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This paper is among the first to investigate the impact of over-education and over-skilling on workers' wages using a unique pan-European database covering twenty-eight countries for the year 2014, namely the CEDEFOP's European Skills and Jobs (ESJ) survey. Overall, the results suggest the existence of a wage penalty associated with over-education. When the educational and the skills mismatches are interacted with each other in order to distinguish apparent over-education from genuine over-education, the results highlight that the workers with the highest wage penalty are those who are both over-educated and over-skilled.

We report on the growth and characterization of a new class of photocathode structures for use as electron sources to produce high brightness electron beams for accelerator applications. The sources are realized using III-nitride materials and are designed to leverage the strong polarization field, which is characteristic of this class of materials when grown in their wurtzite crystal structure, to produce a negative electron affinity condition without the use of Cs, possibly allowing these materials to be operated in radio frequency guns. A Quantum Efficiency (QE) of about 1 × 10 − 3 and an emitted electrons' Mean Transverse Energy (MTE) of about 100 meV are measured at a wavelength of 265 nm. In a vacuum level of 3 × 10 − 10 Torr, the QE does not decrease after more than 24 h of continuous operation. The lowest MTE of about 50 meV is measured at 300 nm along with a QE of 1.5 × 10 − 5 . Surface characterizations reveal a possible contribution to the MTE from surface morphology, calling for more detailed studies. [ABSTRACT FROM AUTHOR]

We report the design and performance of a time-resolved electron diffraction apparatus capable of producing intense bunches with simultaneously single digit micrometer probe size, long coherence length, and 200 fs rms time resolution. We measure the 5d (peak) beam brightness at the sample location in micro-diffraction mode to be 7 × 10 13 A / m 2 rad 2 . To generate high brightness electron bunches, the system employs high efficiency, low emittance semiconductor photocathodes driven with a wavelength near the photoemission threshold at a repetition rate up to 250 kHz. We characterize spatial, temporal, and reciprocal space resolution of the apparatus. We perform proof-of-principle measurements of ultrafast heating in single crystal Au samples and compare experimental results with simulations that account for the effects of multiple scattering. [ABSTRACT FROM AUTHOR]

Experimental observations of the angular distribution of droplets during laser ablation and deposition of Al thin films are presented and discussed. The experimental results, obtained by simply moving the laser spot position with respect to the rotation axis of the target, allow clarification of the unexpected symmetric double peaked angular droplet distribution on the films. These results provide direct evidence that a laser fluence threshold exists, beyond which droplets are generated from a melt displacement and ejection mechanism rather than from a phase explosion. The main directions of particulate ejection are related to the particular geometry of the laser generated tracks, whose profiles depend on the relative position of the incident beam with respect to the rotation axis of the target. [ABSTRACT FROM AUTHOR]

Recent research on photocathodes for photoinjectors has focused on the understanding of the photoemission process at low energy (i.e. at photon energy close to the material's work function) as well as on the study of ordered and innovative photocathode materials, with the aim of minimizing the emittance at the cathode. We here present a preliminary study on low energy photoemission from (100) oriented Ba1−xLaxSnO3 thin films, characterizing their quantum efficiency and the mean transverse energy of the photoelectrons. The aim of the study is to pave the way for future experiments on innovative photocathodes based on perovkite oxides. [ABSTRACT FROM AUTHOR]

The intrinsic emittance of electron beams generated from a multi-alkali photocathode operated in a high voltage DC gun is reported. The photocathode showed sensitivity extending to the infrared part of the spectrum up to 830 nm. The measured intrinsic emittances of electron beams generated with light having wavelength longer than 800 nm are approaching the limit imposed by the thermal energy of electrons at room temperature with quantum efficiencies comparable to metallic photo-cathodes used in operation of modern photoinjectors. [ABSTRACT FROM AUTHOR]

