This PDF file contains the front matter associated with SPIE Proceedings Volume 11776, including the Title Page, Copyright information, and Table of Contents.

X-ray free-electron lasers, delivering x-ray pulses of femtosecond duration, are available for experiments for more than a decade and allow for hitherto unachievable x-ray intensities on sample, reaching up to 1021 W/cm2 for hard x-rays. At these intensities, the probability of a single atom or molecule to absorb a photon of an impinging x-ray pulse reaches unity. Moreover, several interactions of photons and matter within the duration of the x-ray pulse – nonlinear x-ray matter interactions – become possible, opening the pathway to nonlinear x-ray optics. For a macroscopic ensemble of atoms, molecules, nanometer-sized clusters or a solid, the interaction with a strongly focused x-ray beam can create macroscopic, highly excited states of matter, far from equilibrium. In particular, saturated absorption with a high-intensity x-ray pulse can result in transient states, present for roughly one femtosecond, with the characteristic feature, that every single atom in the interaction region is in a population inverted state with missing population in the innermost electronic shell. This macroscopic population inversion can lead to collective radiative decay mechanisms, such as amplified spontaneous emission or superfluorescence. In this presentation I will give you an overview over our experimental and theoretical investigations of these single-pass x-ray laser amplifiers in the x-ray spectral domain. I will address applications of this phenomenon in the area of chemical x-ray emission spectroscopy, a new concept of an x-ray laser oscillator, and will highlight recent theoretical developments to describe collective spontaneous emission in the x-ray spectral domain.

For space-born astronomical X-ray telescopes, iridium-based reflective layer systems are known as highly effective mirrors coatings. During the recent years, Aschaffenburg University of Applied Sciences and the Czech Technical University in Prague jointly developed stress compensated chromium-iridium coatings for this application. To overcome the disturbing re ectivity reduction of the iridium absorption edge around 2 keV photon energy, thin overcoat layers of chromium were applied in addition. Now a prototype of a wide-field, imaging X-ray telescope of Lobster Eye type is assembled at the company RIGAKU. For this purpose a small series of 34 mirrors based on 100 x 50 mm semiconductor grade silicon substrates has been coated at Aschaffenburg University. The applied tri-layer system consists of a stack of 40 nm chromium, which act as adhesive layer and compensates layer stress, a 30 nm iridium thick reflective layer, and an additional overcoat layer of 6 nm chromium. This layer system have been analysed by AFM and TEM images. The mirrors are assembled into an aluminium frame to build a 2D Lobster Eye type telescope. The designed focal length of this wide field X-ray telescope is two meter. To study the performance of the tri-layer coating system, a twin LE telescope with convenient gold coatings was manufactured also. Performance measurements of both telescopes and under same conditions are planned at the PANTER test facility at the Max-Planck Institute for Extraterrestrial Physics. First experimental results, their comparison with theoretical simulations and the comparison between both models will be presented in this contribution.

Mirrors are a subset of optical components essential for the success of current and future space missions. Most of the telescopes for space programs ranging from Earth Observation to Astrophysics and covering all the electromagnetic spectrum from X-rays to Far-Infrared are based on reflective optics. Mirrors operate in diverse and harsh environments that range from Low-Earth Orbit, to interplanetary orbits and the deep space. The operational life of space observatories spans from minutes (sounding rockets) to decades (large observatories), and the performance of the mirrors within the optical system is susceptible to degrade, which results in a transient optical efficiency of the instrument. The degradation that occurs in space environments depends on the operational life on the orbital properties of the space mission, and it reduces the total system throughput and hence compromises the science return. Therefore, the knowledge of potential degradation physical mechanisms, how they affect mirror performance, and how to prevent it, is of paramount importance to ensure the long-term success of space telescopes. In this brief review paper we report an overview on current mirror technology for space missions with a particular focus on the importance of degradation and radiation resistance of the coating materials.

