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Physical review applied 14(3), 034057 (2020). doi:10.1103/PhysRevApplied.14.034057, Solids irradiated by energetic photons can be eroded in two modes, depending on the radiation intensity. High average, low-peak power sources, e.g., synchrotron radiation and high-order harmonics, induce desorption of the material at a low etch rate. In contrast, high-peak-power radiation from extreme ultraviolet and x-ray lasers usually causes a massive removal of the material even by a single shot. In this contribution, an effective material erosion is reported in PMMA exposed to multiple accumulated pulses generated by the free-electron x-ray-laser Linac Coherent Light Source (LCLS, tuned at a photon energy of 830 eV in this study, operated in Menlo Park at Stanford, CA, USA) at a fluence below the single-pulse ablation threshold. The effect is caused by polymer-chain scissions initiated by single photons carrying enough energy to break the C$—$C bounds. High efficiency of the erosion is supposed to occur due to a correlation of the single-photon effects. The subthreshold damage exhibits a nonlinear dose dependence resulting from a competition between chain scissions and cross-linking processes. The cross-linking is proven by Raman spectroscopy of the irradiated polymer. Two theoretical models of the x-ray free-electron-laser-induced erosion are suggested, which provide an excellent agreement with the experimental results., Published by American Physical Society, College Park, Md. [u.a.]

Time-resolved luminescence spectroscopy has been widely used in studies of electronic excitations and their dynamics in insulators. Present paper discusses the potential of vacuum ultraviolet spectroscopy applied for wide band gap crystals using classical synchrotron radiation as well as more recently developed pulsed femtosecond short-wavelength light sources. Optical spectroscopy using novel free electron lasers and laser systems based on higher harmonic generation process allow detailed investigation of the interaction between electronic excitations and behaviour of solids under extreme radiation conditions. Significant changes in the quantum yield and emission decay kinetics as well as surface damage of single crystals were observed for CdWO4 and CaWO4 crystals studied in the temperature range of 8-300 K. However, in the range of excitation densities applied, up to ∼1012 W/cm2, the nature of luminescence centres and their characteristic emission spectra remained similar to those observed at low excitation densities. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Nous presentons les resultats d'etudes de luminescence resolue en temps emise par des cristaux sous excitation par des photons VUV issus de la generation d'harmoniques d'ordre eleve. L'utilisation d'un laser Ti-Sa PLFA a Saclay, qui delivre des impulsions d'une energie de 13 mJ et d'une duree de 35 fs a 1 KHz, pour la generation des harmoniques dans l'Argon et la microfocalisation du faisceau VUV nous ont permis de faire varier la densite du rayonnement sur la cible dans une large gamme. Nous avons etudie l'effet de la densite d'excitation VUV sur la luminescence intrinseque de cristaux a large bande interdite (CdWO 4 et BaF 2 ). En fonction de la densite d'excitation VUV nous avons observe les modifications des spectres d'emission et de la cinetique de la luminescence excitonique. Quand la densite d'excitation VUV croit, le rendement de la luminescence diminue et apparaissent dans la cinetique de luminescence des composantes rapides. Le profil de la courbe de declin devient alors complexe avec une forte deviation de la loi exponentielle dans sa partie initiale.

Recemment nous avons debute des series d'experiences sur l'interaction d'impulsions intenses VUV avec les solides luminescents. Le but principal en est l'etude des mecanismes d'interaction et de transfert d'energie entre les excitations electrcniques dans les solides a large bande interdite. L'application directe de ces etudes sera le developpement de materiaux et de methodes de metrologie pour les faisceaux UVX intenses. Nous presentons les resultats obtenus pour CdWO 4 qui montrent que ce cristal pourra servir comme un scintillateur pour le rayonnement VUV intense.

International audience; Free electron lasers (FEL) are new generation accelerator-based short wavelength light sources providing high pulse intensity and femtosecond pulse duration, which enable investigation of interaction of elementary excitations in solids under extreme conditions. Using the FLASH facility of HASYLAB at DESY (Hamburg, Germany), we investigated the response of different materials with scintillating properties based on intrinsic emissions to the 25.6 and 13.8 nm FEL radiation by means of time-resolved luminescence spectroscopy. FLASH delivered single pulses of 25 fs duration having energy per pulse up to 30 μJ resulting in power densities of 1012 W/cm2 on crystals. As a function of excitation density we observed the shortening of lifetime and non-exponential behaviour of emission decays in CaWO4, while the emission spectra recorded are comparable to those obtained at conventional excitation sources

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/cm 2 (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.

