Photoemission Electron Source Research Articles

Photo-emission Electron Gun and Electron Optical Simulation for Ultrafast Scanning Electron Microscope

Ultrafast scanning electron microscope (USEM) integrates pump-probe technique with microscopic imaging, enabling visualization of photon-induced surface charge dynamics with high spatial and temporal resolution. This capability is crucial for high-resolution detection of semiconductor surface states and optoelectronic devices. This work discusses the parametric design of a thermionic emission electron gun that has been modified into a photoemission electron gun, based on a home-built ultrafast scanning electron microscope. The transition to photoemission requires the removal of the self-bias voltage function of the original electron microscope power supply to ensure proper operation of the wehnelt electrode, given that the dose of the photoemitting electron beam is often significantly lower than that of thermionic emission. We quantitatively analyze the dependencies of bias voltage, cathode, wehnelt electrode, and anode on the position, spot size and divergence angle of crossover point, which aids in parameter adjustments for the modified electron gun. The analysis results indicate that if the distance between the wehnelt and the anode is adjusted from 8 mm to 23 mm, the distance between the filament and wehnelt is adjusted from 0.65 mm to 0.45 mm to cooperate with the bias adjustment, so that the normal use of high-resolution thermionic emission mode, low voltage mode and photoemission mode can be realized. Subsequently, the effect of the mirror's position on the electron optical path was analyzed. It was found that when the anode was raised 1.4 mm above the mirror, the influence on the electron optical path became negligible. Additionally, the zero-of-time and temporal broadening of the photo-electron pulse were further simulated. The results indicate that as the bias voltage increases, the time zero point of photoemission shifts later, and the temporal broadening increases. This study lays a foundation for the future development of ultrafast electron microscope and the design of photoemission electron sources.

Demonstration of sub- GV/m accelerating field in a photoemission electron gun powered by nanosecond X -band radio-frequency pulses

Radiofrequency (RF) electron guns operating at high accelerating gradients offer a pathway to producing bright electron bunches. Such beams are expected to revolutionize many areas of science: they could form the backbone of next-generation compact x-ray free-electron lasers or provide coherent ultrafast quantum electron probes. We report on the experimental demonstration of an RF photoemission electron source supporting an accelerating field close to 400~MV/m at the photocathode surface. The gun was operated in an RF transient mode driven by short $\sim 9$~ns X-band (\SI{11.7}{\giga\hertz}) RF pulses. We did not observe any major RF breakdown or significant dark current over a three-week experimental run at high accelerating fields. The demonstrated paradigm provides a viable path to forming relativistic electron beams with unprecedented brightness.

Single-Crystal Alkali Antimonide Photocathodes: High Efficiency in the Ultrathin Limit.

The properties of photoemission electron sources determine the ultimate performance of a wide class of electron accelerators and photon detectors. To date, all high-efficiency visible-light photocathode materials are either polycrystalline or exhibit intrinsic surface disorder, both of which limit emitted electron beam brightness. In this Letter, we demonstrate the synthesis of epitaxial thin films of Cs_{3}Sb on 3C-SiC (001) using molecular-beam epitaxy. Films as thin as 4nm have quantum efficiencies exceeding 2% at 532nm. We also find that epitaxial films have an order of magnitude larger quantum efficiency at 650nm than comparable polycrystalline films on Si. Additionally, these films permit angle-resolved photoemission spectroscopy measurements of the electronic structure, which are found to be in good agreement with theory. Epitaxial films open the door to dramatic brightness enhancements via increased efficiency near threshold, reduced surface disorder, and the possibility of engineering new photoemission functionality at the level of single atomic layers.

Coulomb interactions in high-coherence femtosecond electron pulses from tip emitters.

Tip-based photoemission electron sources offer unique properties for ultrafast imaging, diffraction, and spectroscopy experiments with highly coherent few-electron pulses. Extending this approach to increased bunch-charges requires a comprehensive experimental study on Coulomb interactions in nanoscale electron pulses and their impact on beam quality. For a laser-driven Schottky field emitter, we assess the transverse and longitudinal electron pulse properties in an ultrafast transmission electron microscope at a high photoemission current density. A quantitative characterization of electron beam emittance, pulse duration, spectral bandwidth, and chirp is performed. Due to the cathode geometry, Coulomb interactions in the pulse predominantly occur in the direct vicinity to the tip apex, resulting in a well-defined pulse chirp and limited emittance growth. Strategies for optimizing electron source parameters are identified, enabling advanced ultrafast transmission electron microscopy approaches, such as phase-resolved imaging and holography.