Iridium-based layer systems are highly effective mirror coatings for space-born X-ray telescopes. During the recent years, Aschaffenburg University of Applied Sciences and its partners developed stress compensated chromium-iridium coatings for such astronomical applications, using chromium as an adhesive layer between iridium reflective layer and mirror substrate. However, there was room for improvement: To overcome the disturbing reflectivity reduction of the iridium absorption edge around 2 keV photon energy, thin overcoat layers of chromium are applied in addition now. This layer system has been analyzed by atomic force microscopy and transmission electron microscopy images. Furthermore, the reflectivity of such innovative X-ray mirrors based on chromium-iridium tri-layer coatings was recently measured at PTB's four-crystal monochromator beamline at the synchrotron radiation facility BESSY II. The experimental results, obtained for photon energies between 1.9 keV and 11 keV at two grazing incidence angles (0.6 degree and 0.9 degree) and their comparison with corresponding simulations are presented in this contribution. When compared to simulations of pure iridium coatings, a significantly higher reflectivity was achieved especially in the soft X-ray regime between 2 keV and 4 keV. Such chromium-iridium tri-layer coatings have high potential to increase the effective area for X-ray telescopes in Lobster Eye Design, for Wolter-I type X-ray optics, and for silicon pore optics as considered for the ATHENA telescope of the European Space Agency.

This paper presents the 2^nd generation of the optical system for Rocket Experiment (REX2). This optical device is based on successful mission REX1. The purpose of REX2 is to verify X-ray optical system consisting of wide- field 2D X-ray Lobster-Eye (LE) optics with an uncooled Quad Timepix3 detector (512x512 px @ 55 um and spectrometer (active area 7 mm², 145 eV @ 5.9 keV ). The 2D X-ray Lobster-Eye (LE) optics is a combination of two one-dimension LE modules with a focal length of up to 1 m and a FOV better than 4.0 x 4.0 deg. The proposed optical system has imaging capabilities (2.5 to 30 keV) and spectroscopy capabilities (0.2 to 10 keV). The optical system was recently tested in the X-ray vacuum chamber. The preliminary test results are presented and discussed in this paper.

We are proposing an X-ray reflecto-interferometry technique using an x-ray microfocus laboratory source for thin-film structure research based on compound refractive lenses. The idea of this technique is to use a very simplified experimental setup where a focused X-ray beam reflected from parallel flat surfaces creates an interference pattern in a wide angular range. Due to this, the interference pattern can be obtained in a single shot without the need to rotate the specimen or the detector. The applicability of this technique has been demonstrated using the MetalJet Excillium microfocus laboratory source, which has GaKα emission line at 9.25 keV. A series of interference patterns for Si₃N₄ membranes and the experimentally obtained film thickness are in good agreement with the declared characteristics. The main advantages and future possible of the reflecto-interferometry technique are discussed.

A novel interferometric approach where the X-ray amplitude splitting interferometer based on the compound refractive lens was used to visualization of thin layers with a thickness of several nanometers applied to the surface of the membrane. The functional capabilities of the proposed technique were theoretically studied and experimentally demonstrated on the example of the gold strip 8 nm thick deposited on the 1000 nm Si₃N₄ membrane. The corresponding numerical simulations were performed. It was shown that the interference pattern is very sensitive to small thickness deviations of the gold layer, resulting in the ability to accurately determine not only its average thickness but also to reconstruct its cross-section profile with a nanometer resolution.

Diffraction gratings are key components in experiments with UV, VUV and soft x-ray radiation at synchrotron radiation and free-electron laser (FEL) facilities. They areused in monochromators to produce beams with narrow energy distribution, and in analyzers to provide high-resolution spectral analysis of scattered radiation. Today, such diffraction gratings consist of a large number of periodic grooves (typically tens to thousands per millimeter) arranged on the reflecting surface on a plane or concave substrate. The performance of the grating is also ultimately limited by the surface quality of the substrate. For best spectral resolution, the substrates need to be ultra-precise and superpolished, with typical lengths of 100 – 300 mm and shape errors of 1-2 nanometers, slope errors in the order of 100 nrad, and a roughness in the Ångstrom scale. Today, the line patterns of x-ray gratings are mostly produced either by holographic exposures of interfering laser beams into a photo resist, or by ruling machines that imprint the grating groves in a line-by-line manner into a soft metal layer. As an alternative to these established methods, we present a strategy of grating fabrication by electron-beam lithography (EBL). This technique can expose arbitrary patterns with high resolution and placement accuracy at a speed sufficient for x-ray grating fabrication. We have performed optical metrology of a low-cost grating substrate, which served as the basis for an aligned exposure of a corrected grating pattern to yield a laminar grating with a constant deflection angle along the substrate surface. We have confirmed that the effects of slope errors were strongly compensated over an extended wavelength range by using synchrotron radiation in the soft x-ray range. This work shows a path for circumventing the limited availability of high quality reflective substrates to obtain gratings with performance beyond what is possible today.

