Photoelectron Angular Distributions Research Articles

An Algorithm to Correct the Sensitivity Distribution of a Retarding Field Analyzer for Photoelectron Holography

Recently, we developed a retarding field analyzer (RFA) with high energy resolution and wide acceptance angles, which was installed at BL25SU in SPring-8, Japan. This apparatus enables us to measure photoelectron angular distributions (photoelectron holograms) with a solid angle of ±49° in a few minutes, and this approach is now being applied to measure the atomic arrangements of dopants and interfaces. However, correcting the sensitivity distribution of the measured images by the RFA is important in processing this data. The sensitivity distribution of the RFA is difficult to measure directly because it depends on the geometry of a sample, the kinetic energy of the photoelectrons, and so forth. Therefore, we developed an image processing algorithm to estimate the sensitivity distribution using spherical harmonics. This approach is more accurate than a Fourier transform and can efficiently correct the sensitivity distribution. Moreover, the algorithm can also be adopted in a wide variety of other applications in addition to RFA measurements.

Shape resonances in angle-resolved photoionization of aligned CF3I molecules

The photoionization of CF3I molecules in the photon energy range between 10 eV and 40 eV is investigated using angle-resolved photoelectron spectroscopy in the molecular frame. Using a femtosecond laser pulse, the molecules are impulsively aligned prior to their ionization by a series of extreme ultra-violet photon energies obtained by high-harmonic generation. The manifestation of molecular alignment in the photoelectron angular distributions (PADs) is well reproduced by non-relativistic quantum-mechanical photoionization calculations carried out with the ePolyScat software package, taking into account all open ionization channels. Our analysis points to two prominent shape resonances that dominate the photoelectron scattering dynamics and that therefore largely explain the observed changes in the PADs as a function of molecular alignment.

Alignment sensitivity of molecular-frame photoelectron momentum and angular distribution of by XUV attosecond pulses

In this work, we investigate the molecular-frame photoelectron momentum and angular distributions (MF-PMD and MF-PAD) of in the presence of XUV Attosecond Pulses. We present the alignment-sensitive effect of MF-PMDs and MF-PADs when the laser field polarization and the molecular axis are close to parallel geometry. Our results show that the MF-PADs and MF-PMDs of single-photon ionization of ion vary drastically when the alignment slightly shifts from parallel geometry to a small alignment angle (e.g. 4°) between the laser polarization direction and the molecular axis at the equilibrium inter-nuclear distance of R c = 2 atomic units. As R c increases to 4 atomic units, such sensitivity is decreased. The R c -dependent transition cross-sections in different channels are found to be the origin of such sensitivity.

Strong configuration-interaction contributions to the angle-resolved 4p photoelectron spectra of atomic xenon

High-resolution photoelectron spectra of xenon (Xe) were measured in the $4p$ photoionization region. The anisotropy parameters \ensuremath{\beta} were obtained for several photoelectron peaks with angle-resolved photoelectron measurements. The various complicated spectral features were observed due to the perturbed $4{p}^{\ensuremath{-}1}$ spin-orbit doublet and adjacent satellite structures. An analysis based on a relativistic multiconfiguration calculation has been carried out. The key role of the strong interactions between the $4{p}^{\ensuremath{-}1}$ core hole and the $4{d}^{\ensuremath{-}2}nl$ configurations has been investigated in detail. Experimental peak energies and intensities are in good agreement with theory. We find that the many of the complicated features are $4{d}^{\ensuremath{-}2}nl$ satellite lines, and their intensities are determined mostly by the amount of $4{p}^{\ensuremath{-}1}$ configurations that are mixed in. The experimental \ensuremath{\beta} values agree quite well with theory, indicating that the photoelectron angular distributions are also strongly influenced by the $4p$ direct-ionization contributions.

