High-energy Photoelectrons Research Articles

Coaxially Bi/ZnO@ZnSe array photocathode enables highly efficient CO2 to C1 conversion via long-lived high-energy photoelectrons.

The key aspect of the photoelectrochemical CO2 reduction reaction (PEC CO2 RR) lies in designing cathode materials that can generate high-energy photoelectrons, enabling the activation and conversion of CO2 into high-value products. In this study, a coaxially wrapped ZnO@ZnSe array heterostructure was synthesized using a simple anion exchange strategy and metallic Bi nanoparticles (NPs) were subsequently deposited on the surface to construct a Bi/ZnO@ZnSe photocathode with high CO2 conversion capability. This array photocathode possesses a large aspect ratio, which simultaneously satisfies a low charge carrier migration path and a large specific surface area that facilitates mass transfer. Additionally, the barrier formed at the n-n heterojunction interface hinders the transfer of high-energy photoelectrons from ZnSe to lower energy levels, resulting in their rapid capture by Bi while maintaining a relatively long lifetime. These captured electrons act as active sites, efficiently converting CO2 into CO with a Faradaic efficiency above 88.9% at -0.9 V vs. RHE and demonstrating superior stability. This work provides a novel approach for synthesizing high-energy photoelectrode materials with long lifetimes.

Surface-Degenerate Semiconductor Photocatalysis for Efficient Water Splitting without Sacrificial Agents via a Reticular Chemistry Approach.

The production of green hydrogen through photocatalytic water splitting is crucial for a sustainable hydrogen economy and chemical manufacturing. However, current approaches suffer from slow hydrogen production (<70 µmol·gcat-1·h-1) due to the sluggish four-electrons oxygen evolution reaction (OER) and limited catalyst activity. Herein, we achieve efficient photocatalytic water splitting by exploiting a multifunctional interface between a nano-photocatalyst and metal-organic framework (MOF) layer. The functional interface plays two critical roles: (1) enriching electron density directly on photocatalyst surface to promote catalytic activity, and (2) delocalizing photogenerated holes into MOF to enhance OER. Our photocatalytic ensemble boosts hydrogen evolution by ~100-fold over pristine photocatalyst and concurrently produces oxygen at ideal stoichiometric ratio, even without using sacrificial agents. Notably, this unique design attains superior hydrogen production (519 µmol·gcat-1·h-1) and apparent quantum efficiency up to 13-fold and 8-fold better than emerging photocatalytic designs utilizing hole scavengers. Comprehensive investigations underscore the vital role of the interfacial design in generating high-energy photoelectrons on surface-degenerate photocatalyst to thermodynamically drive hydrogen evolution, while leveraging the nanoporous MOF scaffold as an effective photohole sink to enhance OER. Our interfacial approach creates vast opportunities for designing next-generation, multifunctional photocatalytic ensembles using reticular chemistry with diverse energy and environmental applications.

Simulating the Upper Hybrid Instability as a Cause for 150 km Echoes

AbstractOne Hundred and Fifty kilometer echoes are a type of strong radar echo observed in the valley region of the equatorial ionosphere, whose origin was long standing mystery. Recently, a new upper hybrid (UH) instability theory, driven by high energy photoelectrons, has been proposed to explain most features of 150 km echoes. However, this instability excites high frequency electron modes, whereas radars observe low frequency ion modes. To explain 150 km echoes, the UH instability must ultimately excite ion acoustic modes. This paper describes a set of particle‐in‐cell simulations used to study how photoelectrons interacting with a cold background plasma generate UH waves, and how these drive ion acoustic waves measured by radars. We implement a new electron‐N2 collision algorithm to better model the bump on tail features of the photoelectron distribution in the valley region. These simulations show that photoelectrons drive unstable UH waves that strongly enhance ion acoustic waves. We also show a strong frequency dependence on power in the ion line, which explains observational differences between radars, most notably the lack of echoes at ALTAIR. The photoelectron driven UH instability successfully reproduces most features of 150 km echoes associated with naturally enhanced incoherent scattering.

