Space-charge effects in high-energy photoemission

Pump-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.Comment: 15 pages, 2 tables, 8 figure

Resonant Soft X-ray Reflectivity in the Study of Magnetic Properties of Low-Dimensional Systems

In 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

Kramers-Kronig relations and precision limits in quantum phase estimation

The ultimate precision in any measurement is dictated by the physical process implementing the observation. The methods of quantum metrology have now succeeded in establishing bounds on the achievable precision for phase measurements over noisy channels. In particular, they demonstrate how the Heisenberg scaling of the precision can not be attained in these conditions. Here we discuss how the ultimate bound in presence of loss has a physical motivation in the Kramers-Kronig relations and we show how they link the precision on the phase estimation to that on the loss parameter

Dichroism of x-­ray fluorescence under standing waves regime in magnetic periodic multilayers

We present the first test of the implementation of a characterization method whose aim is to study the interfaces of magnetic and periodic hetero-structures. The methodology relies on the combination of two techniques, generation of x-ray standing waves and dichroism in x-ray emission. The first one gives the depth selectivity since the maximum of the electric field can be put in specific locations of the stack, the centre of layers or their interfaces, while the second one enables being sensitive to the magnetic character of the atoms present within the stack. To concentrate on the methodology, the well-studied Mg/Co multilayer is analysed by using incident photon of monochromatic energies across the Co L2,3 absorption edge and measuring the intensity of the Co Lαβ emission. Despite large dispersive effects preventing the maxima of the electric field to reach the interfaces of the stack, it has been possible to observe the dichroic signal in the angular distribution of the Co emission intensity, i.e. in the so-called x-ray standing wave curve

Photoemission from the gas phase using soft x-ray fs pulses: An investigation of the space-charge effects

An experimental and computational investigation of the space-charge effects occurring in ultrafast photoelectron spectroscopy from the gas phase is presented. The target sample CF$_3$I is excited by ultrashort (100 fs) far-ultraviolet radiation pulses produced by a free-electron laser. The modification of the energy distribution of the photoelectrons, i.e. the shift and broadening of the spectral structures, is monitored as a function of the pulse intensity. A novel computational approach is presented in which a survey spectrum acquired at low radiation fluence is used to determine the initial energy distribution of the electrons after the photoemission event. The spectrum modified by the space-charge effects is then reproduced by $N$-body calculations that simulate the dynamics of the photoelectrons subject to the mutual Coulomb repulsion and to the attractive force of the positive ions. The employed numerical method allows to reproduce the complete photoelectron spectrum and not just a specific photoemission structure. The simulations also provide information on the time evolution of the space-charge effects on the picosecond scale. Differences with the case of photoemission from solid samples are highlighted and discussed. The presented simulation procedure constitutes an effective tool to predict and account for space-charge effect in time-resolved photoemission experiments with high-intensity pulsed sources.Comment: 18 pages, 4 figures, 1 tabl

Resonant Soft X-ray Reflectivity in the Study of Magnetic Properties of Low-Dimensional Systems

In 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

Soft-X study of buried interfaces in stratified media

Abstrac

Electrical activation of the Fe2+/3+ trap in Fe-implanted InP

We have studied the electrical activation of the Fe2+/3+ trap in Fe-implanted InP by means of capacitance-voltage and deep level transient spectroscopy analyses. Five deep traps have been identified and we have characterized the concentration and depth distribution of the Fe2+/3+ deep trap, located at E-C-0.66 eV. The InP substrate background doping, i.e., the Fermi-level position, plays a crucial role in the Fe activation process by setting an upper limit to the amount of Fe centers electrically activated as deep acceptor traps