The requirement for the high-quality optics for the 4^th generation synchrotrons is becoming particularly urgent nowadays. We present a laboratory complex “SynchrotronLike” designed for the X-ray optics tests and development of the X-ray optical techniques before their use on the synchrotron sources. This complex consists of four main parts: 1) microfocus Xray source MetalJet D2+ 70kV, 2) detector module that contains X-ray spectrometer and three X-ray cameras with different resolution and field of view, 3) Micro-Optics Test Bench for X-ray optics tests and development of the X-ray optical techniques, 4) X-ray Single Crystal Diagnostics Endstation for the X-ray diffraction microscopy experiments. This complex was successfully used for the preliminary tests of the X-ray refractive lenses made from polymer, epoxy, and diamond. Moreover, the ultracompact X-ray transfocators, diamond monochromators, and beam-splitters for the 4th generation synchrotron sources were also tested on the “SynchrotronLike”. The unique parameters of our laboratory source, such as high brightness and high beam coherency make it possible to implement versatile experimental synchrotron techniques. Consequently, we managed to get excellent results in the X-ray reflecto-interferometry and X-ray microradian diffraction. The results of the latter technique are presented in this paper.

On November 8^th, 1895, Professor Wilhelm Conrad Röntgen was still working late on cathode rays in his laboratory at the University of Würzburg in Germany. By chance, he found a new type of radiation that was able to penetrate materials. He himself called them X-rays. In his honour, in Germany we call this type of radiation Röntgen´s rays since 1896. In 2020, Germany was celebrating the Röntgen anniversary year, recognizing the discovery of X-rays 125 years ago and the 175^th birthday of its discoverer Wilhelm Conrad Röntgen, the first Nobel laureate in physics. Financed by a grant from the Klaus Tschira Foundation, the Reiss-Engelhorn-Museums in Mannheim and the German Röntgen Museum in Remscheid developed an “X-periments mobile” with interactive experiments for children and young people. Following the given motto "X-periments - making the invisible visible", this research station with optical experiments started traveling across Germany in the anniversary year and, due to the corona pandemic in the following year. In summer 2021, the “X-periments mobile” will make a guest appearance at Aschaffenburg University. The experimental set-ups enable school classes and students to gain an overview of optical technologies from infrared light to ultraviolet radiation to X-rays and gamma rays. The aim of the campaign is to inspire the young generation and to attract students for courses in science and technology. This contribution presents the historical discovery of X-rays, the “X-periments mobile” and its experiments, its intention, and the experiences from its stays at different places across Germany.

At XRnanotech, a spin-off from the Paul Scherrer Institut (PSI) in Switzerland, we develop diffractive X-ray optics to enable experiments at many large-scale research facilities. Our goal is to push the limits of diffractive X-ray optics by continuously, improving the resolution and efficiency enabling new applications in microscopy to make the invisible visible. In X-ray microscopy, Fresnel zone plates (FZPs) are used as high-resolution lenses. Their resolution depends mainly on the size of their smallest outermost zones and many years of development were required in order to push this value into ever-smaller regimes. Recently, the 10 nm barrier was broken with the line-doubling approach. While the zone-doubling approach is very helpful for achieving ultra-high resolutions, it has a fundamental disadvantage compared to conventional FZPs in terms of efficiency. The efficiency of a FZP depends not only on the wavelength, the material and its optical thickness but also on the duty cycle. For a FZP consisting of structures with a continuously changing period along its radius, the duty cycle of the Ir structures changes accordingly, because the deposited Ir thickness by ALD is uniform. Consequently, the efficiency along the radius changes as a function of the duty cycle and drops to zero towards the center of the zone plate. An elegant method to mitigate this effect is to fabricate hybrid FZPs that consist of a conventional center region with structures made e.g. of gold by electroplating and a zone-doubled outer part, where the deviation in duty cycle is not as pronounced anymore. Here, by fabricating such a hybrid structure, we achieved 7 nm resolution in direct imaging with soft X-rays and are able to fabricate X-ray optics with ultra-high aspect ratios.