Photo-plasmonic effect as the hot electron generation mechanism

Based on the effective Schrödinger–Poisson model a new physical mechanism for resonant hot-electron generation at irradiated half-space metal–vacuum interface of electron gas with arbitrary degree of degeneracy is proposed. The energy dispersion of undamped plasmons in the coupled Hermitian Schrödinger–Poisson system reveals an exceptional point coinciding the minimum energy of plasmon conduction band. Existence of such exceptional behavior is a well-know character of damped oscillation which in this case refers to resonant wave–particle interactions analogous to the collisionless Landau damping effect. The damped Schrödinger–Poisson system is used to model the collective electron tunneling into the vacuum. The damped plasmon energy dispersion is shown to have a full-featured exceptional point structure with variety of interesting technological applications. In the band gap of the damped collective excitation,depending on the tunneling parameter value, there is a resonant energy orbital for which the wave-like growing of collective excitations cancels the damping of the single electron tunneling wavefunction. This important feature is solely due to dual-tone wave-particle oscillations, characteristics of the collective excitations in the quantum electron system leading to a resonant photo-plasmonic effect, as a collective analog of the well-known photo-electric effect. The few nanometer wavelengths high-energy collective photo-electrons emanating from the metallic surfaces can lead to a much higher efficiency of plasmonic solar cell devices, as compared to their semiconductor counterpart of electron–hole excitations at the Fermi energy level. The photo-plasmonic effect may also be used to study the quantum electron tunneling and electron spill-out at metallic surfaces. Current findings may help to design more efficient spasers by using the feature-rich plasmonic exceptional point structure.

Enhancement of Secondary Gamma Radiation Flux Energies in the Energy Region from 1400 Kev to 1500 Kev during Lunar Eclipse on June 16, 2011 at Udaipur, India

Abstract: The lunar eclipse at Udaipur (270 43’ 12.00” N, 750 28’ 48.01” E), India was experimentally observed on June 16, 2011 using ground-based NaI (Tl) scintillation detector. Cadences of data were collected at intervals of half an hour. The analyzed data revealed a significant enhancement in the energy region from 1400 keV to 1500 keV of secondary gamma radiation flux (SGRF) on comparison to pre- and after lunar eclipse days. On June 16, we observed additional peaks in the energy spectrum of SGR flux in the energy region extending from 1461.010 keV to 1472.536 keV during the progress of lunar eclipse. We interpret such enhancement of SGR flux energies on the basis of combined phenomenon of gravitational lensing effect of Sun and Earth as well as the Sun’s magnetic field and the interplanetary magnetic field. Due to such combined effect, primary cosmic and solar radiations bend and may cause strong hitting on the less air surface of the Moon, resulting into emission of secondary particle flux mostly gamma radiation, high energy photo electrons, hard X -radiation, muons, protons and neutrons which may be regarded as back scattering from the Moon surface. Energy of backscattered secondary flux is large enough that it gives such enhancement in the energy region from 1400 keV to 1500 keV during lunar eclipse observation. Also, due to combined gravitational effect, cosmic radiation bent and impinged deep inside the atmosphere of Earth, producing a large shower of secondary radiation particles. These collective effects may give such enhancement of secondary gamma radiation flux energies. Keywords: Lunar eclipse, Solar magnetic field, Interplanetary magnetic field, Gravitational lensing, Bending of primary cosmic and solar radiations.

Hybrid modelling of cavity system generated electromagnetic pulse in low pressure air

