Photoelectron Source Research Articles

Nanoscale photoemission in a plasmon focusing lens with a nanohole explored by time-of-flight photoemission electron microscopy

Propagating surface plasmon polaritons (SPPs) provide an important platform for the design of various photoelectric devices such as nanometer-scale ultrafast electron sources. Here, a high-brightness nanoscale photoelectron source in a plasmon focusing lens with a nanohole is investigated using time-of-flight photoemission electron microscopy (TOF-PEEM). By exploiting the high spatial resolution of TOF-PEEM, it is found that the photoemission was localized at the nanoscale in both x and y directions. In addition, a large multiphoton photoemission enhancement was achieved and a strong-field effect was observed due to the interplay between the SPP and the nanohole. This paper provides a new idea for the development of the high-brightness nanoscale photoelectron sources and a deep understanding of the interplay between SPP and LSP.

A semiempirical model for low energy electron–atom transport cross sections: The case of noble gases

A semiempirical approach to describe low energy electron–atom transport cross sections of easy implementation and reproduction is presented. The heart of the model is an energy independent two-parameter potential that was adjusted to reproduce the accurate total cross sections for He, Ne, Ar and Kr, measured with a threshold photoelectron source technique from meV up to 20 eV. Once the potential was conceived, the model was validated by comparing the values obtained for the calculated scattering lengths and phase shifts with the respective quantities previously reported in the literature. We close the article by presenting the momentum transfer and viscosity cross sections. Good agreement is found when compared to the similar data obtained from swarm experiments, from phase shifts according to differential cross section measurements and to the cross sections reported by sophisticated ab initio relativistic many-body calculations. Tables for the phase shifts and cross sections are provided for direct use and applications.

Retrofitting a Photoelectron Source: Improving Resolution & Functionality

Retrofitting a Photoelectron Source: Improving Resolution & Functionality

Low-Energy Electron Damage to Condensed-Phase DNA and Its Constituents.

The complex physical and chemical reactions between the large number of low-energy (0–30 eV) electrons (LEEs) released by high energy radiation interacting with genetic material can lead to the formation of various DNA lesions such as crosslinks, single strand breaks, base modifications, and cleavage, as well as double strand breaks and other cluster damages. When crosslinks and cluster damages cannot be repaired by the cell, they can cause genetic loss of information, mutations, apoptosis, and promote genomic instability. Through the efforts of many research groups in the past two decades, the study of the interaction between LEEs and DNA under different experimental conditions has unveiled some of the main mechanisms responsible for these damages. In the present review, we focus on experimental investigations in the condensed phase that range from fundamental DNA constituents to oligonucleotides, synthetic duplex DNA, and bacterial (i.e., plasmid) DNA. These targets were irradiated either with LEEs from a monoenergetic-electron or photoelectron source, as sub-monolayer, monolayer, or multilayer films and within clusters or water solutions. Each type of experiment is briefly described, and the observed DNA damages are reported, along with the proposed mechanisms. Defining the role of LEEs within the sequence of events leading to radiobiological lesions contributes to our understanding of the action of radiation on living organisms, over a wide range of initial radiation energies. Applications of the interaction of LEEs with DNA to radiotherapy are briefly summarized.

Precision measurement of the electron energy-loss function in tritium and deuterium gas for the KATRIN experiment

The KATRIN experiment is designed for a direct and model-independent determination of the effective electron anti-neutrino mass via a high-precision measurement of the tritium upbeta -decay endpoint region with a sensitivity on m_nu of 0.2 hbox {eV}/hbox {c}^2 (90% CL). For this purpose, the upbeta -electrons from a high-luminosity windowless gaseous tritium source traversing an electrostatic retarding spectrometer are counted to obtain an integral spectrum around the endpoint energy of 18.6 keV. A dominant systematic effect of the response of the experimental setup is the energy loss of upbeta -electrons from elastic and inelastic scattering off tritium molecules within the source. We determined the energy-loss function in-situ with a pulsed angular-selective and monoenergetic photoelectron source at various tritium-source densities. The data was recorded in integral and differential modes; the latter was achieved by using a novel time-of-flight technique. We developed a semi-empirical parametrization for the energy-loss function for the scattering of 18.6-keV electrons from hydrogen isotopologs. This model was fit to measurement data with a 95% hbox {T}_2 gas mixture at 30 K, as used in the first KATRIN neutrino-mass analyses, as well as a hbox {D}_2 gas mixture of 96% purity used in KATRIN commissioning runs. The achieved precision on the energy-loss function has abated the corresponding uncertainty of sigma (m_nu ^2)< {{10}^{-2}}{hbox {eV}^{2}} [1] in the KATRIN neutrino-mass measurement to a subdominant level.