Brightness of femtosecond nonequilibrium photoemission in metallic photocathodes at wavelengths near the photoemission threshold

The operation of photoemission electron sources with wavelengths near the photoemission threshold has been shown to dramatically decrease the minimum achievable photocathode emittance, but at the cost of significantly reduced quantum efficiency (QE). In this work, we show that for femtosecond laser and electron pulses, the increase in required laser intensities due to the low QE drives the photocathode electronic distribution far from static equilibrium. We adapt an existing dynamic model of the electron occupation under high intensity laser illumination to predict the time-dependent effects of the nonequilibrium electron distribution on the QE, mean transverse energy (MTE), and emission brightness of metal photocathodes. We find that multiphoton photoemission dramatically alters the MTE as compared to thermal equilibrium models, causing the MTE to no longer be a monotonic function of photon excess energy.

A cryogenically cooled high voltage DC photoemission electron source.

Linear electron accelerators and their applications such as ultrafast electron diffraction require compact high-brightness electron sources with high voltage and electric field at the photocathode to maximize the electron density and minimize space-charge induced emittance growth. Achieving high brightness from a compact source is a challenging task because it involves an often-conflicting interplay between various requirements imposed by photoemission, acceleration, and beam dynamics. Here we present a new design for a compact high voltage DC electron gun with a novel cryogenic photocathode system and report on its construction and commissioning process. This photoemission gun can operate at ∼200 kV at both room temperature and cryogenic temperature with a corresponding electric field of 10 MV/m, necessary for achieving high quality electron beams without requiring the complexity of guns, e.g., based on RF superconductivity. It hosts a compact photocathode plug compatible with that used in several other laboratories opening the possibility of generating and characterizing electron beam from photocathodes developed at other institutions.

Spatial control of photoemitted electron beams using a microlens-array transverse-shaping technique

A common issue encountered in photoemission electron sources used in electron accelerators is the transverse inhomogeneity of the laser distribution resulting from the laser-amplification process and often use of frequency up conversion in nonlinear crystals. A inhomogeneous laser distribution on the photocathode produces charged beams with lower beam quality. In this paper, we explore the possible use of microlens arrays (fly-eye light condensers) to dramatically improve the transverse uniformity of the drive laser pulse on UV photocathodes. We also demonstrate the use of such microlens arrays to generate transversely-modulated electron beams and present a possible application to diagnose the properties of a magnetized beam.

Axial heating and temperature of RF-excited non-neutral plasmas in Penning-Malmberg traps

Electro-magnetostatic traps have been used for decades to provide long-term storage of charged particle samples or non-neutral plasmas. The dynamics and equilibrium states of these ideally simple systems can be strongly diverted from the usual working conditions (i.e. single-species, quiescent samples) in the presence of oppositely charged particles or external electric field perturbations. Both these conditions occur when the plasma is generated by means of a radio-frequency (RF) excitation continuously applied on a trap electrode. The application of RF drives of some volts over periods larger than typical collisional time scales leads to residual-gas ionization and to the accumulation of an electron plasma, a process that has previously been exploited as an alternative to thermionic or photoemission electron sources. The analysis of the axial energy distribution shows a deviation of the continuously excited final state from maxwellianity dependent on the radial position and the subsequent relaxation to equilibrium after the interruption of the drive. Systematic measurements also indicate the high sensitivity to the residual gas pressure of both the total confined charge and of the attainable densities and plasma profiles. The results are compared to the information obtained from a very simple one-dimensional electron heating model and show the validity of its most basic features together with its shortcomings.

Design of a high-bunch-charge 112-MHz superconducting RF photoemission electron source.

High-bunch-charge photoemission electron-sources operating in a continuous wave (CW) mode are required for many advanced applications of particle accelerators, such as electron coolers for hadron beams, electron-ion colliders, and free-electron lasers. Superconducting RF (SRF) has several advantages over other electron-gun technologies in CW mode as it offers higher acceleration rate and potentially can generate higher bunch charges and average beam currents. A 112 MHz SRF electron photoinjector (gun) was developed at Brookhaven National Laboratory to produce high-brightness and high-bunch-charge bunches for the coherent electron cooling proof-of-principle experiment. The gun utilizes a quarter-wave resonator geometry for assuring beam dynamics and uses high quantum efficiency multi-alkali photocathodes for generating electrons.