The surface of metal system exposed to ionizing radiation (X-ray and γ-ray) will emit high-energy electrons through the photoelectric effect and other processes. The transient electromagnetic field generated by the high-speed electron flow is called system generated electromagnetic pulse (SGEMP), which is difficult to shield effectively. An ongoing effort has been made to investigate the SGEMP response in vacuum by numerical simulation. However, the systems are usually operated in a gaseous environment. The objective of this paper is to investigate the effect of low-pressure air on the SGEMP. A three-dimensional hybrid simulation model is developed to calculate the characteristics of the electron beam induced air plasma and its interaction with the electromagnetic field. In the hybrid model, the high-energy photoelectrons are modelled as macroparticles, and secondary electrons are treaed as fluid for a balance between efficiency and accuracy. A cylindrical cavity with an inner diameter of 100 mm and a length of 50 mm is used. The photoelectrons are emitted from one end of the cavity and are assumed to be monoenergetic (20 keV). The photoelectron pulse follows a sine-squared distribution with a peak current density of 10 A/cm&lt;sup&gt;2&lt;/sup&gt;, and its full width at half maximum is 2 ns. The results show that the number density of the secondary electrons near the photoelectron emission surface and its axial gradient increase as air pressure increases. The electron number density in the middle of the cavity shows a peak value at 20 Torr (1 Torr = 133 Pa). The electron temperature decreases monotonically with the increase in pressure. The low-pressure air plasma in the cavity prevents the space charge layer from being generated. The peak value of the electric field is an order of magnitude lower than that in vacuum, and the pulse width is also significantly reduced. The emission characteristic of the photoelectrons determines the peak value of the current response. The current reaching the end of the cavity surface first increases and then decreases with pressure increasing. The plasma return current can suppress the rising rate of the total current and extend the duration of current responses. Finally, to validate the established hybrid simulation model, the calculated magnetic field is compared with that from the benchmark experiments. This paper helps to achieve a better prediction of the SGEMP response in a gaseous environment. Compared with the particle-in-cell Monte Carlo collision method, the hybrid model adopted can greatly reduce the computational cost.

Production of secondary electrons and photons and energy absorption mechanisms in amorphous carbon irradiated by photons in the energy range of 0.03 to 17.4 keV

Energy absorption mechanisms in amorphous carbon under irradiation by 0.03–17.4 keV photons is studied by Monte Carlo simulation with accounting for the cascade decays of inner-shell vacancies and tracking all secondary electrons and photons including very-low-energy ones. Energy absorption by the atoms of the sample that underwent initial ionization by incident photons is only important at energies of several tens of eV (UV and XUV range). The principal mechanism of energy absorption on the whole incident photon energy interval is through inelastic processes caused by secondary electrons. At incident photon energies above C1s ionization threshold low-energy KLL Auger electrons produced by the cascade decay of C1s vacancy transfer energy to the medium in the vicinity of the site of initial photoionization. At energies far above the C1s-threshold, high-energy photoelectrons spread the absorbed energy over larger volumes and with smaller density. A great portion of the energy brought by incident photons, about 42 %, is transferred to the medium by low-energy secondary electrons and photons that are incapable of ionizing or exciting the atoms of the sample. The number of low-energy electrons is proportional to the energy of an incident photon, and at incident photon energies of several keV, one initial photoionization produces hundreds of secondary low-energy electrons.

Rescattering time-energy analysis of high-order above-threshold ionization in few-cycle laser fields

A Wigner distributionlike function based on the improved strong-field approximation theory is proposed to calculate the rescattering time-energy distribution (RTED) of high-energy photoelectrons of atomic above-threshold ionization process in few-cycle laser fields with different frequencies. The RTED shows bell-like stripes and the outermost stripe is compared with semiclassical results given by the simple-man model with consideration of different positions of tunnel exit and different initial longitudinal momenta. Analysis indicates the existence of the tunnel exit. However, though it shifts farther away from the core with decreasing frequency, the position of the tunnel exit is significantly less than the prediction by adiabatic theory even for the low-frequency case which is well in the tunneling regime. Our results also imply that the effect of the tunnel exit is more important than that of the initial longitudinal momentum at the tunnel exit for the backward-scattering electrons. Moreover, the inner stripe structures in the RTED are attributed to interference between electrons with the same final energy emitted at different ionization times.

Dependence of the returning electron wave packet on the model potential for high-energy plateau photoelectron spectra

SynopsisIn the present work, we study the two-dimensional momentum distributions of high-energy plateau photoelectrons of a model-atom in a linearly polarized few-cycle laser pulse. Based on the improved strong-field approximation (ISFA), it is found that the returning electron wave packet is dependent of the model potential.