Temporary anion states of fluorine substituted benzenes probed by charge transfer in O2−·C6H6−xFx (x = 0–5) ion–molecule complexes

The broadband photoelectron source realized by detaching O2 -·X (X = neutral unsaturated molecule) complexes offers a unique opportunity to probe temporary anion states of the unsaturated species. Detachment of the ion molecule complex typically accesses a dissociative portion of the neutral potential, creating a continuum electron source that can undergo scattering with X. We present the application of this new approach to electron-neutral scattering toward a study of the series of fluorinated benzenes via photoelectron spectroscopy of O2 -·C6H6-xFx (x = 0-6) measured with several photon energies. We compare these spectra to the reference O2 -·hexane spectrum and observe evidence of temporary anion states of C6H6-xFx for species with x = 0-5 in the form of enhanced signal intensity at electron kinetic energies coinciding with the energies of the temporary anions. Furthermore, we observe autodetachment features in the x = 3, 5 spectra. Results of calculations on the isolated symmetric isomer of C6H3F3 suggest that the molecule cannot support a weakly-bound non-valence state that could be associated with the observed autodetachment. However, C6HF5 - is predicted to support a valence bound state, which, if produced by charge transfer from O2 - with sufficient vibrational energy, may undergo autodetachment. Finally, the [O2·C6F6]- spectrum is unique insofar as the spectrum is substantially higher in binding energy and qualitatively different from the x = 0-5 spectra. This result suggests much stronger interactions and charge delocalization between O2 - and C6F6.

Surface localisation of laser photoelectron emission using Cu2O and Ag film structures

A laser electron source localised on a photocathode surface was developed on the basis of Ag and Cu2O thin films sequentially deposited by magnetron sputtering on a glass substrate. Photoemission of electrons under the action of the fourth (263 nm) harmonic of a femtosecond fibre laser was initiated from the surface of an Ag film deposited on the substrate surface. Photoelectron source localisation on the photocathode surface was provided by sputtering the Cu2O copper oxide film on its surface, which significantly (about 50 times) reduced the photoemission efficiency, and by subsequent removal of the copper oxide layer in the photoemission spot with a diameter of ∼15 μm by means of focused second-harmonic radiation of a femtosecond fibre laser. In conducting research on the photocathode surface, a grid of photoemission spots was formed, which greatly facilitated the laser beam alignment with the photoemission spot. A decrease in the noises of ultrafast transmission electron microscope, associated with spatial fluctuations of the laser beam initiating pulsed photoelectron emission from the microscope photocathode when using a localised electron source on the basis of a structure of thin Cu2O and Ag films, was experimentally demonstrated.

A pulsed, mono-energetic and angular-selective UV photo-electron source for the commissioning of the KATRIN experiment

The KATRIN experiment aims to determine the neutrino mass scale with a sensitivity of 200 {mathrm{meV}/mathrm{c}^2} (90% C. L.) by a precision measurement of the shape of the tritium beta -spectrum in the endpoint region. The energy analysis of the decay electrons is achieved by a MAC-E filter spectrometer. To determine the transmission properties of the KATRIN main spectrometer, a mono-energetic and angular-selective electron source has been developed. In preparation for the second commissioning phase of the main spectrometer, a measurement phase was carried out at the KATRIN monitor spectrometer where the device was operated in a MAC-E filter setup for testing. The results of these measurements are compared with simulations using the particle-tracking software “Kassiopeia”, which was developed in the KATRIN collaboration over recent years.

A pulsed, mono-energetic and angular-selective UV photo-electron source for the commissioning of the KATRIN experiment

A pulsed, mono-energetic and angular-selective UV photo-electron source for the commissioning of the KATRIN experiment

Total cross sections for electron scattering from noble-gas atoms in near- and below-thermal energy collisions

Absolute total cross sections for electron scattering from He, Ne, Ar, Kr and Xe at very low electron energies are presented. The cross sections were obtained using the threshold- photoelectron source, which employs a combination of the penetrating field technique together with the threshold photoionization of atoms by synchrotron radiation. Obtained total cross sections for electron scattering from these noble gas atoms generally agree well with those obtained in the previous experiments above 100 meV, where several experimental works have been reported. Comparison of the measured cross section for He with that of theoretical ones shows very good agreement at very low energies even below 10 meV, which confirms the validity of theoretical cross sections of He which have been regarded as the ‘standard’ cross sections. Scattering lengths for the e- - noble gas scatterings determined from the our cross sections using the modified effective range theory (MERT) showed that scattering lengths for He and Ne agree well with the values obtained in the previous experimental and theoretical studies. On the other hand, in case of heavier noble gas atoms, significant discrepancies were found between the scattering lengths derived from MERT analysis to our total cross sections and those reported in previous studies.

