Photoelectron Energy Distribution Research Articles

Ultrastable, high-repetition-rate attosecond beamline for time-resolved XUV–IR coincidence spectroscopy

The implementation of attosecond photoelectron–photoion coincidence spectroscopy for the investigation of atomic and molecular dynamics calls for a high-repetition-rate driving source combined with experimental setups characterized by excellent stability for data acquisition over time intervals ranging from a few hours up to a few days. This requirement is crucial for the investigation of processes characterized by low cross sections and for the characterization of fully differential photoelectron(s) and photoion(s) angular and energy distributions. We demonstrate that the implementation of industrial-grade lasers, combined with a careful design of the delay line implemented in the pump–probe setup, allows one to reach ultrastable experimental conditions leading to an error in the estimation of the time delays of only 12 as over an acquisition time of 6.5 h. This result opens up new possibilities for the investigation of attosecond dynamics in simple quantum systems.

Electron energy loss and angular asymmetry induced by elastic scattering in superfluid helium nanodroplets.

Helium nanodroplets are ideal model systems to unravel the complex interaction of condensed matter with ionizing radiation. Here we study the effect of purely elastic electron scattering on angular and energy distributions of photoelectrons emitted from He nanodroplets of variable size (10-109 atoms per droplets). For large droplets, photoelectrons develop a pronounced anisotropy along the incident light beam due to a shadowing effect within the droplets. In contrast, the detected photoelectron spectra are only weakly perturbed. This opens up possibilities for photoelectron spectroscopy of dopants embedded in droplets provided they are smaller than the penetration depth of the light and the trapping range of emitted electrons in liquid helium.

Dye retention and desalination behavior of MoS2 doped high-flux β-CD/TDI polyurethane nanofiltration membrane

Molybdenum disulfide nanosheets (MoS2) were dispersed in β-cyclodextrin (β-CD) aqueous solution and interfacially polymerized with toluene-2,4-diisocyanate (TDI) to prepare high flux polyurethane loose nanofiltration membrane (DSPU) with good hydrophilicity and highly negatively charged. Through infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and energy dispersive spectrometer (EDS), the existence and distribution of MoS2 nanosheets in DSPU were characterized and confirmed. Based on the pure water flux, salt, and dye rejection rate, The optimal composition of β-CD and MoS2 in the aqueous phase was investigated. The pure water flux of DSPU-1.2–0.006 reaches 78.44 L/m2•h•bar, which is 2.4 times higher than that of the DPU membrane without MoS2 and can retention the molecular weight >416 g/mol highly negatively charged small dye molecules. The interception behavior of DSPU on 10 kinds of dye molecules with different charge abilities, molecular weight, and size was investigated. The chemical composition, cavity structure, and MoS2 nanosheet spacing of β-CD were analyzed for their correlation with DSPU interface properties, functional layer structure, and separation performance. DSPU worked stably for a long time and had an excellent antifouling performance. It has potential application performance in the field of dye desalination, separation, and purification.

Strong-field ionization of argon: Electron momentum spectra and nondipole effects

We investigate the influence of relativistic nondipole effects on the photoelectron spectra of argon, particularly in the low kinetic energy region (0 eV - 5 eV). In our experiment, we use intense linearly polarised 800 nm laser pulse to ionise Ar from a jet and we record photoelectron energy and momentum distributions using a reaction microscope (REMI). Our measurements show that nondipole effect can cause an energy-dependent asymmetry along the laser propagation direction in the photoelectron energy and momentum spectra. Model simulation based on time-dependent Dirac equation (TDDE) can reproduce our measurement results. The electron trajectory analysis based on classical model reveals that the photoelectron which obtains negative momentum shift along laser propagation direction is caused by the interplay between the Lorenz force induced radiation pressure during its free propagation in continuum and re-scattering by Coulomb potential of the parent ion when it is driven back by the laser field.

