1,721,007 research outputs found
Space-charge effects in high-energy photoemission
No full textPump-and-probe photoelectron spectroscopy (PES) with femtosecond pulsed sources opens new perspectives in the investigation of the ultrafast dynamics of physical and chemical processes at the surfaces and interfaces of solids. Nevertheless, for very intense photon pulses a large number of photoelectrons are simultaneously emitted and their mutual Coulomb repulsion is sufficiently strong to significantly modify their trajectory and kinetic energy. This phenomenon, referred as space-charge effect, determines a broadening and shift in energy for the typical PES structures and a dramatic loss of energy resolution. In this article we examine the effects of space charge in PES with a particular focus on time-resolved hard X-ray (∼10 keV) experiments. The trajectory of the electrons photoemitted from pure Cu in a hard X-ray PES experiment has been reproduced through N-body simulations and the broadening of the photoemission core-level peaks has been monitored as a function of various parameters (photons per pulse, linear dimension of the photon spot, photon energy). The energy broadening results directly proportional to the number N of electrons emitted per pulse (mainly represented by secondary electrons) and inversely proportional to the linear dimension a of the photon spot on the sample surface, in agreement with the literature data about ultraviolet and soft X-ray experiments. The evolution in time of the energy broadening during the flight of the photoelectrons is also studied. Despite its detrimental consequences on the energy spectra, we found that space charge has negligible effects on the momentum distribution of photoelectrons and a momentum broadening is not expected to affect angle-resolved experiments. Strategy to reduce the energy broadening and the feasibility of hard X-ray PES experiments at the new free-electron laser facilities are discussed
Resonant Soft X-ray Reflectivity in the Study of Magnetic Properties of Low-Dimensional Systems
Full text linkIn this review, the technique of resonant soft X-ray reflectivity in the study of magnetic low-dimensional systems is discussed. This technique is particularly appealing in the study of magnetization at buried interfaces and to discriminate single elemental contributions to magnetism, even when this is ascribed to few atoms. The major fields of application are described, including magnetic proximity effects, thin films of transition metals and related oxides, and exchange-bias systems. The fundamental theoretical background leading to dichroism effects in reflectivity is also briefly outlined
Space-charge effect in electron time-of-flight analyzer for high-energy photoemission spectroscopy
No full textThe space-charge effect, due to the instantaneous emission of many electrons after the absorption of a single photons pulse, causes distortion in the, photoelectron energy spectrum. Two calculation methods have been applied to simulate the expansion during a free flight of clouds of mono- and bi-energetic electrons generated by a high energy pulse of light and their results have been compared. The accuracy of a widely used tool, such as SIMION (R), in predicting, the energy distortion caused by the space-charge has been tested and the reliability of its results is verified. Finally we used SIMION (R) to take into account the space-charge effects in the simulation of simple photoemission experiments with a time-of-flight analyzer. (C) 2016 Elsevier B.V. All rights reserved
Nanometer-scale spatial inhomogeneities of the chemical and electronic properties of an ion implanted Mn-Ge alloy
No full textElectron trajectory simulations of time-of-flight spectrometers for core level high-energy photoelectron spectroscopy at pulsed X-ray sources
No full textThe advent of Free Electron Lasers (FELs), able to provide short (2-100 fs) and intense (10(33) photons/s/mm(2)/mrad(2)/0.1%bandwidth) pulses of light also in the hard X-ray regime (h omega> 2000 eV), opens new possibilities to study the ultrafast dynamics of processes, exploiting the capability of Hard X-ray Photoelectron Spectroscopy (HAXPES) to measure core-level spectra of elements with bulk sensitivity. In order to detect the intense bursts of high kinetic energy electrons generated by the X-ray pulses with an energy resolution comparable to the existing category of electron analyzers, a new class of spectrometers must be designed. We present a characterization of two different TOF spectrometers, namely one based on a retarding cylindrical lens and another one based on the spherical reflector geometry. SIMION (R) software has been used in order to evaluate electron trajectories of high kinetic energy electrons (5000-10,000 eV) and extract transmission properties, angular acceptance and energy resolution. It resulted that while the linear system is able to accept a larger solid angle (similar to 50 msr), the spherical mirror offers a better resolving power (around 71,000). Both analyzers are capable of a transmission above 90% within range of kinetic energies wide enough to measure the full line-shape of a core photoionization peak. Furthermore, we proved that both instruments are able to discriminate between two consecutive electron bunches having a temporal separation inferior than 220 ns, which is the distance between two consecutive photon pulses at the European X-ray Free Electron Laser (EXFEL), which is currently under construction in Hamburg
Kramers–Kronig relations and precision limits in quantum phase estimation
Full text linkPhase measurements are of paramount importance in quantum optical sensing. However, the promise of a quantum advantage, the celebrated Heisenberg scaling, is severely curtailed in the presence of noise and loss. Here we investigate systems in which phase and absorption profiles are linked by Kramers–Kronig relations and show that, in the limit of a large photon number, their use connects the uncertainties on the profiles attainable by optimal probes for loss and phase. This underlines a physical motivation for which the Heisenberg scaling for the phase is lost. Our results bear practical implications, revealing the metrological capabilities of absorption measurements in determining phase profiles
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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