Tip-based source of femtosecond electron pulses at 30 keV

We present a nano-scale photoelectron source, optimized for ultrashort pulse durations and well-suited for time-resolved diffraction and advanced laser acceleration experiments. A tungsten tip of several-ten-nanometers diameter mounted in a suppressor-extractor electrode configuration allows the generation of 30 keV electron pulses with an estimated pulse duration of 9 fs (standard deviation; 21 fs full width at half maximum) at the gun exit. We infer the pulse duration from particle tracking simulations, which are in excellent agreement with experimental measurements of the electron-optical properties of the source in the spatial domain. We also demonstrate femtosecond-laser triggered operation of the apparatus. The temporal broadening of the pulse upon propagation to a diffraction sample can be greatly reduced by collimating the beam. Besides the short electron pulse duration, a tip-based source is expected to feature a large transverse coherence and a nanometric emittance.

Ultra-low-energy electron scattering cross section measurements of Ar, Kr and Xe employing the threshold photoelectron source

A new experimental technique for the measurements of the total cross section for electron scattering from atoms and molecules at very low energy is described. Momentum transfer cross sections for scattering from Ar, Kr and Xe at very low energies were carefully derived using the modified effective range theory from the recently measured total cross sections, which were obtained with a new experimental technique utilizing the threshold photoelectron source. A significant discrepancy between the momentum cross sections derived from the present analysis and those determined in the previous electron swarm studies was found at energies below 100 meV. The findings emphasize the need of further high precision experiments in the very low energy region as well as re-analysis of the previous swarm data.

Photoelectrons on closed crustal field lines at Mars

[1] A statistical survey of dayside photoelectrons in the Mars upper ionosphere is presented and discussed. A methodology for isolating photoelectron spectra on strong crustal field lines is developed and used to obtain a database of over 280,000 distributions from the Mars Global Surveyor (MGS) electron reflectometer instrument. The relationship of these electron fluxes to various controlling factors is explored and presented. It is found that much of the flux variation can be explained by a linear fit with the EUV solar radiation proxy, adjusted for the Sun-Mars distance and local solar zenith angle. The state of the lower atmosphere seems to also play a critical role in the photoelectron flux intensity. An interval with a global dust storm shows an increased flux and steepened slope for the relationship with EUV radiation. This implies that the dust storm is altering the density, composition, and/or temperature of the photoelectron source region within the thermosphere and perhaps even the characteristics of the Mars exosphere above the MGS orbit. Other parameters considered had either no influence or a small perturbation compared to this dominant trend.

Prototype of an angular-selective photoelectron calibration source for the KATRIN experiment

The method of direct neutrino mass determination based on the kinematics of tritium beta decay, which is adopted by the KATRIN experiment, makes use of a large, high-resolution electrostatic spectrometer with magnetic adiabatic collimation. In order to target a sensitivity on m(ν) of 0.2eV/c2, a detailed understanding of the electromagnetic properties of the electron spectrometer is essential, requiring comprehensive calibration measurements with dedicated electron sources. In this paper we report on a prototype of a photoelectron source providing a narrow energy spread and angular selectivity. Both are key properties for the characterisation of the spectrometer. The angular selectivity is achieved by applying non-parallel strong electric and magnetic fields: Directly after being created, photoelectrons are accelerated rapidly and non-adiabatically by a strong electric field before adiabatic magnetic guiding takes over.