In the manuscript we report on characterization of single-crystalline (111) plates prepared from type Ib diamonds with nitrogen content of 100-150 ppm and (100) plates prepared from IIa diamond by means of high-resolution rocking curve imaging (RCI). Contrary to a common opinion about intrinsic poor diffraction quality of type Ib diamonds, RCI showed the presence of nearly defect-free areas of several mm2 in the central part of the (111)-oriented diamond plates. In comparison with the (100)-oriented IIa diamond plates prepared by the same HPHT setup the Ib diamonds possess better diffraction quality. The observed broadening of the rocking curves associates with the cutting and polishing processes, causing strains around the edges of the plates and rare defects. An improvement of preparation technique will thus allow to make single-crystalline diamond plates for Laue and Bragg monochromators and beam splitters from type Ib diamond with areas large enough to be used as wavefront-preserving optical elements at 4th generation synchrotron facilities. Contrary to a common opinion about intrinsically poor diffraction quality of type I diamonds, RCI showed the presence of nearly defect-free areas of several mm2 in the central part of the diamond plates. The observed broadening of the rocking curves results from the cutting and polishing processes, causing strains around the edges of the plates and rare defects. An improvement of preparation technique will thus allow to make single-crystalline diamond plates for Laue and Bragg monochromators and beam splitters from type Ib material with areas large enough to be used as optical elements at 4^th generation synchrotron facilities.

We demonstrate the capabilities of ion-beam lithography (IBL) for the manufacturing of X-ray micro-optics from amorphous materials. The feasibility of rapid and direct milling of microlenses in glass using a Ga+ ion beam is presented in this work. A single lens with curvature radii below 3 μm was produced from microstructure- and artifacts-free materials: borosilicate glass. In addition, the use of IBL to create optics with a complex curved surface with a radius of curvature of less than 3 μm from single crystal silicon has been shown. The new generation of micro-optics from new materials will be in demand for coherent applications at modern diffraction-limited light sources.

PAL-XFEL has operated general user program successfully from 2017 in both hard (2.5 – 14 keV) and soft (250 – 1300 eV) X-ray beamlines. Particularly in Soft X-ray Scattering and Spectroscopy (SSS) beamline, X-ray absorption and emission spectroscopy (XAS / XES) endstation has conducted general user programs. In SSS beamline, second experimental instrument, Resonant Soft X-ray Scattering (RSXS) endstation, was built and open from 2020. This endstation mainly supports optical laser pump (800-nm-wavelength and harmonics) soft X-ray probe (400-1300 eV, covers 3d transition metal L2,3-edge, O K-edge, and part of 5f rare-earth element M4,5-edge) time-resolved RSXS experiments, with a time resolution better than 100 fs. Within less than a year, there have been several successful experiments, mainly in 3d transition metal compounds which are important magnetic and strongly correlated materials. The details of the endstation and selected experimental results will be introduced in the presentation.

High gain Free-electron lasers (FELs) are driven by linear accelerators that are currently capable of delivering electron bunches in high-repetition rates from kHz and up to several MHz. Short wavelength FELs are commonly operated in a self-amplified spontaneous emission (SASE) scheme which is, due to its stochastic nature, limited in temporal coherence. External seeding and harmonic generation techniques have been shown to be valid solutions for the generation of fully coherent and stable radiation up to the soft x-ray. However, the required seed lasers in the UV regime and shorter wavelengths cannot generate pulses with sufficient energy at the repetition-rates accessible with superconducting accelerators, hence the seed laser can become the limiting factor which determines the maximum repetition-rate for future seeded FEL based on superconducting linacs. In this paper, we address the need for generation of fully coherent radiation with higher average flux and we propose two different schemes which overcome the limitations and enable the generation of seeded radiation at high-repetitionrates. In one case, an optical cavity is used instead of the seed laser source. It retains a seed laser pulse and recirculates it to seed electron bunches in high-repetition-rates. In the other case, an optical klystron scheme is exploited in the seeding process: a low-intensity seed laser initiates the process in the first modulator and then, a dispersive section increases the density modulation and enables coherent emission in the second modulator which increases further the energy modulation. The resulting energy modulation is several times larger than the one possible with a standard seeding scheme and same laser intensity. Reducing the requirements in seed laser power, the proposed setup makes the use of seed lasers at higher repetition-rates or shorter wavelengths possible. Here, we show simulation results for the optical klystron scheme and we discuss experimental possibilities.

