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Supported metal catalysts have applications in fuel cells, fine chemical production, and emission control, to name a few [1]. SMSI is a phenomenon observed in supported metal catalysts, by which reducible metal oxide supports can form overlayers over the active metal nanoparticles (NPs) when treated with hydrogen (H2) at elevated temperature [2]. SMSI can lead to loss of activity, but if the extent of coverage is controlled, the decoration of the active phase can bring unique catalytic properties. Studying the formation of metal oxide mono- or multilayers on the surface of NPs is usually done using high-resolution electron microscopy, which only focuses on one or a few particles at a time. To unravel SMSI on ensembles of NPs, element-sensitive techniques are needed, able to measure nanoscale changes in atomic profile. X-ray standing wave (XSW) techniques are suited for this, as they can measure changes in atomic distribution profile at the sub-nanometer scale by measuring fluorescence signal. Here, we study a model catalyst system based on Co NPs supported on the reducible oxide TiO2, relevant e.g. for CO2 hydrogenation and Fischer-Tropsch catalysis. To achieve maximum sensitivity towards atomic movements of the TiO2 layer, a multilayer mirror (MLM) of MoN/Si3N4 with thin (~ 3nm) TiO2 layer on top was synthesized to generate XSW. Co NPs of ~ 4 nm in diameter were deposited on top of the TiO2 layer by spark ablation followed by mass selection. The samples were reduced under H2/N2 at 450℃ for 1 hour and characterized by a combination of X-ray reflectivity and XSW. The results of the first measurements and indication of SMSI will be reported.

Absorption of extreme ultraviolet (EUV) and X-ray photons by thin-film optical devices results in the excitation of electrons from various energetic levels, either core shells or valence band of film material. These electrons initiate various processes leading to the degradation of film sensitives. One of such processes is electron emission, which occurs at the initial stage of the electron system response to photon absorption. Emitted electrons reduce the amount of absorbed energy decreasing the heating of a film. However, they may ionize an environment gas that can cause chemical reactions of gas ions with the surface leading to surface degradation [1]. The intensity of surface-related processes is determined by the number of emitted electrons. This number depends on many factors, such as the energy of an absorbed photon, thickness of a film, and its electron structure. Having the information about the distribution of emitted electrons, one can extract parameters of electron cascade, which is the number of secondary electrons excited by one primary photoelectron. In the present work, we study the electron emission caused by 92 eV EUV photon irradiation as a function of film thickness for Si, a-C, and graphite films. We provide a theoretical analysis of emission spectra using Monte Carlo code TREKIS [2]. In the Monte Carlo scheme, we simulate the electron cascade formation in a film and treat emitted electrons as particles that can overcome potential barrier at the film surface. Our simulations predict an increase of the number of secondary electrons with increasing film thickness until a critical thickness value when saturation is reached. For graphite films, we demonstrate that the saturation thickness and saturation value correspond to experimentally measured values [3].

Radiation stability of optical elements exposed to a high dose of laser radiation is an important issue in developing optical elements for x-ray free-electron laser light sources. In order to prevent optics from being damaged, the fundamental mechanisms governing the material response to ultrashort high peak power XFEL pulses must be identified and studied. In this paper we present a study on the interaction of fs laser pulses with Ru films in a wide range of incident photon energies from ~1 up to 104 eV. Since reflective thin metal films operate at grazing incidence conditions, absorption of the light occurs in the top part of the Ru mirrors (~1 – 10 nm). Hybrid multi-scale modeling of target evolution after irradiation is performed. The model takes into account photoabsorption and non-equilibrium electron cascading occurring on a fs timescale, thermal diffusion and electron-phonon energy exchange on a ps timescale and lattice dynamics up to ns timescale. Different photon energies result into qualitatively different absorbed energy profiles by the time of thermalization of the electronic system. The effects of such a difference on the hydrodynamical evolution and eventual damage of Ru are discussed. Details of processes such as melting, cavitation, ablation and recrystallization are revealed for selected photon energies. The results show good qualitative agreement with the experimental observations on Ru ablation.

An accurate transmission measurement of thin foils (usually made of elemental metals and/or semiconductors), which routinely are used as attenuators in soft x-ray beamlines, end-stations and instruments, represents a long standing problem over the wide experimentation field with photon beams, see for example [1-4]. Such foils are also frequently utilized for blocking long wavelength emission, i.e., UV-Vis-IR radiation, from plasma and high order harmonic sources, whilst soft x-rays emitted from the source pass through the foil with only a slight attenuation. Despite the enormous amount of data available in the literature, e.g., Henke’s tables [5], measurements made on real foils often provide surprising results. In this study, a procedure based on the ablation imprints method is utilized for determination of soft x-ray filter transmission, namely the f-scan technique [6,7]. This technique combines the GMD (Gas Monitor Detector) pulse energy measurement and attenuation of the beam by foils (made of different metallic/semiconducting elements of varying thickness) with areas of ablation imprints created on a suitable target, e.g. PMMA – Poly(methyl methacrylate). The results show only a partial overlap with transmission values found in Henke’s tables. Nevertheless, a good agreement with transmission values determined by conventional radiometry techniques at synchrotron radiation beamlines has been found. Such a difference between the experimentally obtained values and transmissions calculated for a pure element is usually explained by spontaneous formation of oxidized layers on the filter surface and in the near-surface layer and their possible alteration by intense FEL radiation. The first results obtained with Al, Nb, Zr and Si filters at FLASH/FLASH2 (Free-electron LASer in Hamburg tuned to 13.5 nm) facilities will be shown and discussed in this presentation.