Dramatic relativistic effects on the ns dipole angular distribution asymmetry parameter, βns , of heavy and superheavy elements

A theoretical study of the dipole photoelectron angular distribution asymmetry parameter, β, of the 6s, 5s and 4s subshells has been conducted for a range of closed-shell heavy, Hg (Z = 80), Rn (Z = 86), Ra (Z = 88) and superheavy, No (Z = 102), Cn (Z = 112), Og (Z = 118), Ubn (Z = 120) elements to understand the deviation of β from the nonrelativistic value of 2 owing to relativistic effects. It was found that the deviation, strongest in Cooper minimum regions, persists over all energy for all ns subshells, and the deviations increase with Z. Three levels of relativistic calculations were performed in order to pinpoint the particular interactions responsible for the details of the behavior of the ns β’s as functions of energy, n, and Z and, in addition to the effects of the (relativistic) spin–orbit interaction, interchannel coupling was found to be of crucial importance in the determination of the β parameter in most of the situations studied.

Partial-wave representation of the strong-field approximation. II. Coulomb asymmetry in the photoelectron angular distribution of many-electron atoms

Strong-field ionization experiments are routinely performed with different atomic targets. While these experiments helped reveal many details about the light-matter interaction in intense laser fields, the target atoms are often modeled in a rather crude manner by neglecting most of their electronic structure. Until the present, therefore, many above-threshold ionization measurements had been understood only qualitatively, and this especially applies to the ionization with elliptically polarized laser pulses, for which the photoelectron angular distributions are known to exhibit a quite strong (Coulomb) asymmetry. This asymmetry arises from the Coulomb and short-range forces between the photoion and the outgoing field-dressed electron in the continuum. We demonstrate here how the strong-field approximation (SFA) can be combined with atomic structure theory for modeling such target-resolved observations. Using a partial-wave representation of the SFA, we show that this combination reproduces the Coulomb asymmetry for argon and xenon targets in good agreement with previous experiments and especially if a distorted-wave Volkov continuum is applied for the active electron. We therefore conclude that a better account of the initial and final electron waves in strong-field theories will enhance our understanding of ionization phenomena in intense fields.

Core-Level Photoelectron Angular Distributions at the Liquid-Vapor Interface.

ConspectusPhotoelectron spectroscopy (PES) is a powerful tool for the investigation of liquid-vapor interfaces, with applications in many fields from environmental chemistry to fundamental physics. Among the aspects that have been addressed with PES is the question of how molecules and ions arrange and distribute themselves within the interface, that is, the first few nanometers into solution. This information is of crucial importance, for instance, for atmospheric chemistry, to determine which species are exposed in what concentration to the gas-phase environment. Other topics of interest include the surface propensity of surfactants, their tendency for orientation and self-assembly, as well as ion double layers beneath the liquid-vapor interface. The chemical specificity and surface sensitivity of PES make it in principle well suited for this endeavor. Ideally, one would want to access complete atomic-density distributions along the surface normal, which, however, is difficult to achieve experimentally for reasons to be outlined in this Account. A major complication is the lack of accurate information on electron transport and scattering properties, especially in the kinetic-energy regime below 100 eV, a pre-requisite to retrieving the depth information contained in photoelectron signals.In this Account, we discuss the measurement of the photoelectron angular distributions (PADs) as a way to obtain depth information. Photoelectrons scatter with a certain probability when moving through the bulk liquid before being expelled into a vacuum. Elastic scattering changes the electron direction without a change in the electron kinetic energy, in contrast to inelastic scattering. Random elastic-scattering events usually lead to a reduction of the measured anisotropy as compared to the initial, that is, nascent PAD. This effect that would be considered parasitic when attempting to retrieve information on photoionization dynamics from nascent liquid-phase PADs can be turned into a powerful tool to access information on elastic scattering, and hence probing depth, by measuring core-level PADs. Core-level PADs are relatively unaffected by effects other than elastic scattering, such as orbital character changes due to solvation. By comparing a molecule's gas-phase angular anisotropy, assumed to represent the nascent PAD, with its liquid-phase anisotropy, one can estimate the magnitude of elastic versus inelastic scattering experienced by photoelectrons on their way to the surface from the site at which they were generated. Scattering events increase with increasing depth into solution, and thus it is possible to correlate the observed reduction in angular anisotropy with the depth below the surface along the surface normal.We will showcase this approach for a few examples. In particular, our recent works on surfactant molecules demonstrated that one can indeed probe atomic distances within these molecules with a high sensitivity of ∼1 Å resolution along the surface normal. We were also able to show that the anisotropy reduction scales linearly with the distance along the surface normal within certain limits. The limits and prospects of this technique are discussed at the end, with a focus on possible future applications, including depth profiling at solid-vapor interfaces.