During the last year, a variety of advanced FEL techniques were investigated at the European XFEL by the Operation and FEL R&D teams, which led to several scientific achievements. The Hard X-ray Self-Seeding (HXRSS) system installed at the SASE2 line produced seeded beams with more than 700uJ within 0.7 eV FWHM bandwidth at 9keV. High-repetition rate capabilities were enabled with the production of up to 4000 seeded pulses per second, and seeded (as well as unseeded) FEL spectra were monitored in single-shot-resolved manner with MHz-rep-rate. Almost no effects due to crystal heat loading was detected on pulse intensities and spectral properties along the pulse train. Two-color pulses were generated and delivered in the Soft X-Ray range, by means of a newly installed 2-Color Pump-Probe (2CPP) setup. Electron bunching manipulations beyond the SASE mode, like e.g. frequency mixing, began to be experimentally investigated. In this talk we will review the current status of these techniques at the European XFEL, with particular emphasis on the Hard X-ray Self Seeding system.

Temporal diagnostics are essential to characterize and optimize the performance of free-electron lasers (FELs). We propose and demonstrate a new method to measure the temporal profile of electron and photon beams in FEL facilities. The approach utilizes the transverse wakefields generated by the electrons traveling through a corrugated structure installed after an undulator section of an FEL. Compared to the more standard radio-frequency deflector approach, the method is more cost-effective, less sensitive to arrival time jitter, but reconstruction of the beam profiles becomes more complicated. We present numerical and experimental results that demonstrate the feasibility of the method. The experiments were carried out at SwissFEL, the x-ray FEL facility at the Paul Scherrer Institute in Switzerland.

For LCLS-II, we have developed fluorescence intensity monitors and power meters as intensity monitors. The Fluorescence Intensity Monitor (FIM) provides the non-invasive, pulse-by-pulse normalization of experiments. For the LCLS-II instruments, the diagnostic was constructed with an array of four microchannel plate assemblies and four avalanche photodiodes. The diagnostics are being installed in each Kirkpatrick Baez mirror chamber. The noise of the diagnostic will be evaluated against a goal of 1 %. The X-ray power meter delivers average power values. For the LCLS-II instruments, a power meter was selected compatible with high average power. In the LCLS-II instruments, power meters are being installed with each profile monitor in order to evaluate the transmission along the X-ray transport. A calibration of a set of power meters was carried out against a gas monitor detector at FLASH. In addition for all the power meters, a relative calibration was performed with a visible light source. At the endstations, a power meter will determined the pulse energy at the sample.

The stochastic nature of the self-amplified spontaneous emission (SASE) process in hard x-ray free electron laser (FEL) photon pulse generation often requires online spectral monitoring on a pulse-to-pulse basis. Experimenters and machine operators use the spectral information to better understand their data and guide the experiment, or to optimize the machine parameters through feedbacks to achieve a desired stability. Even experiments that use monochromators benefit from online spectral measurements as it allows the FEL operators to use machine feedbacks on the generated spectra so that the light would not jitter too far out of the monochromator’s acceptance bandwidth. Online spectrometers have existed for several years in several facilities like LCLS, SACLA, and SwissFEL, and have mostly been used to characterize hard x-rays at photon energies typically above 4 keV. This abstract presents ongoing work on the development of the online tender x-ray spectrometer (TXS)], meant to cover the energy range between 2000 and 4000 eV and fill a gap for online spectral measurements at SwissFEL.

A split-and-delay unit for the XUV and soft X-ray spectral range has been installed at beamlines FL23 and FL24 at the FLASH2 Free-Electron Laser at DESY. It enables time-resolved pump-probe experiments covering the whole spectral range of FLASH2 from 30 eV up to 1500 eV. Using wavefront beam splitting and grazing incidence mirrors a sub-fs resolution with a relative pulse delay of -5 ps ≤ ▵τ ≤ +18 ps is achieved. Two different mirror coatings are required to cover the complete spectral range and thus, a design that is based on a three dimensional beam path was developed. This allows the choice between different sets of mirrors with either coating for the fixed branch. In the variable branch two coatings are available on the same set of mirrors. A Ni coating allows a total transmission above T > 0.50 for photon energies between hν ≈ 30 eV and 650 eV at a grazing angle of a ϑ_variable = 1.8° in the beam path with variable delay. With a Pt coating a transmission of T > 13 % is possible for photon energies up to hν = 1500 eV. In the fixed beam path at a grazing angle of ϑ_fixed = 1.3° a transmission of T > 60 % with a Ni coating and T > 28 % with a Pt coating is possible.

