Electron Emission Spectra Research Articles

Single electron ionization of multishell atoms: dynamic screening and post–prior discrepancies in the CDW-EIS model

A complete formulation of the post-version of the continuum distorted wave-eikonal initial state (CDW-EIS) model is used to investigate single ionization of multishell atoms by fast bare proton beams. The influence of the non-ionized electrons on the dynamic evolution is studied for each of the different shells of the targets. Its inclusion was made by means of the parametric Green–Sellin–Zachor potential. In this way, it is shown that discrepancies between the prior- and post-versions of the CDW-EIS model are avoided for any nl states of the systems studied here. The present analysis is supported by comparisons with existing experimental electron emission spectra.

Sub-GeV Electron and Positron Channeling in Straight, Bent and Periodically Bent Silicon Crystals

Preliminary results of numerical simulations of electron and positron channeling and emission spectra are reported for straight, uniformly bent and periodically bent silicon crystal. The projectile trajectories are computed using the newly developed module [1] of the MBN Explorer package [2,3]. The electron and positron channeling along Si(110) and Si(111) crystallographic planes are studied for the projectile energies 195-855 MeV.

Direct Measurement of Auger Electrons Emitted from a Semiconductor Light-Emitting Diode under Electrical Injection: Identification of the Dominant Mechanism for Efficiency Droop

We report on the unambiguous detection of Auger electrons by electron emission spectroscopy from a cesiated InGaN/GaN light-emitting diode under electrical injection. Electron emission spectra were measured as a function of the current injected in the device. The appearance of high energy electron peaks simultaneously with an observed drop in electroluminescence efficiency shows that hot carriers are being generated in the active region (InGaN quantum wells) by an Auger process. A linear correlation was measured between the high energy emitted electron current and the "droop current"--the missing component of the injected current for light emission. We conclude that the droop phenomenon in GaN light-emitting diodes originates from the excitation of Auger processes.

Ionization of water molecules by proton impact: Two nonperturbative studies of the electron-emission spectra

Two nonperturbative methods are applied to obtain total and singly differential (in the electron energy) cross sections of electron emission in proton collisions with H${}_{2}$O at impact energies in the range 10 keV $\ensuremath{\le}{E}_{\mathrm{p}}\ensuremath{\le}$ 5 MeV. Both methods, one classical and one semiclassical, combine an independent particle treatment with a multicenter model potential description of the target. The excellent agreement obtained with experimental data supports the usefulness of the approximations involved and encourages the study of more complex systems.

Study of the hard component of pulsed X-ray emission of micropinch discharge plasma

A set of spectrometers that allow simultaneous measurements of the spectra of pulsed X-ray and electron emission from a micropinch discharge in a wide energy range (1.5–500 keV) is described. The experimental results of the study of electron and X-ray spectra of micropinch discharge plasma are discussed. The mechanism for the formation of hard X-rays is caused by acceleration processes in micropinch discharge plasma.

Two-Center Resonant Photoionization

Photoionization of an atom A, in the presence of a neighboring atom B, can proceed via resonant excitation of B with subsequent energy transfer to A through two-center electron-electron correlation. We demonstrate that this two-center mechanism can strongly outperform direct photoionization at nanometer internuclear distances and possesses characteristic features in its time development and the spectrum of emitted electrons.

Electron emission in laser-nanotip interaction

The interaction of ultrashort laser pulses with nano-scale metal tips provides a unique testing ground for strong-field physics in a confined solid state environment. We theoretically investigate the spectrum of emitted electrons, which is directly related to the quantum dynamics at the surface.

Convoy electron emission following ionization of highly-charged ions excited by resonant coherent excitation

Projectile ionization of highly-charged Ar and Fe ions in the excited states passing relativistically fast through a thin crystalline foil was experimentally studied. We selectively controlled the population of the excited states of the projectiles, and their alignment by choosing a specific m-state through three-dimensional resonant coherent excitation technique by periodical electric fields in a crystalline. We measured energy-differential spectra of electron emission released from projectiles at zero degree. Under the resonance condition, we found an evident enhancement of the convoy electron yield, which reflects the electron momentum distribution of the initial bound state of the excited ions.

Photoelectron emission from LiF surfaces by ultrashort pulses

Energy- and angle- resolved electron emission spectra produced by incidence of ultrashort electromagnetic pulses on a LiF(001) surface are studied by employing a distorted-wave method. The model, named crystal surface-Volkov (CSV) approximation, is applied to investigate the effects introduced by the crystal lattice when the electric field is oriented parallel to the surface plane. These effects are essentially governed by the vector potential of the external field, while the influence of the crystal orientation was found to be negligible.