Cross‐Terminator Variations of the Photoelectron Energy Distribution in the Martian Ionosphere

AbstractPhotoelectrons are produced on the dayside of Mars via solar Extreme Ultraviolet (EUV) and X‐ray ionization and are also frequently observed on the nightside as a result of field‐aligned transport. Based on the Solar Wind Electron Analyzer (SWEA) measurements made on board the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft, we show that the shape of the photoelectron energy distribution exhibits interesting cross‐terminator variations, in that the He II peak becomes less pronounced and the 10 − 50 eV smooth background becomes hardened, both more evident at lower altitudes. These observations could be understood as the outcome of “atmospheric absorption” via photoelectron collisions with ambient neutrals during day‐to‐night transport. Such a scenario could successfully interpret the diversity of the MAVEN SWEA observations such as the dawn‐dusk and north‐south asymmetries, the impacts of crustal magnetization, upstream solar wind conditions, and solar EUV and X‐ray radiation, all using the 10–50 eV spectral hardness as a diagnostic. In particular, the magnetic control of the cross‐terminator spectral reshaping is proposed as the outcome of varying day‐to‐night magnetic connectivity which naturally implies varying field‐aligned atmospheric column for “absorption.” Crude estimates suggest that an “absorbing” atmospheric column of ≈1.5 × 1015 cm−2 is required for a significant photoelectron spectral reshaping beyond the EUV terminator. Therefore the observations reported here may shed light on the extension of the cross‐terminator magnetic field lines, as complementary to the day‐to‐night magnetic connectivity inferred from the occurrence of nightside photoelectrons.

Using photoelectron spectroscopy to measure resonant inelastic X-ray scattering: a computational investigation.

Resonant inelastic X-ray scattering (RIXS) has become an important scientific tool. Nonetheless, conventional high-resolution (few hundred meV or less) RIXS measurements, especially in the soft X-ray range, require low-throughput grating spectrometers, which limits measurement accuracy. Here, the performance of a different method for measuring RIXS, i.e. photoelectron spectrometry for analysis of X-rays (PAX), is computationally investigated. This method transforms the X-ray measurement problem of RIXS to an electron measurement problem, enabling use of high-throughput, compact electron spectrometers. X-rays to be measured are incident on a converter material and the energy distribution of the resultant photoelectrons, the PAX spectrum, is measured with an electron spectrometer. A deconvolution algorithm for analysis of such PAX data is proposed. It is shown that the deconvolution algorithm works well on data recorded with ∼0.5 eV resolution. Additional simulations show the potential of PAX for estimation of RIXS features with smaller widths. For simulations using the 3d levels of Ag as a converter material, and with 105 simulated detected electrons, it is estimated that features with a few hundred meV width can be accurately estimated in a model RIXS spectrum. For simulations using a sharp Fermi edge to encode RIXS spectra, it is estimated that one can accurately distinguish 100 meV FWHM peaks separated by 45 meV with 105 simulated detected electrons that were photoemitted from within 0.4 eV of the Fermi level.

Relativistic ionization probabilities and photoelectron distributions of hydrogenlike ions in superstrong electromagnetic fields

The ionization probabilities and photoelectron distributions of the hydrogen atom and hydrogenlike ions ${\mathrm{He}}^{+}$ and ${\mathrm{Ne}}^{9+}$ in superstrong electromagnetic fields are obtained by solving the time-dependent Dirac equation with the help of the generalized pseudospectral method in spherical coordinates. A simple transformation of the original radial Dirac Hamiltonian is suggested that removes the spurious states. The ionization probabilities are calculated both within and beyond the dipole approximation for two pulse durations and various peak field strengths scaled with respect to the nuclear charge for each target. Performance of two analytic parameters previously suggested to estimate the nondipole effects is assessed against the numerical data obtained in this study. The photoelectron energy and angular distributions exhibit the above-threshold ionization peaks as well as the low-energy structure. In the superstrong field regime, the low-energy structure builds up with increasing field strength while the above-threshold ionization peaks decrease. The nondipole effects in the photoelectron distributions are also revealed and analyzed.

