Auger electron angular distributions following excitation or ionization from the Xe 3d and F 1s levels in xenon difluoride

Electron spectroscopy following Xe 3d and F 1s ionization in XeF2 elucidates the influence of core electrons on molecular bonding.

Measuring the photoelectron emission delay in the molecular frame

How long does it take to emit an electron from an atom? This question has intrigued scientists for decades. As such emission times are in the attosecond regime, the advent of attosecond metrology using ultrashort and intense lasers has re-triggered strong interest on the topic from an experimental standpoint. Here, we present an approach to measure such emission delays, which does not require attosecond light pulses, and works without the presence of superimposed infrared laser fields. We instead extract the emission delay from the interference pattern generated as the emitted photoelectron is diffracted by the parent ion’s potential. Targeting core electrons in CO, we measured a 2d map of photoelectron emission delays in the molecular frame over a wide range of electron energies. The emission times depend drastically on the photoelectrons’ emission directions in the molecular frame and exhibit characteristic changes along the shape resonance of the molecule.

Ultrafast electron energy-dependent delocalization dynamics in germanium selenide

Ultrafast scattering process of high-energy carriers plays a key role in the performance of electronics and optoelectronics, and have been studied in several semiconductors. Core-hole clock spectroscopy is a unique technique for providing ultrafast charge transfer information with sub-femtosecond timescale. Here we demonstrate that germanium selenide (GeSe) semiconductor exhibits electronic states-dependent charge delocalization time by resonant photo exciting the core electrons to different final states using hard-x-ray photoemission spectroscopy. Thanks to the experiment geometry and the different orbital polarizations in the conduction band, the delocalization time of electron in high energy electronic state probed from Se 1s is ~470 as, which is three times longer than the delocalization time of electrons located in lower energy electronic state probed from Ge 1s. Our demonstration in GeSe offers an opportunity to precisely distinguish the energy-dependent dynamics in layered semiconductor, and will pave the way to design the ultrafast devices in the future.

Probes: Sources and Their Interactions with Matter

Probes are generated using laboratory sources, or in large user facilities. Photon sources include incandescence and plasma discharge lamps. Electron beams are generated using thermionic or field-emission sources. RF plasma sources generate ions that are accelerated and used for scattering experiments. Specimens should be probed first with light, as it causes the least damage. Electron interaction with matter causes beam broadening, atomic displacements, sputtering, or radiolysis leading to mass loss and local contamination. Neutrons are heavier than electrons, penetrate more deeply in materials, and require more sample for analysis. Protons (positive charge, heavier than electrons) go a longer way in the specimen without significant broadening. Ions in solids undergo kinematic collisions with conservation of energy and momentum; they also lose energy continuously as they propagate. In the back-scattering geometry, they form important methods of Rutherford backscattering spectroscopy (RBS) and low-energy ion scattering spectroscopy (LEISS). Medium energy ions generate secondary ions by sputtering that can be analyzed by mass spectrometers to determine specimen composition (SIMS). Alternatively, its composition is analyzed (ICP-MS), by creating an aqueous dispersion and converting it to a plasma. Finally, interaction of high-energy ions with core electrons can lead to inner shell ionization and characteristic X-ray emission (PIXE).

Non-linear self-driven spectral tuning of Extreme Ultraviolet Femtosecond Pulses in monoatomic materials

Self-action nonlinearity is a key aspect – either as a foundational element or a detrimental factor – of several optical spectroscopies and photonic devices. Supercontinuum generation, wavelength converters, and chirped pulse amplification are just a few examples. The recent advent of Free Electron Lasers (FEL) fostered building on nonlinearity to propose new concepts and extend optical wavelengths paradigms for extreme ultraviolet (EUV) and X-ray regimes. No evidence for intrapulse dynamics, however, has been reported at such short wavelengths, where the light-matter interactions are ruled by the sharp absorption edges of core electrons. Here, we provide experimental evidence for self-phase modulation of femtosecond FEL pulses, which we exploit for fine self-driven spectral tunability by interaction with sub-micrometric foils of selected monoatomic materials. Moving the pulse wavelength across the absorption edge, the spectral profile changes from a non-linear spectral blue-shift to a red-shifted broadening. These findings are rationalized accounting for ultrafast ionization and delayed thermal response of highly excited electrons above and below threshold, respectively.