Ionization and fragmentation of RNA base molecule uracil in collisions with carbon ions of energies between 100 keV and 60 MeV

We report here the first measurement of absolute single ionization cross-section of uracil (C4H4N2O2, m=112) in collisions with highly charged C ions of energy ranging between 100 keV to 60 MeV i.e. in the range of Bragg peak which is relevant for high energy hadron therapy. An ECR based low energy accelerator along with a 14 MV Pelletron accelerator were used to obtain a wide range of energies. Energy and charge state (representing perturbation strength) dependence of io-nization cross-section has been studied using a ToF mass spectrometer. In the low energy range, cross-section increases with energy and then saturates while in the high energy range it decreases with energy. Ionization cross-section found to increase linearly with charge-state. The CTMC and CDW-EIS models are used to compare with the data. The complementary experiment was also carried out to measure the low energy electron emission spectrum at different angles.

Modeling of biexciton and trion emission line shapes in quantum wells

Energy levels and wave functions were calculated numerically for a system of two to four interacting electrons and holes in a quantum well by a direct solution of the many-body Schroedinger equation. It was found that long-wavelength tails of the emission spectrum of electron–hole complexes decayed exponentially. Analytical expressions were proposed to describe them. The spontaneous emission line widths in the trion and biexciton spectra were estimated.

Primary energy dependence of secondary electron emission from graphene adsorbed on Ni(111)

We measured energy distribution and yields of electrons emitted from graphene adsorbed on a Ni(111) surface under the impact of 200–1200 eV electrons. The adsorption of graphene results in significant changes of the spectra of emitted electrons but not of the total emission yields. The electronic structure of the graphitic overlayer determines the final states of excited electrons transported by collision cascade from the bulk of the substrate.

Synthesis and luminescent properties of Zn0.890Nb2O6:Eu0.053+, Bi0.0053+, M0.055+ (M = Li, Na, K) phosphors

Zn1–xNb2O6:Eux3+, Zn0.95−yNb2O6:Eu0.053+, Biy3+, and Zn0.890Nb2O6:Eu0.053+, Bi0.0053+, M0.055+ (M = Li, Na, K) red-emitting phosphors were synthesized via sol–gel method. X-ray power diffraction, scanning electron microscopy, photoluminescence excitation, and emission spectra (PL) were used to characterize the phosphors. The obtained Zn1−xNb2O6:Eux3+ phosphor showed a stronger excitation band near 400 nm. When Eu3+ and Bi3+ were incorporated into the ZnNb2O6 lattice, a broad band from 300 nm to 350 nm with the center at 329 nm appeared. Taking the ion size difference of Li+ (59 pm), Na+ (99 pm), K+ (137 pm), Eu3+ (95 pm), and Zn2+ (60 pm), the emitting intensity of the phosphor increased observably by adding Li+ and Na+ as charge compensators. The PL intensity of the K+-doped in Zn0.945Nb2O6:Eu0.053+, Bi0.0053+ was slightly less than those of Zn0.945Nb2O6:Eu0.053+, Bi0.0053+. The chromaticity coordinates of Zn0.890Nb2O6:Eu0.053+, Bi0.0053+, Li0.055+ (x = 0.67, y = 0.34) were close to the standard of National Television Standard Committee values (x = 0.670, y = 0.330). The fabricated light-emitting diode (LED) further confirmed that the Zn0.890Nb2O6:Eu0.053+, Bi0.0053+, Li0.055+ phosphors can efficiently absorb up to 400 nm irradiation and emit red light, and are potential candidates as red-emitting components for white LED.

Tunable white light emission from γ-irradiated Ag/Eu co-doped phosphate glass phosphor

We report a new tunable white light emission from γ-irradiated Ag/Eu co-doped phosphate glass phosphor. The appearance of Eu2+ and silver aggregates after γ-irradiation was confirmed via Electron paramagnetic resonance (EPR), absorption, and emission spectra. The effects of Ag/Eu doping concentration and excitation wavelengths on white light emission were discussed. A pure white light emission with the chromaticity coordinates (X = 0.331, Y = 0.329) was generated using the optimal sample under excitation at 360 nm.

Viable thermionic emission from graphene-covered metals

Thermionic emission from monolayer graphene grown on representative transition metals, Ir and Ru, is characterized by low-energy electron microscopy. Work functions were determined from the temperature dependence of the emission current and from the electron energy spectrum of emitted electrons. The high-temperature work function of the strongly interacting system graphene/Ru(0001) is sufficiently low, 3.3 ± 0.1 eV, to have technological potential for large-area emitters that are spatially uniform, efficient, and chemically inert. The thermionic work functions of the less strongly interacting system graphene/Ir(111) are over 1 eV larger and vary substantially (0.4 eV) between graphene orientations rotated by 30°.