Experimental setup combining in situ hard X-ray photoelectron spectroscopy and real-time surface X-ray diffraction for characterizing atomic and electronic structure evolution during complex oxide heterostructure growth.

We describe the next-generation system for in situ characterization of a complex oxide thin film and heterostructure growth by pulsed laser deposition (PLD) using synchrotron hard X-rays. The system consists of a PLD chamber mounted on a diffractometer allowing both real-time surface X-ray diffraction (SXRD) and in situ hard X-ray photoelectron spectroscopy (HAXPES). HAXPES is performed in the incident X-ray energy range from 4 to 12 keV using a Scienta EW4000 electron energy analyzer mounted on the PLD chamber fixed parallel with the surface normal. In addition to the standard application mode of HAXPES for disentangling surface from bulk properties, the increased penetration depth of high energy photoelectrons is used for investigation of the electronic structure changes through thin films grown deliberately as variable thickness capping layers. Such heterostructures represent model systems for investigating a variety of critical thickness and dead layer phenomena observed at complex oxide interfaces. In this new mode of operation, in situ HAXPES is used to determine the electronic structure associated with unique structural features identified by real-time SXRD during thin film growth. The system is configured for using both laboratory excitation sources off-line and on-line operation at beamline 33-ID-D at the Advanced Photon Source. We illustrate the performance of the system by preliminary scattering and spectroscopic data on oxygen vacancy ordering induced perovskite-to-brownmillerite reversible phase transformation in La2/3Sr1/3MnO3 films capped with oxygen deficient SrTiO3-δ (100) layers of varying thickness.

Transition from strong-field sequential to nonsequential double ionization at near-infrared wavelengths and low intensities

Within a quantum-mechanical model, we investigate strong-field double ionization of a model helium atom by near-infrared, linearly polarized laser pulses at intensities far below the recollision threshold. The quantum simulations show a clear mechanism change from sequential to nonsequential double ionization (NSDI) as the laser intensity increases. For NSDI, the two-electron correlated momentum distribution exhibits a strong final-state Coulomb repulsion effect for high-energy photoelectrons, but absent for low-energy photoelectrons. This repulsion effect is ascribed to field double ionization from doubly-excited states populated by recollision of the first ionized electron when it returns to the parent ion. Such recollision-induced excited states are absent at ultraviolet wavelengths due to the very low returning kinetic energies, resulting to the absence of final-state repulsion effect in NSDI.

Ratios of double to single ionization of He and Ne by strong 400-nm laser pulses using the quantitative rescattering theory

We present numerical simulations of the ratio between double and single ionization of He and Ne by intense laser pulses at wavelengths of 390 and 400 nm, respectively. The yields of doubly charged ions due to nonsequential double ionization (NSDI) are obtained by employing the quantitative rescattering (QRS) model. In this model, the NSDI ionization probability is expressed as a product of the returning electron wave packet (RWP) and the total scattering cross sections for laser-free electron impact excitation and electron impact ionization of the parent ion. According to the QRS theory, the same RWP is also responsible for the emission of high-energy above-threshold ionization photoelectrons. To obtain absolute double-ionization yields, the RWP is generated by solving the time-dependent Schr\odinger equation (TDSE) within a one-electron model. The same TDSE results can also be taken to obtain single-ionization yields. By using the TDSE results to calibrate single ionization and the RWP obtained from the strong-field approximation, we further simplify the calculation such that the nonuniform laser intensity distribution in the focused laser beam can be accounted for. In addition, laser-free electron impact excitation and ionization cross sections are calculated using the state-of-the-art many-electron $R$-matrix theory. The simulation results for double-to-single-ionization ratios are found to compare well with experimental data and support the validity of the nonsequential double-ionization mechanism for the covered intensity region.

