Angular Photoelectron Research Articles

Unraveling Atomic and Electronic Surface Structure and Dynamics from Angular Photoelectron Distributions

Angle-resolved photoelectron spectroscopy (ARPES) is a powerful tool in solid state sciences. Beside the direct measurement of the energy-momentum dispersion relation, the angular distribution of the photoelectron current reveals the structural environment of the emitting atoms via photoelectron diffraction effects. Moreover, in the case of molecular layers, the angular distribution of emission from molecular orbitals can be directly related to their charge density distribution via so-called orbital tomography. In the present paper we summarize our efforts undertaken over the past 12 years to add the dimension of time to these two methods via pump-probe experiments with femtosecond resolution. We give a comprehensive introduction to standard ARPES and time-resolved two photon photoemission and then focus on our efforts towards time-resolved versions of photoelectron diffraction and orbital tomography. Both, optimization of experimental parameters and data acquisition procedures, as well as new numerical tools are needed in order to realize such challenging full stop missing after experiments.

Ionization in intense laser fields beyond the electric dipole approximation: concepts, methods, achievements and future directions

The electric dipole approximation is widely used in atomic, molecular and optical physics and is typically related to a regime for which the wavelength is much larger than the atomic structure. However, studies have shown that in strong laser fields another regime exists where the dipole approximation breaks down. During the ionization process in intense laser fields and at long wavelengths the photoelectrons can reach higher velocities such that the magnetic field component of the laser field becomes significant. The ionization dynamics and the final momentum of the electron is therefore modified by the entire Lorentz force. In contrast the magnetic field interaction is neglected in the dipole approximation. Rapid developments in laser technology and advancements in the accuracy of the measurements techniques have enabled the observation of the influence of such non-dipole effects on the final angular photoelectron momentum distributions. More recently the number of studies on ionization beyond the dipole approximation has increased significantly, providing more important insight into fundamental properties of ionization processes. For example we have shown that the final three dimensional photoelectron momentum spectra is significantly affected by the non-dipole drift with the parent–ion interaction, the linear multiphoton momentum transfer on a sub-cycle time scale and the sharing of the transferred linear photon momenta between the electron and the ion. In this article we present an overview of the underlying mechanisms and we review the experimental techniques and the achievements in this field. We focus on ionization in strong laser fields in the regime where the dipole approximation is not valid but a fully relativistic description is not required.

Attosecond all-optical control and visualization of quantum interference between degenerate magnetic states by circularly polarized pulses.

Controlling coherence and interference of quantum states is one of the central goals in quantum science. Different from energetically discrete quantum states, however, it remains a demanding task to visualize coherent properties of degenerate states (e.g., magnetic sublevels). It becomes further inaccessible in the absence of an external perturbation (e.g., Zeeman effect). Here, we present a theoretical analysis of all-optical control of degenerate magnetic states in the molecular hydrogen ion, $ {\rm H}_2^ + $H2+, by using two time-delayed co- and counterrotating circularly polarized attosecond extreme-ultraviolet (XUV) pulses. We perform accurate simulations to examine this model by solving the three-dimensional time-dependent Schrödinger equation. A counterintuitive phenomenon of quantum interference between degenerate magnetic sublevels appears in the time-dependent electronic probability density, which is observable by using x-ray-induced transient angular and energy-resolved photoelectron spectra. This work provides an insight into quantum interference of electron dynamics inside molecules at the quantum degeneracy level.

Attoclock Ptychography

Dedicated simulations show that the application of time-domain ptychography to angular photo-electron streaking data allows shot-to-shot reconstruction of individual X-ray free electron laser pulses. Specifically, in this study, we use an extended ptychographic iterative engine to retrieve both the unknown X-ray pulse and the unknown streak field. We evaluate the quality of reconstruction versus spectral resolution, signal-to-noise and sampling size of the spectrogram.