Carrier-envelope phase dependence of high-order above-threshold ionization by few-cycle laser pulses

With a stereo above-threshold ionization (ATI) apparatus, we systematically investigate the carrier-envelope phase (CEP) dependence of the photoelectron kinetic energy spectra from high-order ATI of Xe atom subject to a strong few-cycle laser field. Our measurements reveal a prominent step structure and a maximum in the photoelectron kinetic energy dependence of the yield asymmetry. The observed structures can be well reproduced by the numerical solution of time-dependent Schrödinger equation (TDSE) and the improved strong field approximation (ISFA) theory. According to our analysis, the step structure can be ascribed to the interference of long and short orbits, while the maximum is closely related to the distinct kinetic energy distributions of photoelectrons which elastically scatter on the core at odd and even returns. Our work provides a more comprehensive understanding of the CEP effects on high energy photoelectron emissions in a few-cycle laser field.

Angular distribution and energy spectra of photoelectrons from tetrahydrofuran illuminated by VUV photon source

Photoelectron emission from the valence shell of biologically relevant tetrahydrofuran (THF) molecule is studied. Energy and angular distributions of photoelectrons from THF ionized by a He(I) vacuum ultraviolet (VUV) photon source are measured. Theoretical calculations performed for the binding energies of orbitals and photoelectron intensities are in reasonable agreement with measurement. The observed angular distributions show the determining role of dipole transition mechanism.

“EXPERIMENTAL STUDY OF PHOTOELECTRONIC SPECTRA OF NIOBIUM-HAFNIUM-ZIRCONIUM ALLOY”

Crystals of niobium and its alloys obtained by low-energy implantation of hafnium and zirconium ions have been studied in a multifunctional experimental setup. Distribution profiles of implanted hafnium and zirconium atoms, energy distributions of photoelectrons N (E), and spectral dependences of the quantum yield of photoelectron emission before and before and after heating of the niobium-hafnium-zirconium alloy have been investigated. The contribution of surface states and bands formed by hafnium and zirconium atoms to photoelectron emission in the photon energy range of 8–10 eV is analyzed. The experimental results are discussed on the basis of theoretical calculations of the surface, photoelectron emission in which the dominant factors are indirect optical transitions. The experiments were carried out under ultrahigh vacuum conditions using photoelectron and Auger electron spectroscopy. Krypton and xenon resonance lamps were used as radiation sources.

Compton scattering of keV photons at helium atom near the He$$^+$$(1s) threshold for small momentum transfer

We study Compton scattering at helium atom exposed to an electromagnetic field with a central frequency of 80 a.u. ( $$\sim 2.18$$ keV). We consider the situation where the incident light and scattered light are polarized along the same direction with small relative propagation angle $$\beta $$ . The energy of the emitted electron ranges from 0.2 to 2.1 a.u. ( $$\sim 5.44$$ –57.14 eV). The approach is based on previous works on stimulated Compton scattering (see Bachau et al. Phys Rev Lett 112:073001, 2014). We consider a field intensity of $$3.51\times 10^{16}$$ W/cm $$^2$$ , where stimulated Compton scattering can be treated in perturbative regime. In lowest order perturbation theory, the process results from the contribution of $$\mathbf{A\cdot \mathbf{P}}$$ in second order and $$\mathbf{A}^2$$ in first order ( $$\mathbf{P}$$ is the electron momentum operator and $$\mathbf{A}$$ the vector potential of the field); both terms induce two-photon transitions. The Compton matrix element $$|{{\mathcal {M}}}_{fg}|^2$$ is extracted numerically resolving the time-dependent Schrodinger equation, and in perturbation theory, emphasis is put on the calculation of the second-order amplitude associated with $$\mathbf{A\cdot \mathbf{P}}$$ . We investigate the cases of relative propagation angle $$\beta =0$$ and 10 degrees. The photoelectron energy distributions are dominated by the nondipole term $$\mathbf{A}^2$$ ; they increase by orders of magnitude when $$\beta $$ grows from 0 to 10 degrees. At $$\beta =0$$ degree, both $$\mathbf{A\cdot \mathbf{P}}$$ and $$\mathbf{A}^2$$ are at play and we show that the electrons are emitted in the (forward) direction of the momentum transfer $${{\mathcal {Q}}}$$ and to a lesser extent in the backward direction. When $$\beta $$ increases, the nondipole contribution $$\mathbf{A}^2$$ tends to dominate and the forward/backward asymmetry vanishes.