The requirement of a short response time of a cathode with respect to the laser beam for the generation of the photoelectrons limits, in the case of SPARC photoinjectors, the choice of the cathode materials to metals that usually present response time on the order of a few tens of femtoseconds. While copper and magnesium have been extensively studied, new research activities were devoted to study the photoemission properties of yttrium and to explore the feasibility of a photocathode based on this metal. Nevertheless, to date little data has been reported in literature. We present, in this work, the results of our investigation of Y films as suitable materials for the production of bright electron beams. Bulk and thin films of Y, prepared by pulsed laser deposition on Si substrates, have been tested as photocathodes in an ultra-high vacuum photodiode chamber at 10-7 Pa. Systematic laser cleaning treatments improved the film quantum efficiency (QE) from 10-6 to 4.5×10-4 at 266 nm, comparable to those of the corresponding bulk. Preliminary QE measurements carried out in the visible range at 406 nm will be illustrated. [ABSTRACT FROM AUTHOR]

A 250-TW laser system (FLAME - Frascati laser for acceleration and multidisciplinary experiments) is now in its commissioning phase in a new laboratory at LNF-INFN in the framework of the PLASMONX (Plasma acceleration and monochromatic X-ray generation) project. The laser will deliver<25 fs duration pulses with an energy up to 6 J, at a 10 Hz repetition rate. An ad hoc target area has also been designed and is currently being set up, allowing the first test experiments of electron laser wakefield acceleration to be carried out over the next few months in a safe, radiation-protected environment. An overview of the main features of the laser system and target area is given, along with a survey of the design and set-up of the self-injection test experiment, which is expected to reach the production of sub-GeV electron bunches. [ABSTRACT FROM AUTHOR]

We report the results of preliminary tests on Mg film photocathodes deposited by pulsed laser ablation deposition on various substrates. A comparison with other deposition techniques is presented. We illustrate briefly the technique of deposition of thick films (up to several microns). Scanning electron microscope analysis of the films before and after laser cleaning has been performed to characterize the changes occurred on the surface. Measurements of the distribution and integrated value of quantum efficiency over the illuminated surface are reported. Plans for the deposition on the back flange of an RF gun are illustrated. [ABSTRACT FROM AUTHOR]

Abstract: Pd-doped-SnO2 thin films were grown on Si (100) substrates using pulsed laser deposition at room temperature. Scanning electron microscopy and atomic force microscopy studies showed that the film morphologies are strongly related to two major parameters: first, high O2 background pressure, namely ≥10 Pa, induces the formation of nanoparticles by condensation in the gas phase; second, when the target–substrate distance d t–s is close to the plume length L p , considering an adiabatic expansion, textured thin films are obtained. Their morphologies depend therefore on the O2 pressure, since by increasing the pressure from 10 to 100 Pa the film morphology changes from a cracked structure to a well developed cauliflower-like-structure with a columnar growth. X-ray photoelectron spectroscopy, energy dispersive X-ray and grazing incidence X-ray diffraction were also employed to analyze, respectively, the chemical bonding state, the chemical composition and the structure in the deposited films. Sub-stoichiometric thin films are also synthesized with a Pd content of about 2.3 at.% under O2 background pressures higher than 10 Pa. [Copyright &y& Elsevier]

Abstract: Nanostructured tin oxide films were deposited on Si(100) substrates at room temperature using the PLAD method. Depositions were achieved by using a XeCl* laser, with a fluence of 10J/cm2 to ablate a SnO2 target either in vacuum or in an atmosphere containing a relatively low partial pressure (10−2 to 100Pa) of O2. A range of spectroscopic diffraction and real space imaging techniques, SEM, EDS, XRD, and XPS were used in order to characterize the surface morphology, structure, and composition of films. XPS results indicate that the film deposited in vacuum comprises a mixture of Sn, SnO and SnO2. In contrast, films deposited in oxygen comprise only SnO and SnO2. This indicates that the deposition of tin oxide films in relatively low partial pressures of O2 prevents the accumulation of unreacted tin metal within the resulting film. SEM studies reveal a sub-micron grain size structure, while XRD analyses indicate that the samples are crystalline, with no evidence for the presence of amorphous material. In this paper we discuss the significance and implications of these findings in terms of the optimization of tin oxide films for use in gas sensing devices and other technological applications. [Copyright &y& Elsevier]