Theoretical calculation of the photoelectron angular distribution of neon

The general formula of the angular distribution of photoelectron is derived by using the density matrix theory and Racah algebra method. For comparing with the experimental data, the general formula in this paper is matched to the parametric formula and the non-dipole parameters of the photoelectron angular distribution associated with the terms of the second order for both unpolarized and polarized incident light are given explicitly. From the formula of these parameters we can see that the contribution to the non-dipole parameter is from the interference between dipole amplitude and multipole amplitude. And then, the relativistic calculation program for photoelectron angular distribution is further developed with the help of the program packages GRASP2K and RATIP which are based on the multi-configuration Dirac-Fock method. By using this program, the dipole and non-dipole angular-distribution parameters for neon 2s and 2p photoelectrons are calculated concretely. The good agreement between the results of this paper and the available theoretical data is obtained in a 50-5000 eV photoelectron-energy range studied. On this basis, the angular photoelectron distributions for neon 2s and 2p are calculated with and without considering the second non-dipole terms at the photoelectron energy E=600 eV and E=5000 eV, respectively. Special attention is paid to the effects of the polarization property of incident light and the non-dipole terms of photo-electron interaction on the angular distribution of photoelectrons. The results show that 1) the dipole and non-dipole parameters of the photoelectron angular distribution are sensitive to the ionized electron orbital, it can bring out considerable diversities among the photoelectron angular distributions of the different shells; 2) non-dipole effects make the photoelectron forward distribution in the direction of incident light, the polarization property of incident light will strengthen the asymmetric distribution of photoelectrons.

Modelling the effect of nuclear motion on the attosecond time-resolved photoelectron spectra of ethylene

Using time-dependent density functional theory we examine the energy, angular and time-resolved photoelectron spectra (TRPES) of ethylene in a pump–probe setup. To simulate TRPES we expose ethylene to an ultraviolet femtosecond pump pulse, followed by a time delayed extreme ultraviolet probe pulse. Studying the photoemission spectra as a function of this delay provides us direct access to the dynamic evolution of the molecule’s electronic levels. Further, by including the nuclei’s motion, we provide direct chemical insight into the chemical reactivity of ethylene. These results show how angular and energy resolved TRPES could be used to directly probe electron and nucleus dynamics in molecules.

An investigation into low-lying electronic states of HCS₂ via threshold photoelectron imaging.

Low-energy photoelectron imaging spectra of HCS2(-) are reported for the first time. Vibrationally resolved photodetachment transitions from the ground state of HCS2(-) to the ground state and low-lying excited states of HCS2 are observed. Combined with the ab intio calculations and Franck-Condon simulations, well-resolved vibrational spectra demonstrate definitive evidence for the resolution of the ground-state and excited states of HCS2 radical in the gaseous phase. The ground state and two low-lying excited states of HCS2 radical are assigned as (2)B2, (2)A2, and (2)A1 states, respectively. The adiabatic electron affinity is determined to be 2.910 ± 0.007 eV. And the term energies of the excited states, T0 = 0.451 ± 0.009 eV and 0.553 ± 0.009 eV, are directly measured from the experimental data, respectively. Angular filtering photoelectron spectra are carried out to assist in the spectral band assignment.

Strong-field ionization of heteronuclear diatomic molecules in circularly polarized laser fields

We theoretically investigate the strong-field ionization of heteronuclear diatomic molecules, CO and NO, by calculating the photoelectron angular distribution and the photoelectron momentum distribution in circularly polarized fields. We find that the photoelectron angular distribution and the photoelectron momentum distribution of CO are inversion asymmetric, but symmetric about the molecular axis, most of the photoelectrons are ejected from the neighborhood of the C core. In contrast, for NO the photoelectron angular distribution and photoelectron momentum distribution are nearly fourfold symmetric about the molecular axis and its vertical axis, which means that the photoelectrons are equally ejected from the neighborhood of the N and O cores. By our results, the photoelectron angular distribution and photoelectron momentum distribution can be used to imprint the molecular orbitals.