Survivability of optical elements exposed to high doses of XUV laser radiation is an important issue in the context of rapidly developing x-ray free-electron laser (XFEL) light sources. In order to prevent optics from being damaged, the fundamental mechanisms governing the material response to ultrashort high peak power XFEL pulses must be identified and studied. We present computational study of the interaction of femtosecond XUV (13.5 nm wavelength) laser pulses with 50 nm thin Ru films. With our calculations we model the damage experiments that was performed at Free-Electron LASer in Hamburg (FLASH) [1]–[3]. Ru is chosen as optically favorable material for grazing incidence reflective mirrors. The performed simulations consist of two parts. First, the effect of electron cascading induced after absorption of XUV photons is studied using an event-by-event Monte Carlo code XCASCADE [4]. Time of cascading and ballistic range of non-thermalized electrons are calculated. Second, the evolution of electron and lattice temperatures in the regime of thermal non-equilibrium together with atomic motion in irradiated Ru are modeled with a combination of two temperature hydrodynamics [5] and molecular dynamics [6]. Our calculations showed that the mechanism responsible for the ablation of Ru observed in the experiment is spallation in the stress confinement regime. The processes of melting, cavitation, spallation and recrystallization are modeled. The results show good agreement with the experimental observations.

In this article we present an overview of the optimization of LaN/B multilayers that enabled the deposition of a multilayer with a normal incidence reflectance of 57.3 % at 6.6 nm wavelength, the highest value reported to date. Two different ways of nitridation of the La layers were investigated: firstly N-ion post treatment of the La layer and secondly reactive magnetron sputtering of La in N2 atmosphere. Initially the optimization of the multilayers was performed for 50 period test multilayers, followed by the selection of the best process to study the stability of the full stack deposition and the optical performance of the mirrors. The scaling of reflectivity with increasing number of periods for LaN/B multilayer mirrors will also be discussed.

Object: La/B-based multilayersApplications: EUV lithography, X-ray fluorescenceanalysis, space research, high-intensity freeelectronlasers.Our reflectance world record [1]: 64.1% at 1.5 offnormalAOI, λ=6.65 nmApplication-desired reflectance: ~70.0%Purpose: Investigation of roughness influence on R

The physics investigations described in this thesis have been inspired by the perspective of the application of La/B-based multilayer mirrors in photolithography. This perspective however, demands the highest possible reflectance of the mirrors and full control over the multilayer growth process. The corresponding research in this thesis therefore covers the following topics: a theoretical and experimental analysis of the spectral properties of La/B-based multilayer stacks, an experimental layer composition and growth study of the candidate La/B-based multilayer optics, and the development of techniques to enable characterization of the internal structure of such multilayers.

Optics for extreme ultraviolet radiation of 6.5-6.9 nm wavelength are of interest for many applications, such as photolithography, x-ray free electron lasers, x-ray spectroscopy or space research. Photolithography using a wavelength of ~6 nm is considered to be a potential extension of the currently introduced 13.5 nm, or EUV lithography. EUVL poses extremely high requirements on the quality of the multilayer optics. The performance of a single multilayer mirror (MLM) will affect the throughput of a multi-mirror optical system, proportionally to the power of the number of mirrors. The currently highest reflectivity of a single MLM of about 50 % is significantly lower than a desired reflectance of above 70 %. To reach this performance we first optimize the multilayer design by selecting the materials that shows the best optical contrast. The deposition process should be optimized to enable the lowest possible interface roughness to maximize layer contrast. The best material combination for 6.7 nm wavelength is based on lanthanum as a reflector and boron as a spacer material. In this presentation we will discuss the optimization of the optical design of La/B MLM, optimization of the deposition process and the analysis of the MLMs.

Lithography based on the wavelength of 6.x nm is considered to be a potential extension of the current 13.5 nm EUV lithography. Light sources might be based on Tb or Gd. The published spectra of plasmas created from these materials show highest intensities at 6.5 and 6.8 nm respectively. Multilayer mirrors (MLMs) based on La and B show the highest optical contrast for the 6.6-7 nm wavelength range and a maximum reflectance at 6.64 nm, where the B absorption is minimal. It is not possible to design high reflective B-based mirrors for 6.5 nm because this would be below the B-K absorption edge. In theory the reflectivity of La/B MLMs at 6.8 nm is only a few percent lower than at the optimal wavelength of 6.64 nm, but measurements of real multilayer mirrors show a significantly larger reduction of reflectivity at 6.8 nm. The origin of this larger drop is the reduced optical contrast of the deposited La/B mirror with respect to theoretical case. In the presentation we will discuss how the multilayer structure influences its reflectivity profile and the ways to optimize La/B mirrors for 6.8 nm.

The spectral properties of LaN/B and LaN/B4C multilayer mirrors have been investigated in the 6.5-6.9 nm wavelength range, based on measured B and B4C optical constants. We show that the wavelength of optimal single mirror reflectance for boron based optics is between 6.63 and 6.65 nm, depending on the boron chemical state. The wavelength of the maximum reflectance of the LaN/B4C multilayer system is confirmed experimentally. Calculations of the wavelengthintegrated reflectance for ideal 10-multilayer-mirror stacks show that a B-based optical column can be optimized for a wavelength larger than 6.65 nm.