Nonlocal heat transport and improved target design for x-ray heating studies at x-ray free electron lasers

The extremely high power densities and short durations of single pulses of x-ray free electron lasers (XFELs) have opened new opportunities in atomic physics, where complex excitation-relaxation chains allow for high ionization states in atomic and molecular systems, and in dense plasma physics, where XFEL heating of solid-density targets can create unique dense states of matter having temperatures on the order of the Fermi energy. We focus here on the latter phenomena, with special emphasis on the problem of optimum target design to achieve high x-ray heating into the warm dense matter (WDM) state. We report fully three-dimensional simulations of the incident x-ray pulse and the resulting multielectron relaxation cascade to model the spatial energy density deposition in multicomponent targets, with particular focus on the effects of nonlocal heat transport due to the motion of high energy photoelectrons and Auger electrons. We find that nanoscale high-Z/low-Z multicomponent targets can give much improved energy density deposition in lower-Z materials, with enhancements reaching a factor of 100. This has three important benefits. First, it greatly enlarges the thermodynamic parameter space in XFEL x-ray heating studies of lower-Z materials. Second, it allows the use of higher probe photon energies, enabling higher-information content X-ray diffraction (XRD) measurements such as in two-color XFEL operations. Third, while this is merely one step toward optimization of x-ray heating target design, the demonstration of the importance of nonlocal heat transport establishes important common ground between XFEL-based x-ray heating studies and more traditional laser plasma methods.

Identifying the contributions of multiple-returning recollision orbits in strong-field above-threshold ionization

We calculate the photoelectron momentum distributions (PMDs) from strong-field above-threshold ionization of Ar in the co-linearly polarized two-color laser fields consisting a strong fundamental component and a much weaker second harmonic by solving the time-dependent Schrodinger equation. Utilizing the recently introduced phase-of-the-phase (PP) spectroscopy, we analyze the relative phase dependence of the PMDs of the recollision electrons and the jumps in the PP spectroscopy are observed. With the semi-classical model, we demonstrate that the phase jumps originate from the competition between the orbits where recollision occurs at different returnings. Thus, the relative contribution of the multiple-returning recollision orbits is unambiguously identified with the PP spectroscopy. Additionally, we show that the relative contribution of the multiple-returning recollision orbits depends on the laser wavelength and ellipticity. In elliptically polarized laser field, the yield of the high-energy photoelectrons favors the contribution of the second-returning recollision orbits. In 1600-nm laser field, the PP spectroscopy indicates that the relative contribution of multiple-returning recollision orbits is strongly sensitive to the electron energy.

Imaging the square of the correlated two-electron wave function of a hydrogen molecule

The toolbox for imaging molecules is well-equipped today. Some techniques visualize the geometrical structure, others the electron density or electron orbitals. Molecules are many-body systems for which the correlation between the constituents is decisive and the spatial and the momentum distribution of one electron depends on those of the other electrons and the nuclei. Such correlations have escaped direct observation by imaging techniques so far. Here, we implement an imaging scheme which visualizes correlations between electrons by coincident detection of the reaction fragments after high energy photofragmentation. With this technique, we examine the H2 two-electron wave function in which electron–electron correlation beyond the mean-field level is prominent. We visualize the dependence of the wave function on the internuclear distance. High energy photoelectrons are shown to be a powerful tool for molecular imaging. Our study paves the way for future time resolved correlation imaging at FELs and laser based X-ray sources.

Carrier-envelope phase dependent photoelectron energy spectra in low intensity regime.

We study the carrier-envelope phase (CEP) dependent photoelectron energy spectra from above-threshold ionization by numerically solving the time-dependent Schrödinger equation of hydrogen atom in a few-cycle laser field at intensities in the range of (2-10) × 1013 W/cm2. Depending on the electron energy and the laser intensity, the yield of the photoelectron reveals clear oscillations with respect to the CEP. At high laser intensities (larger than ~3 × 1013 W/cm2), the yield of the high-energy photoelectrons (larger than 2Up, with Up being the ponderomotive potential) shows two kinds of oscillations with the CEP for different electron energies. There is a clear phase jump for those two kinds of oscillations. In contrast, at low laser intensities (smaller than ~3 × 1013 W/cm2), the phase of the oscillation for the high-energy photoelectron yield with the CEP is nearly independent on the electron energy, which will reduce the sensitivity of the retrieval of single-shot CEP using the method reported by T. Wittmann et al. [Nat. Phys. 5, 357 (2009)] at low laser intensities. We further show that the low-energy photoelectrons display distinct CEP-dependent intercycle interference fringes, providing an alternative approach to retrieve the CEP with high sensitivity in a few-cycle laser field with low intensity.