How surface charging affects the photoelectron energy distribution on the moon

Sunlight has large impacts on the electrostatic potential of the lunar surface, but the effect has previously been neglected in studies of the photoelectron distribution within the lunar sheath.

Photoelectron distribution on sunlit surface of the Moon: A formalism

An analytical formulation describing the characteristic energy distribution of photoelectrons over the sunlit regolith of the Moon has been presented. In deriving the distribution function, Fowler's approach based on fermionic lattice electrons for photoemission has been coupled with observed solar spectra, photoelectric quantum yield, and latitude dependent surface temperature. It is noticed that the dominant contribution in the photoelectron distribution function comes from extreme ultraviolet photons (3 eV–300 eV) of the solar spectra. The analysis also illustrates that the photoelectron distribution for the observed solar spectra is significantly different from the spectrum based on Planck's radiation law. In results, the photoelectron distribution is found to be a significant function of the lunar surface potential—the photoelectron distribution in the steady state considerably differs from that of the uncharged surface. In calculations, the variation in the photoelectron distribution function with the lunar latitude, quantum yield, and work function of the surface material has been parametrically analyzed, and it is found to be sensitive to the constituent parameters.

Small recoil momenta double ionization of He and two-electron ions by high energy photons

We calculate various differential and double differential characteristics of ionization by a single photon for H−, He and for the two-electron ions with Z = 3, 4, 5 in the region of the so-called quasi-free mechanism (QFM) domination. We employ highly accurate wave functions at the electron-electron coalescence line where coordinates of both ionized electrons coincide. We trace the Z dependence of the double differential distributions. For all considered targets we discuss the dependence of the photoelectron energy distribution on the photon energy. Our calculation demonstrated the rapid decrease of QFM contribution with increase of the difference in energy of two outgoing electrons, and with decrease of the angle between two outgoing momenta. As a general feature, we observe the decrease of QFM contribution with nuclear charge growth.

Photoelectron imaging on vibrational excitation and Rydberg intermediate states in multi-photon ionization process of NH3 molecule**Project supported by the National Natural Science Foundation of China (Grant Nos. 11574116, 11534004, and 10704028).

The ionization processes of NH3 molecule are studied by photoelectron velocity map imaging technique in a linearly polarized 400-nm femtosecond laser field. The two-dimensional photoelectron images from ammonia molecules under different laser intensities are obtained. In the slow electron region, the values of kinetic energy of photoelectrons corresponding to peaks 1, 2, 3, and 4 are 0.27, 0.86, 1.16, and 1.6 eV, respectively. With both the kinetic energy and angular distribution of photoelectrons from NH3 molecules, we can confirm that the two-photon excited intermediate Rydberg state is A∼1 () state for photoelectron peaks 2, 3, 4, and the three peaks are marked as 1223 (2 + 2), 1123 (2 + 2), and 1023 (2 + 2) multi-photon processes, respectively. Then, peak 1 is found by adding a hexapole between the source chamber and the detection chamber to realize the rotational state selection and beam focusing. Peak 1 is labeled as the 1323 (3 + 1) multi-photon process through the intermediate Rydberg state . The phenomena of channel switching are found in the slow electron kinetic energy distributions. Our calculations and experimental results indicate that the stretching vibrational mode of ammonia molecules varies with channels, while the umbrella vibration does not. In addition, we consider and discuss the ac-Stark effect in a strong laser field. Peaks 5 and 6 are marked as (2 + 2 + 1) and (2 + 2 + 2) above threshold ionization processes in the fast electron region.