Searching for optimal THz generation through calculations of the asymmetry of photoelectron momentum distributions by an improved strong-field-approximation method

From the solution of the time-dependent Schr\"odinger equation (TDSE), it has been shown that in a two-color laser pulse the relative phase that optimizes terahertz (THz) radiation also optimizes the asymmetry of the angular distribution of photoelectrons. Here, we show that a second-order strong-field approximation can accurately calculate the asymmetry of photoelectrons and thus can locate the phase delay that optimizes THz generation, doing so four orders of magnitude faster than TDSE calculations. We further trace that this is possible because THz emission originates from a free-free radiative transition between the rescattered electron and the target ion, similar to other electron-ion collisions in the laser field, e.g., high-harmonic generation and nonsequential double ionization. Our results pave the way to locate the optimal phase delay for any typical laser pulses for maximal THz generation in the laboratory.

Strongly coupled intermediate electronic states in one-color two-photon single valence ionization of O2.

We present an experimental and theoretical energy- and angle-resolved investigation on the non-dissociative photoionization dynamics of near-resonant, one-color, two-photon, single valence ionization of neutral O2 molecules. Using 9.3eV femtosecond pulses produced via high harmonic generation and a 3-D momentum imaging spectrometer, we detect the photoelectrons and O2 + cations produced from one-color, two-photon ionization in coincidence. The measured and calculated photoelectron angular distributions show agreement, which indicates that a superposition of two intermediate electronic states is dominantly involved and that wavepacket motion on those near-resonantly populated intermediate states does not play a significant role in the measured two-photon ionization dynamics. Here, we find greater utility in the diabatic representation compared to the adiabatic representation, where invoking a single valence-character diabat is sufficient to describe the underlying two-photon ionization mechanism.

Photoelectron momentum distributions with twisted attosecond X waves carrying orbital angular momentum

We theoretically investigate the photoelectron momentum distributions of 1s and 2px,y states of hydrogen by twisted attosecond X waves carrying orbital angular momentum based on first-order perturbation theory. The photoionization spectra as a function of photoelectron energy and emission angle are analyzed respectively. The results indicate that there are interference fringes in the energy spectra and more nodes in the angular distributions. These angular nodes are attributed to both orbital structure and the temporal-spatial structure of X waves. We derive an equation that can quantitatively describe the angular nodes in the photoelectron angular distributions. Our results and analyses indicate that the angular distribution is an important observation for the investigation of the information of both orbitals and X waves.

Fast and precise chiroptical spectroscopy by photoelectron elliptical dichroism.

The photoionization of chiral molecules by elliptically polarized femtosecond laser pulses produces photoelectron angular distributions which show a strong and enantio-sensitive forward/backward asymmetry along the light propagation direction. We report on high precision measurements of this photoelectron elliptical dichroism (PEELD). Using an optical cavity to recycle the laser pulses and increase the signal-to-noise ratio, we determine enantiomeric excesses with a 0.04% precision with a low-power femtosecond laser (4 W) in a compact scheme. We perform momentum-resolved PEELD measurements in 16 molecules, from volatile terpenes to non-volatile amino acids and large iodoarenes. The results demonstrate the high structural sensitivity of PEELD, confirming the spectroscopic interest of this technique. Last, we show how a convolutional neural network can be used to retrieve the chemical and enantiomeric composition of a sample from the momentum-resolved PEELD maps.

Observation of an electron-precise metal boryne complex: [BiBH

Metal-boron triple bonds are rare due to the electron deficiency of boron. This study uncovers a simple electron-precise metal boryne complex, [BiBH]-, which is produced within an ion trap through chemical reactions of the open-shell BiB- anion with H2. Photoelectron imaging is used to investigate the electronic structure and chemical bonding of the BiBH- complex. The B atom in the linear closed-shell BiBH- is found to undergo sp hybridization, forming a B-H single bond and a BiB triple bond. Photoelectron imaging reveals three detachment transitions from the BiBH- (1Σ+) anion to the neutral BiBH, including the ground state (2Π3/2) and two excited states (2Σ+ and 2Π1/2). Strong vibronic coupling is observed between the 2Π3/2 and 2Σ+ states, evidenced by the appearance of bending vibrations and their unique photoelectron angular distributions. The BiBH- complex not only stands as the simplest metal boryne complex, but also serves as an ideal molecular system to investigate both spin-orbit and vibronic couplings.