Attosecond-controlled photoemission from metal nanowire tips in the few-electron regime

Metal nanotip photoemitters have proven to be versatile in fundamental nanoplasmonics research and applications, including, e.g., the generation of ultrafast electron pulses, the adiabatic focusing of plasmons, and as light-triggered electron sources for microscopy. Here, we report the generation of high energy photoelectrons (up to 160 eV) in photoemission from single-crystalline nanowire tips in few-cycle, 750-nm laser fields at peak intensities of (2-7.3) × 1012 W/cm2. Recording the carrier-envelope phase (CEP)-dependent photoemission from the nanowire tips allows us to identify rescattering contributions and also permits us to determine the high-energy cutoff of the electron spectra as a function of laser intensity. So far these types of experiments from metal nanotips have been limited to an emission regime with less than one electron per pulse. We detect up to 13 e/shot and given the limited detection efficiency, we expect up to a few ten times more electrons being emitted from the nanowire. Within the investigated intensity range, we find linear scaling of cutoff energies. The nonlinear scaling of electron count rates is consistent with tunneling photoemission occurring in the absence of significant charge interaction. The high electron energy gain is attributed to field-induced rescattering in the enhanced nanolocalized fields at the wires apex, where a strong CEP-modulation is indicative of the attosecond control of photoemission.

Extracting conformational structure information of benzene molecules via laser-induced electron diffraction.

We have measured the angular distributions of high energy photoelectrons of benzene molecules generated by intense infrared femtosecond laser pulses. These electrons arise from the elastic collisions between the benzene ions with the previously tunnel-ionized electrons that have been driven back by the laser field. Theory shows that laser-free elastic differential cross sections (DCSs) can be extracted from these photoelectrons, and the DCS can be used to retrieve the bond lengths of gas-phase molecules similar to the conventional electron diffraction method. From our experimental results, we have obtained the C-C and C-H bond lengths of benzene with a spatial resolution of about 10 pm. Our results demonstrate that laser induced electron diffraction (LIED) experiments can be carried out with the present-day ultrafast intense lasers already. Looking ahead, with aligned or oriented molecules, more complete spatial information of the molecule can be obtained from LIED, and applying LIED to probe photo-excited molecules, a “molecular movie” of the dynamic system may be created with sub-Ångström spatial and few-ten femtosecond temporal resolutions.

Numerical simulation of the double-to-single ionization ratio for the helium atom in strong laser fields

We present calculations on the ratio between double and single ionization of helium by a strong laser pulse at a wavelength of 780 nm using the quantitative rescattering (QRS) model. According to this model, the yield for the doubly charged ion $\mathrm{He}^{2+}$ can be obtained by multiplying the returning electron wave packet (RWP) with the total cross sections (TCSs) for electron impact ionization and electron impact excitation of $\mathrm{He}^{+}$ in the singlet spin channel. The singlet constraint was imposed since the interaction of the helium atom with the laser and the recollision processes both preserve the total spin of the system. An $R$-matrix (close-coupling) code is used to obtain accurate TCSs, while the RWPs, according to the QRS, are calculated by the strong-field approximation for high-energy photoelectrons. The laser field, which lowers the required energy for the electron to escape from the nucleus at the time of recollision, is also taken into account. The simulated results are in good agreement with the measured $\mathrm{He}^{2+}\text{/}\mathrm{He}^{+}$ ratio over a broad range of laser intensities. The result demonstrates that the QRS approach based on the rescattering model is fully capable of quantitatively interpreting nonsequential double ionization processes.

Retrieving the ionization dynamics of high-energy photoelectrons in elliptically polarized laser fields

Retrieving the ionization dynamics of high-energy photoelectrons in elliptically polarized laser fields