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The growth modes of SrF2on Si(001) are investigated by AFM and ultraviolet photoemission. Two growth regimes are identified according to the substrate temperature during deposition, resulting in flat and ordered fluoride layers or in nano-patterned films with characteristic triangular islands. The flat layer growth obtained at high temperature is accompanied by molecular dissociation at the interface.

Extreme ultraviolet and X-ray free-electron lasers (FELs) produce short-wavelength pulses with high intensity, ultrashort duration, well-defined polarization and transverse coherence, and have been utilized for many experiments previously possible only at long wavelengths: multiphoton ionization, pumping an atomic laser and four-wave mixing spectroscopy. However one important optical technique, coherent control, has not yet been demonstrated, because self-amplified spontaneous emission FELs have limited longitudinal coherence. Single-colour pulses from the FERMI seeded FEL are longitudinally coherent, and two-colour emission is predicted to be coherent. Here, we demonstrate the phase correlation of two colours, and manipulate it to control an experiment. Light of wavelengths 63.0 and 31.5 nm ionized neon, and we controlled the asymmetry of the photoelectron angular distribution by adjusting the phase, with a temporal resolution of 3 as. This opens the door to new short-wavelength coherent control experiments with ultrahigh time resolution and chemical sensitivity.

The performance of multilayer optics depends on the quality of the buried interfaces between materials, whose intermixing strongly affects their behavior. We present an experimental method to determine, in a non destructively way, the amount of material intermixing at interfaces of multilayer structures. The reflection mechanism is related to the build up in the multilayer of a standing wave field, whose peaks and the valleys move as a function both of wavelength and of incidence angle. Exploiting this fact it is possible to modulate the electric field inside the multilayer in order to have different parts of the multilayer structure excited at a different extent and in particular the buried interfaces regions. The excitation is directly proportional to the intensity of the electric field and to the concentration of a given element in the sample. The excitation can be detected with different techniques, f.i. electron core level photoemission, fluorescence, luminescence, total electron yield. The flexibility of the experimental apparatus of the BEAR beamline (Elettra Trieste, Italy) allowed us to study some important classes of layered structures in the soft X-ray energy range, using the above mentioned techniques together with the determination of the Bragg conditions through the measurement of the specular reflectivity. We demonstrate the possibility of obtaining quantitative information on the width of the intermixing region, strongly related to the interface roughness, through the comparison with a phenomenological model of the intermixing and a numerical simulation of the standing field inside the multilayer.

We present the results of the thermal stability of Mg/Co multilayers in the EUV range. The annealing study is performed up to a temperature of 400°C. The X-ray reflectivity at 0.154 nm is used in order to determine the structural parameters (thickness, roughness and density) of the layers. The measurements of the EUV reflectivity around 25 nm show that the reflectivity decreases when the annealing temperature increases above 300°C. X-ray emission spectroscopy is performed to determine the chemical state of the Mg atoms within the Mg/Co multilayer. The results show a small oxidation after annealing at 305°C, which increases greatly at 400°C. Scanning electron microscopy images of cross sections of the multilayer show a change of the surface morphology above 305°C. This large change of morphology and the oxidation explain the large reflectivity loss.

NO molecules adsorbed on a Pt(111) surface from dipping in an acidic nitrite solution are studied by near-edge X-ray absorption fine structure spectroscopy (NEXAFS), X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), and scanning tunnelling microscopy (STM) techniques. LEED patterns and STM images show that no long-range ordered structures are formed after NO adsorption on a Pt(111) surface. Although the total NO coverage is very low, spectroscopic features in N K-edge and O K-edge absorption spectra have been singled out and related to the different species induced by this preparation method. From these measurements, we concluded that the NO molecule is adsorbed trough the N atom in an upright conformation. The maximum saturation coverage is about 0.3 ML, and although nitric oxide is the major component, nitrite and nitrogen species are slightly coadsorbed on the surface. The results obtained from this study are compared with those previously reported in the literature for NO adsorbed on Pt(111) under UHV conditions.

