to the electrons by Auger-processes on femtosecond timescales.

Point defects strongly influence optical properties of synthetic amorphous silica (synthetic a-SiO₂) used in excimer laser photolithography and their properties are intensively studied. Decomposition of an Si-O-Si bond into a pair of oxygen vacancy and interstitial oxygen species is an intrinsic defect process in a-SiO₂. It is similar to the creation of vacancy-interstitial pairs in crystalline materials and is regarded as "Frenkel defect process" in an amorphous material. Oxygens are also known to be emitted from a-SiO₂ surfaces under irradiation with vacuumultraviolet (VUV) light or electron beam. However, the anion part of the Frenkel pair in a-SiO₂, interstitial oxygen atom, lacks reliable spectroscopic signatures. Therefore, Frenkel process has been studied much less than another intrinsic defect process in a-SiO₂, a simple cleavage of an Si-O bond, yielding a pair of silicon and oxygen dangling bonds. Interstitial oxygen molecule (O2), a common form of the interstitial oxygen species in a-SiO₂, exhibits characteristic infrared photoluminescence (PL) at 1272 nm. This PL band allows interstitial O₂ to be detected selectively with a high sensitivity, and is useful in studying Frenkel defect processes in both a-SiO2 and crystalline SiO₂. The Frenkel process is dominant over the formation of the dangling bond pairs in highpurity synthetic a-SiO₂. Both these processes are influenced by the degree of the structural disorder of a-SiO₂characterized by distribution of Si-O-Si angles. Fluorine doping promotes the structural relaxation and is useful in decreasing the concentration of "strained" Si-O-Si bonds, which have Si-O-Si bond angles widely different from the relaxed angle and are vulnerable to radiation. Moderate fluorine doping is effective in improving both UV-VUV transparency and radiation hardness, whereas heavy fluorine doping tends to enhance defect processes involving the Frenkel mechanism and to degrade the radiation hardness.

We investigate the effects of VUV and UV radiation on a number of low-k dielectric films. Two different systems were used to investigate the effects: (1) a synchrotron radiation system as a pure VUV radiation source, and (2) an electron-cyclotron resonance (ECR) plasma system as a plasma source (VUV plus ions). Using the synchrotron-radiation system, we find VUV causes trapped charge accumulation within low-k dielectric films by electron depopulation from the defect states. For organosilicate glass (SiCOH) , the defect states were located 1 eV above the valence band of the dielectric. A model was developed to calculate in-situ charge accumulation. Trapped charges can be depleted with UV-lamp exposure. We examined the use of different energy barriers between layers so that less charge will be accumulated. Using the ECR plasma system, the photon effects can be separated from charged particle effects using a capillary-array window to cover the low-k dielectric films so that only photons from plasma can reach the dielectric. Photon fluence from the plasma can be predicted with a model and measured with a VUV monochromator. Photons from the plasma were shown to be responsible for trapped-charge accumulation within the dielectric, while it was found that ions bombardment results primarily on charge on the surface of the dielectrics.

We demonstrated two high-throughput beam splitters for high-order harmonics in soft-x-ray region, an extremely-flat free-standing thin-film filter and a high-damage-threshold reflective beam splitter. The Al thin-film filter, which is coated with a SiC film to increase its oxidation tolerance, has a transmittance of 29% at 30 nm and the Si plate beam splitter set at Brewster's angle with respect to the pump wavelength has a reflectivity of 56% at 30 nm.

Excitation density effects have a pronounced influence on relaxation processes in solids. They come into play in scintillating and dosimetric materials exposed to ionizing radiation or in laser materials operating in intense ultraviolet light fields. The scientific understanding of the underlying process is poor, mainly because most of the studies of light emitting materials under short wavelength excitation have been performed at weak and moderate excitation intensities due to limited availability of powerful light sources. Disembodied data on excitation density effects have been reported for wide-gap dielectrics studied by luminescence spectroscopy, by using such excitation sources as powerful ion beams,^1,2pulsed electron beams,^2,3 and wide-band hard X-ray synchrotron radiation.4 It is obvious that such non-selective excitation is a good tool for revealing density-related phenomena in these materials in general, but for investigating specific features of relaxation processes in insulators, light sources with well defined parameters are necessary. Since the shortwavelength free electron laser (FEL) technology has been devised by an international consortium at HASYLAB of DESY, resulting in the development of TESLA Test facility (TTF)5 and later in the construction of a dedicated FEL source FLASH in Hamburg,6 more advanced studies became possible. The range of interests towards this light source covers the fields from material science and various other branches of physics to structural biology. The pioneering luminescence study revealed excitation density effects in the decay of Ce³⁺ 5d-4f luminescence in Y3Al5O12 crystals and luminescence of BaF2 crystals in UV-visible range.7 These results motivated systematic investigations of excitation density effects in wide gap crystals using FEL8,9 and high-harmonic-generated VUV radiation,10 and, at lower energies, femtosecond laser pulses in the UV.^11,12 The main goal of the present work is to analyze the same phenomenon in wide-band gap BaF2 crystals, where luminescence centres of different origin (self-trapped excitons and cross-luminescence) are present. Using models developed for explaining the non-linear behaviour of luminescence and exciton-exciton interaction effects causing scintillator non-proportionalities,^10,13 simulations of luminescence decay curves are performed. Possible quenching effects in the cross-luminescence decay of BaF2 under XUV excitation have been analyzed by Terekhin et al.¹⁴