An (e, 2e + ion) study of low-energy electron-impact ionization and fragmentation of tetrahydrofuran with high mass and energy resolutions.

We study the low-energy (E0 = 26 eV) electron-impact induced ionization and fragmentation of tetrahydrofuran using a reaction microscope. All three final-state charged particles, i.e., two outgoing electrons and one fragment ion, are detected in triple coincidence such that the momentum vectors and, consequently, the kinetic energies for charged reaction products are determined. The ionic fragments are clearly identified in the experiment with a mass resolution of 1 amu. The fragmentation pathways of tetrahydrofuran are investigated by measuring the ion kinetic energy spectra and the binding energy spectra where an energy resolution of 1.5 eV has been achieved using the recently developed photoemission electron source. Here, we will discuss the fragmentation reactions for the cations C4H8O(+), C4H7O(+), C2H3O(+), C3H6(+), C3H5(+), C3H3(+), CH3O(+), CHO(+), and C2H3(+).

Novel method for state selective determination of electron-impact-excitation cross sections from 0° to 180°

We use an improved target recoil momentum spectroscopy setup to determine differential cross sections for excited metastable state production in atoms and molecules by electron impact and show its capabilities for an atomic helium target. A crossed beam setup with a supersonic helium jet and a pulsed electron beam at energies close to the excitation threshold of 19.82 eV was used. Measuring the recoil momentum vector of the target instead of the momentum of the scattered electron removes common restrictions to the accessible scattering angles while the microchannel plate detector ensures a high counting efficiency. Using a photoemission electron source we reach an energy resolution of about 200 meV at 1 µA peak current. Results are compared with simulations using theoretical convergent-close-coupling (CCC), R-matrix with pseudo-states (RMPS) and B-spline R-matrix (BSR) calculations and show good agreement.

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.

Thermal emittance and response time measurements of negative electron affinity photocathodes

The thermal emittance and temporal response of a photocathode set an upper limit on the maximum achievable electron beam brightness from a photoemission electron source, or photoinjector. We present measurements of these parameters over a broad range of laser wavelength for two different negative electron affinity (NEA) photocathodes. The thermal emittance of NEA GaAs and GaAsP has been measured by two techniques—a measurement of the beam size downstream from a solenoid, whose strength was varied, and a double slit transmission measurement—for different laser spot sizes and shapes. The effect of space charge on the beam spot size allows a good estimation of the photoemission response time from these cathodes. Both cathodes show a subpicosecond response for laser wavelengths shorter than 520 nm.

New material for photoemission electron source: semiconductor alloy InGaAsP grown on GaAs substrate

Wide-bandgap epitaxial In x Ga 1− x As y P 1− y layers grown on GaAs substrates were investigated as a material for photoemission electron sources for the first time. By variation of x and y, both thick (of about one μm) lattice-matched unstrained layers and thin (of about 0.2 μm) lattice-mismatched strained layers with direct energy gap from 1.78 to 1.92 eV were grown. The layers were chemically treated in a glove-box, loaded into UHV via a loading chamber without exposure to air, heat cleaned, and activated to the state of negative electron affinity (NEA). For InGaAsP it was possible to reach the NEA-state by an activation with cesium only, without oxygen. The estimated e-folding lifetime of InGaAsP photocathodes operating at DC current of 400 μA was above one thousand hours in the regime with automatical additional cesiations and above one hundred hours without additional cesiations. Lifetime limitation factors are discussed. Strain-induced splitting of the valence band up to 32 meV was observed in the lattice-mismatched films; this observation gives prospects for an increase of spin polarization beyond the 50% limit.

Operating experience with a GaAs photoemission electron source

We report on the development of several operating procedures that promise to make GaAs photoemission electron sources easier to construct, more reliable to operate, and more amenable to use in dynamic vacuum systems. We describe in particular a method for ‘‘ohmically’’ heating a 〈100〉 crystal of GaAs under vacuum to approximately 600 °C. We also discuss our observations of the role of oxygen in the activation of the crystal surface, the use of continuous cesiation, and of the performance of the crystal under varying vacuum conditions.

Measurement of channel electron multiplier efficiency using a photoemission electron source

A description is given of a new method for absolute measurements of electron multiplier efficiency for electrons. The method, which is based on the use of a photoemission electron source and calibrated optical attenuation filters to control the electron intensity, has been used to measure the efficiency of a Mullard B419BL channel electron multiplier in the electron energy range of 40-1000 eV.