The evolution of the electronic properties of a thin film of copper phthalocyanine deposited on Al(100) and progressively intercalated with rubidium atoms was followed by photoemission and X-ray absorption spectroscopies. Electron donation from the Rb atoms to the C32H16N8Cu molecules results in the lifting of the degeneracy in the egligand-derived molecular orbital and the lowering of the molecular symmetry. For Rb∼2C32H16N8Cu, spectral evidence indicates that both donated electrons reside in the first split-off eg-derived level, thus creating an electronic inequivalence between the C atoms in the benzene rings. For higher Rb concentrations, a reduction of the Cu oxidation state is observed, together with a new Cu-derived state in valence-band photoemission spectra, testifying to the filling of the b1gorbital. Thus, even though b1gis the lowest unoccupied orbital of the neutral molecule, in the film, the Cu-derived b1g-derived states are occupied only after a partial filling of the eg-derived band has taken place. Despite the fact that the eg-derived spectral weight becomes larger as the rubidium content in the RbxC32H16N8Cu compound increases, no spectral density was observed at the Fermi level, showing that the film remains insulating for all of the investigated stoichiometries.

The conical diffraction mounting in which the direction of incident light belongs to a plane parallel to the direction of the grooves has the unique property of maintaining high diffraction efficiency, even in the extreme-ultraviolet (EUV) region. This property is useful for designing high-throughput time-delay-compensated monochromators for the spectral selection of ultrashort EUV pulses as the high-order harmonics generated by the interaction between an ultrashort laser pulse and a gas jet. The time compensation allows one to exploit the femtosecond scale duration of the harmonics both to have high intensity and to reach an unprecedented temporal resolution for pump and probe experiments. Because two gratings have to be used for time compensation, the high diffraction efficiency becomes an essential requirement, which can be fulfilled by the conical diffraction mounting. Measurements recently accomplished at the Bending Magnet for Emission Absorption and Reflectivity (BEAR) beam line (ELETTRA Synchrotron, Trieste, Italy) for three gratings in the 10-90 nm region are reported here that show a peak efficiency of as much as 0.7 in the first order. A model computing the electromagnetic propagation and the grating efficiency, implemented and tested with the experimental data, permits the study and design of rather complex systems operating in the conical mounting. Basic physical principles and mathematical aspects of the model are discussed here.

A Mo-Si multilayer mirror has been used for determination of the ellipticity and higher-order content of a synchrotron beam. The method is based on the angular measure of multilayer reflectivity in the region of Bragg first- and second-order reflections. Beam parameters were derived by a fitting procedure.

The growth modes of SrF2 on Si(001) are investigated by AFM and ultraviolet photoemission. Two growth regimes are identified according to the substrate temperature during deposition, resulting in flat and ordered fluoride layers or in nano-patterned films with characteristic triangular islands. The flat layer growth obtained at high temperature is accompanied by molecular dissociation at the interface.

We have developed and elaborated a series of Mg/Co-based periodic multilayers to build efficient mirrors for the extreme ultraviolet (EUV) range. For s-polarized light and at 45° of grazing incidence, the reflectivity of as-deposited Mg/Co is 42.6% at 25.1 nm. X-ray emission spectroscopy and nuclear magnetic resonance measurements do not indicate any noticeable interdiffusion at the interfaces between layers. Scanning transmission electronic microscopy images attest the high structural quality of the stack. X-ray reflectivity (XRR) curves in the hard x-ray and EUV domains confirm this description and estimate a weak interfacial roughness (~ 0.5 nm). Taking advantage of the magnetic character of Co, we have performed resonant magnetic reflectivity measurements by scanning the photon energy around the Co L absorption edge for opposite circular polarizations. The magnetization profile of the Co layers within Co/Mg determined with an expected depth resolution of one monolayer confirms the interface abruptness. Scanning electron microscopy images and XRR curves give evidence of the thermal stability of Mg/Co up to 300 °C. From that value, a strong change in the sample morphology due to the delamination of the multilayer from the substrate occurs. This should account for the drastic reflectivity drop observed above this temperature. Starting from Mg/Co, we have inserted a Zr layer at one or at the other interface or at both interfaces to estimate the effect of the introduction of a third material within the period. We have found that Mg/Co/Zr is more efficient (50% of reflectivity) than Mg/Zr/Co and Mg/Zr/Co/Zr (~ 40%). Through time-of-flight secondary ion mass spectrometry depth profiling and NMR measurements, we have assigned this difference to an intermixing process when Co layers are deposited onto Zr layers.

The irradiation effects of multiple ultrafast shots of laser beams with estimated fluences of some tens of mJ/cm2on 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/cm2and 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.