Ionizing radiation induces a variety of DNA damages including single-strand breaks (SSBs), double-strand breaks (DSBs), abasic sites, modified sugar and bases. Most theoretical and experimental studies have been focused on DNA strand scissions, in particular production of DNA double-strand breaks. DSBs have been proven to be a key damage at a molecular level responsible for the formation of chromosomal aberrations, leading often to cell death. The complexity of lesions produced in DNA by ionizing radiations is thought to depend on the amount of energy deposited at the site of each lesion. We have studied the nature of DNA damage induced directly by the pulsed 46.9 nm radiation provided by a capillary-discharge Ne-like Ar laser (CDL). Different surface doses were delivered with a repetition rate of a few Hz and an average pulse energy ~ 1 μJ. A simple model DNA molecule, i.e., dried closed-circular plasmid DNA (pBR322), was irradiated. The agarose gel electrophoresis method was used for determination of both SSB and DSB yields. Results are compared with a previous study of plasmid DNA irradiated with a single sub-nanosecond 1-keV X-ray pulse produced by a large-scale, double-stream gas puff target, illuminated by sub-kJ, near-infrared (NIR) focused laser pulses at the PALS facility (Prague Asterix Laser System).

As EUV lithography is on its way into production stage, studies of optics contamination and cleaning under realistic conditions become more and more important. Due to this fact an Exposure Test Stand (ETS) has been constructed at XTREME technologies GmbH in collaboration with Fraunhofer IOF and with financial support of Intel Corporation. This test stand is equipped with a pulsed DPP source and allows for the simultaneous exposure of several samples. In the standard set-up four samples with an exposed area larger than 35 mm² per sample can be exposed at a homogeneous intensity of 0.25 mW/mm². A recent update of the ETS allows for simultaneous exposures of two samples with intensities up to 1.0 mW/mm². The first application of this alternative set-up was a comparative study of carbon contamination rates induced by EUV radiation from the pulsed source with contamination rates induced by quasicontinuous synchrotron radiation. A modified gas-inlet system allows for the introduction of a second gas to the exposure chamber. This possibility was applied to investigate the efficiency of EUV-induced cleaning with different gas mixtures. In particular the enhancement of EUV-induced cleaning by addition of a second gas to the cleaning gas was studied.

Temperature stability of Mo/B4C multilayers with normal incidence peak reflectivity at 7 nm was investigated in a temperature range between 100°C and 900°C. With this multilayer pair we achieved up to 24% reflectivity in the asdeposited state at 6.7 nm. We investigated the effect of post-deposition annealing temperature and time on intrinsic stress, period and normal incidence reflectivity. We observed that stress alters almost linearly with temperature up to 600°C. In this temperature range the multilayer period expands by <1%. The major change in stress and period occurs in the first minutes of heat treatment. The stress relaxation is accompanied with volume and packing density increase (period expansion). This process, which happens within minutes, is followed by a diffusion controlled process.

The irradiation effects of multiple ultrafast shots of laser beams with estimated fluences of some tens of mJ/cm² on a EUV Mo/Si multilayer have been studied. Irradiation damage has been induced by multiple shots of two different lasers (100 fs 400 nm the first, 1.5 ns 46.9 nm the second). The study has been motivated by the need of multilayer Mo/Si optics for the delay lines of the FEL source FERMI@Elettra, where these mirrors will be used to reflect 100 fs pulses at 13 nm with a fluence of some mJ/cm². The analysis has been performed by means of different techniques as EUV and soft X-ray reflectivity, XPS, and Standing wave enhanced XPS. Simulations have been carried on by means of an indigenously developed software OPAL (Optical Properties of Anisotropic Layers) for the calculation of the absorbed energy by the stratified medium. AFM and SEM surface images have been also acquired. In the irradiation at 400 nm, we observed a significant change in the multilayer performance at fluences of 100 mJ/cm² and above with a significant reduction of reflectivity. Spectroscopic analysis allowed to correlate the decrease of reflectivity with the degradation of the multilayer stacking, ascribed to Mo-Si intermixing at the Mo/Si interfaces of the first few layers, close to the surface of the mirror. Preliminary tests have been also performed on the sample irradiated at 46.9 nm.