High-energy molecular-frame photoelectron angular distributions: a molecular bond-length ruler.

We present a study on molecular-frame photoelectron angular distributions (MFPADs) of small molecules using circularly polarized synchrotron light. We find that the main forward-scattering peaks of the MFPADs are slightly tilted with respect to the molecular axis. This tilt angle is directly connected to the molecular bond length by a simple, universal formula. We apply the derived formula to several examples of MFPADs of C 1s and O 1s photoelectrons of CO, which have been measured experimentally or obtained by means of ab initio modeling. In addition, we discuss the influence of the back-scattering contribution that is superimposed over the analyzed forward-scattering peak in the case of homo-nuclear diatomic molecules such as N2.

Effect of nonresonant states in near-resonant two-photon ionization of hydrogen

By numerically solving the three-dimensional time-dependent Schr\"odinger equation, two-photon ionization of hydrogen is investigated at the near-resonant frequencies of the $1s\text{\ensuremath{-}}2p$ transition. Due to the Rabi oscillations between the $1s$ and $2p$ states, the photoelectron energy spectra exhibit the Autler-Townes (AT) doublets and we focus on the energy spacing and the asymmetry of the doublets. Our results show that the laser frequency for the minimum energy spacing of AT doublets locates at the resonant frequency of the ac-stark-shifted states, while the symmetry of the AT doublets is affected both by the ac-stark shift and the nonresonant ionization pathway. Developing the minimal three-state model including all of the nonresonant (nonessential) states, the effects of the ac-stark shift and the nonresonant ionization pathway on the AT doublets due to the nonresonant states are identified. Furthermore, due to the nonresonant ionization pathway, the photoelectron angular distributions are distinctly different for the lower- and higher-energy peaks in the AT doublets and these angular distributions sensitively depend on the laser intensity and pulse duration. These results are well reproduced by our minimal three-state model.

Observation of Nondipole-Induced Asymmetry in the Angular Emission Distribution of Photoelectrons from Fixed-in-Space CO Molecules.

We investigate experimentally and theoretically the C and O 1s photoionization of fixed-in-space CO molecules at a photon energy of 905eV. We find a significant dependence of the photoelectron angular distributions on the direction of propagation of the ionizing radiation. It results from an interplay of nondipole effects, on one hand, and molecular effects, on the other. The nondipole effects lead to an increase of the emission probability in the forward direction along the light propagation, and the photoelectron wave being scattered by the molecular potential gives rise to a strong peak in the direction of the atom neighboring the emitter site. These effects can either conspire or extenuate each other, depending on the photoelectron emission direction and molecular orientation in space.

Photoelectron tomography with an intra-cavity velocity-map imaging spectrometer at 100MHz repetition rate.

We present a compact velocity-map imaging (VMI) spectrometer for photoelectron imaging at 100MHz repetition rate. Ultrashort pulses from a near-infrared frequency comb laser are amplified in a polarization-insensitive passive femtosecond enhancement cavity. In the focus, multi-photon ionization (MPI) of gas-phase atoms is studied tomographically by rotating the laser polarization. We demonstrate the functioning of the VMI spectrometer by reconstructing photoelectron angular momentum distributions from xenon MPI. Our intra-cavity VMI setup collects electron energy spectra at high rates, with the advantage of transferring the coherence of the cavity-stabilized femtosecond pulses to the electrons. In addition, the setup will allow studies of strong-field effects in nanometric tips.