Exciton-exciton interaction is experimentally revealed and quantitatively analyzed in a wide band-gap scintillator material ${\text{CdWO}}_{4}$. Under high-intensity femtosecond vacuum ultraviolet excitation, the ${\text{CdWO}}_{4}$ luminescence is quenched, while its decay becomes essentially nonexponential. We propose an analytical model, which successfully reproduces the decay kinetics recorded in a wide range of excitation densities. The dipole-dipole interaction between excitons leading to their nonradiative decay is shown to be the main cause of a nonproportional response common for many scintillators.

Light sources capable to deliver intense and ultrashort pulses in the VUV domain, based on free electron lasers or on the high order harmonic generation have appeared recently. They bring the possibility to explore a new domain in the field of laser matter interaction. Such sources are available in the visible or near IR range -specially at 800 nm, thanks to Ti-Sa lasers - since more than ten years, and the interaction of femtosecond pulses with solids has been studied in great details. In this paper we will discuss how the knowledge which has been acquired in the visible domain can be used for the VUV studies. I will concentrate on the case of wide band gap dielectric materials (SiO 2 , MgO, Al 2 O 3 ), and on the intensity domain around breakdown and ablation threshold. This type of material is interesting not only because they are involved in numerous applications, but above all because their band gap (Eg) lying in the range 6 to 10 eV, a clear distinction can be made for what concern their interaction with visible (hν Eg). We discuss here two important aspects that must taken into account to understand the energy balance of the interaction. The first is the energy distribution of photoexcited carriers, which are clearly different in the case of visible or VUV light. Photoemission spectroscopy demonstrate that the distribution highly depends upon the incident intensity in the visible and near IR, and can be "warmer" than the one observed by irradiation with VUV photon, despite their much larger energies. The second important parameter is the excitation density achieved during the excitation. Experiments carried out in the IR using the technique of time resolved interferometry allow to measure the density of electrons excited in the conduction band at intensities above and below the optical breakdown threshold. The results show that in the process of laser breakdown multiphoton excitation dominates the avalanche process for picosecond and subpicosecond pulses. The simulations performed to interpret these measurements can be used to predict the damaging mechanism of wide band gap dielectrics submitted to ultra intense VUV pulses.

The multi-mJ, 21-nm soft-x-ray laser at the PALS facility was focused onto the surfaces of amorphous carbon (a-C) coatings, developed for heavily loaded XUV/x-ray optical elements. AFM (Atomic Force Microscopy) images show a 3-micrometer expansion of the irradiated material. Raman spectra, measured with an Ar + laser microbeam in both irradiated and unirradiated areas, confirm a high degree of graphitization in the irradiated layer. In addition to this high-fluence (∼ 1 J/cm 2 ), single-shot experiment, it was necessary to carry out an experiment to investigate the consequences of prolonged XUV irradiation at relatively low fluence. A high-order harmonic (HH) beam generated at the LUCA facility of the CEA/Saclay Research Center was used as a source of short-wavelength radiation delivering high-energy photons to the surfaces at a low single-shot fluence but with high-average power. a-C irradiated at low fluence, (< 0.1 mJ/cm 2 ) by many HH shots exhibits an expansion of several nanometers. Although it is a less dramatic change of surface morphology than that due to single-shot x-ray-laser exposures, even the observed nanometer-sized changes caused on an a-C surface by an HH beam could influence the reflectivity of a grazing incidence optical element. These results seem to be important for estimating damage to surfaces of highly irradiated optical elements developed for guiding and focusing the ultra-intense XUV/x-ray beams provided by the new generation of sources (VUV PEL and XFEL in Hamburg; LCLS in Stanford) because, up to now, only melting and vaporization, and not graphitization, have been taken into account.

The last few years have seen a large development of mm technology and ultra-sensitive detectors devoted to microwave astronomy and astrophysics. The possibility to deal with large numbers of these detectors assembled into multi--pixel imaging systems has greatly improved the performance of microwave observations, even from ground--based stations, especially combining the power of multi--band detectors with their new imaging capabilities. Hereafter, we will present the development of a multi--pixel solution devoted to Sunyaev--Zel'dovich observations from ground--based telescopes, that is going to be operated from the Millimetre and Infrared Testagrigia Observatory., 5 pages, 3 figures, to be published in th eProceedings of the 2K1BC Workshop, July 9-13, 2001 - Breuil-Cervinia

Nineteen-year-old Swiss Army conscripts were questioned in 1992 (n = 1361) and 1995 (n = 1050) about their opinions regarding pain and euthanasia. In 1995, 85% (1992: 71%) considered pain as a fundamental part of life, 73% (1992: 77%) thought that the patient himself should decide how to control pain, and 50% (1992: 51%) would accept the idea of euthanasia at the request of a family member with an incurable disease and in pain. Significant differences were found only in correlation to the degree of religious belief.