Before the first photon beam was delivered at the SOLEIL synchrotron, scientists tried to anticipate the problem of carbon contamination on optical components, with for instance the outgasing of chambers by prior exposure to the beam with dummy optics. In spite of these efforts, deterioration of optical performance by carbon contamination has remained an outstanding issue: on the low-energy beamlines at SOLEIL. For example, carbon contamination results in significant photons flux losses (practically at the Carbon K edges), and modifications of the horizontal-to-vertical polarization transmission ratio, which degrade with time as the thickness of the carbon layer builds up. This contamination is visible and consists of a gray/black line over the entire photon beam footprint. Addressing the carbon contamination issue, two cleaning processes have been tested quite successfully on two SOLEIL beamlines (in the UV-VUV and soft X-ray ranges), namely in-situ oxygen plasma and in-situ ozone generation via UV lamps. A dedicated group is currently working on the improvement of the cleaning processes, the metrology of the optics before and after cleaning and the study of the carbon coating in order to propose possible strategies to prevent or slow down the contamination process.

This paper is concerned with mapping the characteristics of blistering induced on Mo/Si multilayers as a result of irradiation by hydrogen species generated in a thermal capillary cracker. The nature and extent of the damage observed is dependent on exposure conditions such as the sample temperature, the hydrogen isotope used and the total fluence. Increasing the sample temperature leads to fewer but larger blisters. When D₂ is used as the working gas, blisters are ~5 times smaller in diameter than in the case of H₂ exposure, but more blisters are formed. Increasing the gas flow induces more and bigger blisters and blisters were observed to develop in two distinct size distributions.

Polydimethylsiloxane (PDMS) is fundamental materials in the field of biotechnology. Because of its biocompatibility, microfabricated PDMS sheets are applied to micro-reactors and microchips for cell culture. Conventionally, the microstructures were fabricated by means of cast or imprint using molds, however it is difficult to fabricate the structures at high aspect ratios such as through-holes/vertical channels. The fabrication of the high-aspect structures would enable us to stack sheets to realize 3D fluidic circuits. In order to achieve the micromachining, direct photo-ablation by short wavelength light is promising. In the previous works, we investigated ablation of transparent materials such as silica glass and poly(methyl methacrylate) induced by irradiation with laser plasma EUV light. We achieved smooth and fine nanomachining. In this work, we applied our technique to PDMS micromachining. We condensed the EUV light onto PDMS surfaces at high power density up to 108 W/cm2 using a Au coated ellipsoidal mirror. We found that PDMS sheet was ablated at a rate up to 440 nm/shot. It should be emphasized that through hole with a diameter of 1 μm was fabricated in a PDMS sheet with a thickness of 4 μm. Thus we demonstrated the micromachining of PDMS sheets using laser plasma EUV light.

We describe theoretically the interaction of an ultrashort VUV-XUV laser pulse (FWHM = 10fs, photon energy of 100eV) with liquid water. Incident photons ionize water molecules and create free electrons. These excited electrons interact via elastic collisions with other water molecules and produce secondary electrons due to impact ionization. To track each free electron and its collisions event by event, we use the Monte Carlo method. This approach allows us to describe the non-equilibrium behaviour of electrons in irradiated water on femtosecond timescales. As results we present the transient electron particle- and energy-distributions. Furthermore, we exhibit a time resolved description of the total amount of electrons and we also show the corresponding energy redistribution: change in the kinetic energy of excited electrons, increase of the energy of holes, and energizing of water molecules via elastic collisions.

The recent commissioning of a X-ray free-electron laser triggered an extensive research in the area of X-ray ablation of high-Z, high-density materials. Such compounds should be used to shorten an effective attenuation length for obtaining clean ablation imprints required for the focused beam analysis. Compounds of lead (Z=82) represent the materials of first choice. In this contribution, single-shot ablation thresholds are reported for PbWO₄ and PbI² exposed to ultra-short pulses of extreme ultraviolet radiation and X-rays at FLASH and LCLS facilities, respectively. Interestingly, the threshold reaches only 0.11 mJ/cm2 at 1.55 nm in lead tungstate although a value of 0.4 J/cm² is expected according to the wavelength dependence of an attenuation length and the threshold value determined in the XUV spectral region, i.e., 79 mJ/cm² at a FEL wavelength of 13.5 nm. Mechanisms of ablation processes are discussed to explain this discrepancy. Lead iodide shows at 1.55 nm significantly lower ablation threshold than tungstate although an attenuation length of the radiation is in both materials quite the same. Lower thermal and radiation stability of PbI₂ is responsible for this finding.

Single crystals of two fluorides (LiF and CaF2) and a tungstate (PbWO4) were irradiated by nanosecond pulses of 46.9- nm radiation provided by 10-Hz capillary-discharge Ne-like Ar laser (CDL). The damage threshold was determined in LiF using the CDL beam focused by a Sc/Si multilayer-coated spherical mirror. Irradiated samples have been investigated by Nomarski (DIC - Differential Interference Contrast) microscopy and optical (WLI - white light intereferometry) profiler. After an exposure by a certain number of CDL pulses, an ablation rate can be calculated from WLI measured depth of the crater created by the XUV ablation. Potential use of XUV ablation of ionic crystals in pulsed laser deposition (PLD) of thin layers of such a particular material, which is difficult to ablate by conventional UV-Vis- NIR lasers, is discussed in this contribution.