Dynamical interference of H and H2+ in one-photon ionization

The non-Hermitian Floquet theory is applied to systematically calculate both the ac-Stark shift $\ensuremath{\delta}$ and the decay rate $\ensuremath{\gamma}$ for hydrogen atoms and hydrogen molecular ions, when placed in a high-frequency laser field with an intensity ranging from the perturbation to the stabilization region. This allows us to trace the appearance of dynamical interference in ionization. We find that both the stabilization and the dynamical interference are easier to achieve at a frequency close to the ionization threshold. For the molecular case, the effects of nuclear motion are discussed based on the Born-Oppenheimer approximation. Although the dynamical interference condition $\ensuremath{\delta}g\sqrt{\ensuremath{\pi}}\ensuremath{\gamma}$ seems to be satisfied at a low frequency for a fixed-nuclei ${{\mathrm{H}}_{2}}^{+}$ in the perturbation region, the effects of the nuclear motion are shown to destroy the interference patterns in this region. Meanwhile, for the sake of confirmation, the photoelectron energy distributions are calculated by solving the time-dependent Schr\odinger equation for the parameters where the dynamical interference is predicted by the non-Hermitian Floquet theory.

Universal High-Energy Photoelectron Emission from Nanoclusters Beyond the Atomic Limit.

Rescattering by electrons on classical trajectories is central to understand photoelectron and high-harmonic emission from isolated atoms or molecules in intense laser pulses. By controlling the cluster size and the quiver amplitude of electrons, we demonstrate how rescattering influences the energy distribution of photoelectrons emitted from noble gas nanoclusters. Our experiments reveal a universal dependence of photoelectron energy distributions on the cluster size when scaled by the field driven electron excursion, establishing a unified rescattering picture for extended systems with the known atomic dynamics as the limit of zero extension. The result is supported by molecular dynamics calculations and rationalized with a one-dimensional classical model.

Filling gap of combination of gauge and analytical method in KFR-like theory**Project supported by the National Natural Science Foundation of China (Grant Nos. 11274149 and 11304185) and the Program of Shenyang Key Laboratory of Optoelectronic Materials and Technology, China (Grant No. F12-254-1-00).

Bauer recently presented a formula for the ionization rate of a hydrogen atom in a strong linearly polarized laser field [J. Phys. B 49 145601 (2016)]. He started from the Keldysh probability amplitude in the length gauge and utilized Reiss’s method in the velocity gauge. Instead, according to the Reiss probability amplitude in the velocity gauge and Keldysh’s derivation for the length gauge, we derive a formula for the ionization rate of a ground-state hydrogen atom or a hydrogen-like atom in a strong linearly polarized laser field. We compare the numerical results of the total ionization rate and the photoelectron energy distribution calculated from our formula with the results from Keldysh, Reiss, and Bauer. We find that the apparent discrepancies in the ionization rate are caused not only by different gauges, but also by different analytical methods used to derive the ionization rate.

Practical guides for x-ray photoelectron spectroscopy: Analysis of polymers

XPS is widely used to identify and quantify the elements present at the surface of polymeric materials. The energy distribution of photoelectrons emitted from these elements contains information about their chemical state, potentially allowing the analyst to identify and quantify specific functional groups. These functional groups may originate from the synthesis and processing of the polymers, from postsynthetic modifications such as surface grafting, or indeed may be unrelated to the polymer (additives and contaminants). Extracting reliable and meaningful information from XPS data is not trivial and relies on careful and appropriate experimentation, including experimental design, sample preparation, data collection, data processing, and data interpretation. Here, the authors outline some of these challenges when performing XPS analysis of polymers and provide practical examples to follow. This guide will cover all relevant aspects over the course of a typical experiment, including tips and considerations when designing the experiment, sample preparation, charge neutralization, x-ray induced sample damage, depth profiling, data analysis and interpretation, and, finally, reporting of results. Many of these topics are more widely applicable to insulating organic materials, and the recommendations of this guide will help to ensure that data is collected and interpreted using current best practices.

Energy and Time Distribution of Photoelectrons Released in Water under Ultra-Short X-Ray Pulse Irradiation

Energy and Time Distribution of Photoelectrons Released in Water under Ultra-Short X-Ray Pulse Irradiation