Asymmetry parameters in single ionization of He, Ne by XUV pulses

Fully differential cross-sections of single ionization of He and Ne atoms are studied by linearly polarized extreme ultraviolet (XUV) photons in the energy range of 22.1 eV–43.7 eV, using a reaction microscope. Photoelectron angular distributions and the β asymmetry parameters for He 1s2 electrons prove the reliability of our experiment, and the β asymmetry parameters extracted from the angular distributions of Ne 2p6 electrons are obtained. By comparing with different theoretical calculations, it is found that the contribution of the electron correlation effect in Ne 2p6 single ionization becomes increasingly important as the incident photon energy increases, while the relativistic effect is relatively low in the whole incident energy range. Our experimental β asymmetry parameters may serve as a significant reference to test the most elaborated theories in the field. The datasets presented in this paper, including the photoelectron angular distributions and β asymmetry parameters, are openly available at https://doi.org/10.57760/sciencedb.j00113.00073.

MCDAD investigation to characterize the magnetic behavior of thin Pd films on Co(0001)

The magnetic characteristics of Pd thin films on Co(0001) were investigated by means of Magnetic Circular Dichroism in the Angular Distribution of photoelectrons (MCDAD). For uncovered Co it was found that significant photoelectron intensity differences occur for electrons emitted from the 3d band near the Fermi level in the situation of a fixed helicity of the incoming circularly polarized radiation and opposite magnetization of the magnetic layer.After deposition of Pd in the submonolayer regime up to a thickness of a few layers, the new features which are present in the photoemission spectra exhibit asymmetries, too. A strong hybridization takes place at the interface which is due to the intermixing of Pd 4d and Co 3d states. This interface state shows no dispersion and nearly no change of the asymmetry value for different photon energies. Additionally, the sign of the asymmetry is opposite to that of the ferromagnetic substrate. The reason is the strong hybridization which results not in a rigid band shift as found for less-interacting adsorbates but in new states being different in the majority and minority spin band. As a consequence, the interface layer exhibits an induced magnetic polarization. The structure near the Fermi energy exhibit the same sign of the asymmetry but with lower values. For a thickness of more than 2 monolayers the spectrum is comparable to bulk measurements and shows no MCDAD asymmetries in the entire binding energy region.

Differential Measurement of Electron Ejection after Two-Photon Two-Electron Excitation of Helium.

We report the measurement of the photoelectron angular distribution of two-photon single-ionization near the 2p^{2} ^{1}D^{e} double-excitation resonance in helium, benchmarking the fundamental nonlinear interaction of two photons with two correlated electrons. This observation is enabled by the unique combination of intense extreme ultraviolet pulses, delivered at the high-repetition-rate free-electron laser in Hamburg (FLASH), ionizing a jet of cryogenically cooled helium atoms in a reaction microscope. The spectral structure of the intense self-amplified spontaneous emission free-electron laser pulses has been resolved on a single-shot level to allow for post selection of pulses, leading to an enhanced spectral resolution, and introducing a new experimental method. The measured angular distribution is directly compared to state-of-the-art theory based on multichannel quantum defect theory and the streamlined R-matrix method. These results and experimental methodology open a promising route for exploring fundamental interactions of few photons with few electrons in general.

High Resolution Photoelectron Spectroscopy of the Acetyl Anion.

High-resolution photoelectron spectra of cryogenically cooled acetyl anions (CH3CO-) obtained using slow photoelectron velocity-map imaging are reported. The high resolution of the photoelectron spectrum yields a refined electron affinity of 0.4352 ± 0.0012 eV for the acetyl radical as well as the observation of a new vibronic structure that is assigned based on ab initio calculations. Three vibrational frequencies of the neutral radical are measured to be 1047 ± 3 cm-1 (ν6), 834 ± 2 cm-1 (ν7), and 471 ± 1 cm-1 (ν8). This work represents the first experimental measurement of the ν6 frequency of the neutral. The measured electron affinity is used to calculate a refined value of 1641.35 ± 0.42 kJ mol-1 for the gas-phase acidity of acetaldehyde. Analysis of the photoelectron angular distributions provides insight into the character of the highest occupied molecular orbital of the anion, revealing a molecular orbital with strong d-character. Additionally, details of a new centroiding algorithm based on finite differences, which has the potential to decrease data acquisition times by an order of magnitude at no cost to accuracy, are provided.