Tunable white light-emission of a CaW1−x Mo x O4:Tm3+, Tb3+, Eu3+ phosphor prepared by a Pechini sol–gel method

A white light-emitting CaW1−x Mo x O4:Tm3+, Tb3+, Eu3+ phosphor was prepared by a Pechini sol–gel method. The incorporation of Mo6+ into the CaWO4 host matrix can broaden its excitation range and promote tunability to its emission. When the CaW1−x Mo x O4 system is triply-doped with Tm3+, Tb3+, and Eu3+ ions, energy transfer occurs from both WO4 2− and MoO4 2− groups to Tm3+ and Tb3+ ions. A significant red-shift in the excitation of Eu3+ allows the resulting emission to be tunable between cool, natural, and warm white light by varying the excitation wavelength. The undoped and triply-doped CaW1−x Mo x O4 phosphors were characterized by X-ray diffraction, scanning electron microscopy, photoluminescence excitation and emission spectra, and CIE chromaticity (x, y) coordinates.

Auger spectrum of a water molecule after single and double core ionization

The high intensity of free electron lasers opens up the possibility to perform single-shot molecule scattering experiments. However, even for small molecules, radiation damage induced by absorption of high intense x-ray radiation is not yet fully understood. One of the striking effects which occurs under intense x-ray illumination is the creation of double core ionized molecules in considerable quantity. To provide insight into this process, we have studied the dynamics of water molecules in single and double core ionized states by means of electronic transition rate calculations and ab initio molecular dynamics (MD) simulations. From the MD trajectories, photoionization and Auger transition rates were computed based on electronic continuum wavefunctions obtained by explicit integration of the coupled radial Schrödinger equations. These rates served to solve the master equations for the populations of the relevant electronic states. To account for the nuclear dynamics during the core hole lifetime, the calculated electron emission spectra for different molecular geometries were incoherently accumulated according to the obtained time-dependent populations, thus neglecting possible interference effects between different decay pathways. We find that, in contrast to the single core ionized water molecule, the nuclear dynamics for the double core ionized water molecule during the core hole lifetime leaves a clear fingerprint in the resulting electron emission spectra. The lifetime of the double core ionized water was found to be significantly shorter than half of the single core hole lifetime.

Kinetic energy release in fragmentation processes following electron emission: A time-dependent approach

A time-dependent approach for the kinetic energy release (KER) spectrum is developed for a fragmentation of a diatomic molecule after an electronic decay process, e.g., Auger process. It allows one to simulate the time-resolved spectra and provides more insight into the molecular dynamics than the time-independent approach. Detailed analysis of the time-resolved emitted electron and KER spectra sheds light on the interrelation between wave packet dynamics and spectra.

Elaboration and Luminescent Properties of Eu/Tb Co‐Doped GdPO4‐Based Glass Ceramics for White LEDs

Eu/Tb single‐doped and co‐doped oxyfluoride aluminosilicate glasses and glass ceramics containing GdPO4 nanocrystals were elaborated, and their structural and luminescent properties were investigated by X‐ray diffraction, Transmission electron microscopy, excitation and emission spectra, as well as decay lifetime measurement. Excited by 350 nm light, a combination of blue (5d→4f of Eu2+), green (5D4→7F5 of Tb3+) and red (5D0→7F2 of Eu3+) emissions emerges to achieve white emission in Eu/Tb co‐doped precursor glasses and glass ceramics. Our research shows that Eu/Tb co‐doped precursor glasses and GdPO4‐based glass ceramics may be applied in lighting and luminescent aspects.

Solvothermal Synthesis and Optical Performance of One-dimensional Strontium Hydroxyapatite Nanorod

One-dimensional strontium hydroxyapatite (Sr-HAp) nanorods were successfully synthesized by a simple solvothermal method. The products were characterized via X-ray diffraction (XRD), Fourier transform infrared (FT-IR), cold field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), photoluminescence (PL) excitation and emission spectra. The experimental results indicated that oleic acid as a surfactant played a key role in confining the growth of the Sr-HAp powders. A possible formation mechanism of the one-dimensional nanorod was proposed and elaborated. Moreover, the as-obtained Sr-HAp samples showed an intense and bright emission band centered at 460 nm under long-wavelength UV light excitation and the contents of NaOH used in the synthetic process had an obvious impact on the optical performance of Sr-HAp powders. The possible luminescent mechanism of the Sr-HAp samples was discussed.