Measurement of transient optical properties (reflectivity and transmissivity) is frequently performed in X-ray/XUV pump – optical probe experiments. The optical properties reflect the transient state of irradiated materials. Here we propose to extend the material diagnostics with an additional measurement of the transient phase change of the optical probe pulse. It can be recorded in parallel to other transient optical properties, enabling an access to the information on the complex refractive index and the thickness of the radiation-modified material layer. The latter is necessary for investigations of phase transitions progressing in XUV (and X-ray) irradiated materials. In the talk, I present the results of a computational study of XUV irradiated silicon and diamond samples performed in [1]. It demonstrates that the measurement of the optical phase from a probe pulse at correctly tuned pulse parameters can provide a signal strong enough to extract information on transient material properties. Such phase measurement, feasible with modern experimental setups, can also be a basis for an improved diagnostics tool for the temporal characteristics of an ultrashort XUV pulse.

The recent development of nanofocused X-ray beams has paved the way to use them to directly pattern electronic devices at the nanoscale without the use of any photoresist, which is also known as the X-ray nanopatterning (XNP) technique. This novel patterning method is expected to become even more important with the advent of the 4th generation synchrotrons. Here we report on an X-ray nano-patterning and nano-diffraction experiment performed on the high-temperature superconducting oxide Bi_2Sr_2CaCu_2O_8+δ (Bi-2212) aiming at clarifying the correlation between the changes in crystal structure and in electrical properties. The experiment has been carried out at the beamline ID13 at ESRF, where an X-ray beam 250×250 nm^2 in size at 14.85 keV was available. The Bi-2212 single crystals were mounted on electrical chips and have been monitored upon successive irradiation sessions up to a cumulative dose of about 1.9 x 10^11 Gy. Nano-XRD mapping has been performed between the irradiation sessions, allowing to resolve the spatial localization of the different crystal domains. Depending on the initial conditions of the crystals, our localized irradiations have induced some stress leading to a physical bending of the crystal, or, in case the crystal was already bent, have induced some stress release by creating a very small region with a low degree of crystallinity. Concerning the electrical properties, both the superconducting critical temperature and the normal state resistivity have been determined by means of a simple in-series resistor model. This has allowed us to observe for the first time an exponential dependence of the Bi-2212 resistivity on the irradiation dose, which gives access to highly underdoped superconducting regimes for this material, which cannot be obtained in any other way. We think that these results represent an important step towards the development and the reliability of the XNP technique.

Boron carbide (B₄C) is a commonly used coating material in X-ray mirrors in Free-Electron Laser (FEL) beamlines for its good qualities such as high reflectivity, high hardness and high damage threshold. The B₄C coating film fabricated by direct-current magnetron sputtering has large intrinsic compressive stress, which leads to poor adhesion on the substrate. In a traditional way, an adhesive layer of chromium is inserted between the B₄C film and the substrate to improve the adhesion, which may also degrade the anti-damage resistance because of the enhanced photoabsorption inside the metal layer. For applications in FEL, the B₄C film is generally fabricated under high Argon sputtering gas pressure to reduce the intrinsic stress. However, it is unclear about the impact of the Cr adhesive layer and the sputtering gas pressure on the damage resistances of B₄C films. In this study, using a table-top nanosecond EUV damage instrument, single-shot damage experiments were performed on B₄C films fabricated at different sputtering gas pressures with or without Cr adhesive layer. The single-shot damage thresholds are reported and the possible damage mechanisms are discussed based on the measurements using scanning electron microscope (SEM) and atomic force microscope (AFM).

In this work we study the mechanisms of photoinduced emergence and dissociation of defects in silica glass of Er - and Al-codoped optical fiber. Defects responsible for the degradation of amplification properties of the fiber are the main focus. Photoinduced changes in UV, visible and near IR optical range of transmission spectrum of the fiber under the 193 nm UV irradiation and the subsequent 976 nm exposure are investigated. The significant role of the interstitial molecular hydrogen in the process under study is shown. All the observed processes are explained in detail by means of photochemical reactions.