Threshold photoelectron source for the study of low-energy electron scattering: Total cross section for electron scattering from krypton in the energy range from 14 meV to 20 eV

An experimental technique for the measurement of the total cross section for electron scattering from atoms and molecules at high resolution is described. The total cross sections for electron scattering from Kr in the energy range from 14 meV to 20 eV obtained with the technique are also reported. The present technique employs a combination of the penetrating field technique and the threshold photoionization of rare-gas atoms using synchrotron radiation as an electron source in order to produce a high-resolution electron beam at very low energy. The characteristics of the electron sources were determined by measuring the ionizing photon energy dependence of photoelectron yield. Absolute total cross sections for electron scattering are obtained by the attenuation method. The measured absolute values of the total cross sections for electron scattering from Kr agree with those obtained by other groups down to 175 meV, above which several experimental works have been reported. Below 175 meV, the present results generally agree with theoretical cross sections down to 14 meV. The resonant structures in the total cross sections due to Kr${}^{\ensuremath{-}}$ ($4{p}^{5}5{s}^{2 2}{P}_{3/2}$) and the Kr${}^{\ensuremath{-}}$ ($4{p}^{5}5{s}^{2 2}{P}_{1/2}$) Feshbach resonances are also reported. The resolution of the present setup has been estimated from a fit of the measured profile of the Kr${}^{\ensuremath{-}}$ ($4{p}^{5}5{s}^{2 2}{P}_{3/2}$) resonance by the theoretical curve obtained from the resonant scattering theory.

An angular resolved pulsed UV LED photoelectron source for KATRIN

The KArlsruher TRItium Neutrino experiment KATRIN aims to determine the mass of the electron antineutrino with a sensitivity of 0.2 eV/c 2 on m ( ν e ) at 90% confidence level by measuring the kinematics of the tritium beta decay with a spectrometer of MAC-E filter-type. To investigate the properties of the spectrometer, electron sources with special properties are needed. A pulsed UV LED photoelectron source with angular selectivity has been developed for this purpose. The principle has been successfully tested during measurements at the spectrometer of the Mainz neutrino mass experiment.

A GaAs photoemission electron source combined with a Reaction Microscope

Negative electron affinity GaAs photoemission electron sources are the electron sources of choice in many applications. We have built a GaAs photoemission source to be used in a reaction microscope for near threshold and kinematically complete experiments [1]. In this study, we present the current status of the photoelectron source, a precise characterization of the electron beam and first results of electron-atom scattering experiments.

A UV LED-based fast-pulsed photoelectron source for time-of-flight studies

We report on spectroscopy and time-of-flight measurements using an 18 keV fast-pulsed photoelectron source of adjustable intensity, ranging from single photoelectrons per pulse to 5 photoelectrons per μs at pulse repetition rates of up to 10 kHz. Short pulses between 40 ns and 4 μs in length were produced by switching light emitting diodes with central output wavelengths of 265 and 257 nm, in the deep ultraviolet (or UV-C) regime, at kHz frequencies. Such photoelectron sources can be useful calibration devices for testing the properties of high-resolution electrostatic spectrometers, like the ones used in current neutrino mass searches.

High resolution low-energy dielectronic recombination rate coefficients of beryllium-like germanium: QED test bench for two-valence-electron systems

Low-energy dielectronic recombination resonances of beryllium-like Ge28+ have been studied by electron collision spectroscopy in the heavy-ion storage ring TSR using a twin-electron beam technique combined with an ultracold photoelectron source. Rich resonance structure below 1.7 eV is observed which mainly arises from resonant capture of the free electron to 91 Rydberg states accompanied by excitation of inner valence electrons from the 2s2(1S0) ground state to 2s2p(1P1). The positions of resonances below 0.3 eV are measured with an accuracy better than 0.5 meV. Accurate calculations of Rydberg binding energies for these resonances are still required in order to derive a precise value for the 1S0 – 1P1 transition energy, which may give the opportunity for high-accuracy testing of QED corrections to atomic energy levels for a two valence electron system.

Monte Carlo model of electron transport for the calculation of Mars dayglow emissions

A model of the photoelectron collision‐induced component of the Mars dayglow using recent cross sections and solar flux is described. The calculation of the photoelectron source of excitation is based on a stochastic solution of the Boltzmann equation using the direct simulation Monte Carlo method. The neutral atmosphere is taken from outputs of a global circulation model, and recent inelastic collision cross sections are adopted. The calculated vertical profiles of the CO Cameron bands and CO2+ doublet emissions integrated along the line of sight compare well with the Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars (SPICAM) limb profiles observed with the SPICAM spectrograph on board Mars Express made at Ls = 166° during the summer season at northern midlatitudes. The comparison shows agreement to within the uncertainties of the excitation cross sections. Seasonal changes in the brightness and the altitude of the emission peaks are predicted with intensity variations in the range 15–20%.