The conventional configuration (substrate parallel and frontal to the target surface) of the pulsed laser ablation deposition technique has been modified for improving the film uniformity with acceptable deposition rates, leading to the ‘dynamic deposition configuration’. The novelty of this arrangement is the ability to place by computer-control the substrate synchronously in front of the plume axis compensating the plume deflection effect. The new geometrical configuration was used to deposit films of Ti, Al and Cu. All the deposited films exhibited both high thickness uniformity and remarkable deposition rates. Initial results of the experiments are explained and compared with the results obtained in the conventional configuration, in otherwise similar experimental conditions. [Copyright &y& Elsevier]

The deposition process of Al thin films by nanosecond laser irradiation with a special hemi-cylindrical holder, containing an array of silicon substrates, is studied. The experimental results, obtained by scanning electron microscopy investigation, show that the ablated material produces an unexpected component in a direction that is approximately specular to that of the incident direction of the laser beam. This component, never before reported in the literature, is mainly formed of micrometer droplets. Simple calculations and qualitative assessments show that the forces induced by radiation pressure, thermoelastic waves, or shockwave formation cannot explain the direction of this new component. [ABSTRACT FROM AUTHOR]

Sodium potassium antimonide photocathodes with Quantum Efficiency (QE) in the range of few percent have been grown, and their photoemission properties are measured. We report the intrinsic emittance and response time of electron bunches extracted from this material. It is possible to recover the QE of an overheated cathode by simple potassium addition, and the cathode is rugged enough to deliver tens of mA of average current with no or minimal degradation. [ABSTRACT FROM AUTHOR]

Ultra-short electron beams, produced through the velocity bunching compression technique, are used to drive the SPARC linear accelerator based source, which relies on the emission of coherent transition radiation in the terahertz range. This paper reports on the main features of this radiation, as terahertz source, with spectral coverage up to 5 THz and pulse duration down to 200 fs, with an energy per pulse of the order of several micro-joule, and as electron beam longitudinal diagnostics. [ABSTRACT FROM AUTHOR]

The injection of a seed in a free-electron laser (FEL) amplifier reduces the saturation length and improves the longitudinal coherence. A cascaded FEL, operating in the high-gain harmonic-generation regime, allows us to extend the beneficial effects of the seed to shorter wavelengths. We report on the first operation of a high-gain harmonic-generation free-electron laser, seeded with harmonics generated in gas. The third harmonics of a Ti:sapphire laser, generated in a gas cell, has been amplified and up-converted to its second harmonic (λrad = 133 nm) in a FEL cascaded configuration based on a variable number of modulators and radiators. We studied the transition between coherent harmonic generation and superradiant regime, optimizing the laser performances with respect to the number of modulators and radiators. [ABSTRACT FROM AUTHOR]

The growth of thin films by pulsed-laser deposition involves extremely complex physical processes. The study of different aspects of the basic mechanisms of the laser ablation is still in progress, with special emphasis on the modelling of the plume deflection effect and the structure formation on the irradiated surface. In this work, the initial stages of the laser ablation process have been investigated considering the surface roughness and the continuous morphological changes of the surface produced by laser irradiation. Assuming a thermal ablation model, a computational approach of the structure formation on the irradiated surface by finite elements using ansys® (6.1) has been developed. Complementary, different ablation experiments on silicon wafers using a XeCl excimer laser (308 nm) impinging at 45° with respect to the target were carried out. Scanning electron microscope analyses were performed to study the morphological changes of the Si surface. The validity of the model and the agreement of the numerical results are discussed. [Copyright &y& Elsevier]