Solar Wind Plasma Particles Organized by the Flow Speed

Recent reports of the first data from Parker Solar Probe (PSP) have pointed to a series of links, correlations or anti-correlations between the solar wind bulk speed ($V_{\mathrm{SW}}$ V SW ) and physical properties of plasma particles from less than 0.25 AU in the corona. In the present paper, we describe corresponding and additional links of solar wind properties, at 0.4 AU and 1.0 AU, in an attempt to complement the PSP data and understand their evolution. A detailed analysis is carried out for the main electron populations, comparing the low-energy (thermal) core and the collisionless suprathermal halo. We show that the anti-correlation observed at 0.4 AU between $V_{\mathrm{SW}}$ V SW and the number density (average value) is maintained also at 1 AU for both the core and halo electrons. On the contrary, only the core electrons manifest a clear anti-correlation of the temperature with $V_{\mathrm{SW}}$ V SW , while the halo temperature does not vary much. We also describe the ions, protons and helium, which have a more reduced mobility and their properties exhibit different variations with the solar wind speed. The results are used to shed more light on the mechanisms leading to a differential acceleration of these species and the origin of slow and fast wind modulation.

Clarifying the quantum mechanical origin of the covalent chemical bond

Lowering of the electron kinetic energy (KE) upon initial encounter of radical fragments has long been cited as the primary origin of the covalent chemical bond based on Ruedenberg’s pioneering analysis of H$${}_{2}^{+}$$ 2 + and H2 and presumed generalization to other bonds. This work reports KE changes during the initial encounter corresponding to bond formation for a range of different bonds; the results demand a re-evaluation of the role of the KE. Bonds between heavier elements, such as H3C–CH3, F–F, H3C–OH, H3C–SiH3, and F–SiF3 behave in the opposite way to H$${}_{2}^{+}$$ 2 + and H2, with KE often increasing on bringing radical fragments together (though the total energy change is substantially stabilizing). The origin of this difference is Pauli repulsion between the electrons forming the bond and core electrons. These results highlight the fundamental role of constructive quantum interference (or resonance) as the origin of chemical bonding. Differences between the interfering states distinguish one type of bond from another.

Rayleigh and Raman Scattering from Alkali Atoms

Two computational methods developed recently [McNamara, Fursa, and Bray, Phys. Rev. A 98, 043435 (2018)] for calculating Rayleigh and Raman scattering cross sections for atomic hydrogen have been extended to quasi one-electron systems. A comprehensive set of cross sections have been obtained for the alkali atoms: lithium, sodium, potassium, rubidium, and cesium. These cross sections are accurate for incident photon energies above and below the ionization threshold, but they are limited to energies below the excitation threshold of core electrons. The effect of spin-orbit interaction, importance of accounting for core polarization, and convergence of the cross sections have been investigated.

Simple and Accurate Exchange Energy for Density Functional Theory

A non-empirical exchange functional based on an interpolation between two limits of electron density, slowly varying limit and asymptotic limit, is proposed. In the slowly varying limit, we follow the study by Kleinman from 1984 which considered the response of a free-electron gas to an external periodic potential, but further assume that the perturbing potential also induces Bragg diffraction of the Fermi electrons. The interpolation function is motivated by the exact exchange functional of a hydrogen atom. Combined with our recently proposed correlation functional, tests on 56 small molecules show that, for the first-row molecules, the exchange-correlation combo predicts the total energies four times more accurately than the presently available Quantum Monte Carlo results. For the second-row molecules, errors of the core electrons exchange energies can be corrected, leading to the most accurate first- and second-row molecular total energy predictions reported to date despite minimal computational efforts. The calculated bond energies, zero point energies, and dipole moments are also presented, which do not outperform other methods.