Electron-momentum distributions and photoelectron spectra of atoms driven by an intense spatially inhomogeneous field

We use the three-dimensional time-dependent Schr\"odinger equation (3 D-TDSE) to calculate angular electron momentum distributions and photoelectron spectra of atoms driven by spatially inhomogeneous fields. An example for such inhomogeneous fields is the locally enhanced field induced by resonant plasmons, appearing at surfaces of metallic nanoparticles, nanotips, and gold bow-tie shaped nanostructures. Our studies show that the inhomogeneity of the laser electric field plays an important role on the above-threshold ionization process in the tunneling regime, causing significant modifications on the electron momentum distributions and photoelectron spectra, while its effects in the multiphoton regime appear to be negligible. Indeed, through the tunneling above-threshold ionization (ATI) process, one can obtain higher energy electrons as well as a high degree of asymmetry in the momentum space map. In this study we consider near infrared laser fields with intensities in the mid- ${10}^{14}$ W/cm${}^{2}$ range and we use a linear approximation to describe their spatial dependence. We show that in this case it is possible to drive electrons with energies in the near-keV regime. Furthermore, we study how the carrier envelope phase influences the emission of ATI photoelectrons for few-cycle pulses. Our quantum mechanical calculations are fully supported by their classical counterparts.

Dealloying of CuxAu studied by hard X-ray photoelectron spectroscopy

We studied pristine and leached ultra-thin CuxAu (x≈4) films by hard X-ray photoelectron spectroscopy. The Au 4f and Cu 3s core levels show a shift in binding energy which is opposite to expected from the nobility of the elements, which is explained by charge transfer involving differently screening s and d valence levels of the elements [W. Eberhardt, S.C. Wu, R. Garrett, D. Sondericker, F. Jona, Phys. Rev. B 31 (1985) 8285]. The magnitude of the chemical shifts of the metal lines is strongly influenced by the finite size and disorder of the films. Angular dependent photoelectron emission allowed to assess the alloy composition as a function of depth larger than 5nm. The potential controlled dealloying proceeds into depth like a spinodal decomposition with Cu going into solution and the remaining Au accumulating in the surface region. The compositional gradient did not lead to a significant broadening of the metal photoelectron lines suggesting a non-local screening mechanism.

Dark field photoelectron emission microscopy of micron scale few layer graphene

We demonstrate dark field imaging in photoelectron emission microscopy (PEEM) of heterogeneous few layer graphene (FLG) furnace grown on SiC(000-1). Energy-filtered, threshold PEEM is used to locate distinct zones of FLG graphene. In each region, selected by a field aperture, the k-space information is imaged using appropriate transfer optics. By selecting the photoelectron intensity at a given wave vector and using the inverse transfer optics, dark field PEEM gives a spatial distribution of the angular photoelectron emission. In the results presented here, the wave vector coordinates of the Dirac cones characteristic of commensurate rotations of FLG on SiC(000-1) are selected providing a map of the commensurate rotations across the surface. This special type of contrast is therefore a method to map the spatial distribution of the local band structure and offers a new laboratory tool for the characterisation of technically relevant, microscopically structured matter.

Polarization and dipole effects in hard X-ray photoelectron spectroscopy

Hard X-ray photoelectron spectroscopy (HXPS) using X-rays in the 1.5–15keV energy range generated by synchrotron sources becomes an increasingly important analysis technique due to its potential for bulk sensitive measurements. However, besides their high energy, another characteristic of photons generated by synchrotron sources is their linear polarization while X-rays from Al Kα or Mg Kα for instance are unpolarized. This difference implies a possible variation in total path travelled by the photoelectrons generated by the X-rays inside the medium and consequently a modification of the resulting spectrum shape. We show the influence of the polarization on the partial intensity distributions, namely the number of electrons escaping after n inelastic scattering events, for photoelectron with energies of 0.5, 1, 2, 3, 4 and 5keV and originating from Si 1s1/2, Cu 1s1/2, Cu 2p3/2, Au 4d3/2 and Au 4f7/2 subshells. Moreover, we point out the influence of the dipole approximation leading to an underestimation of the partial intensity distributions due to the neglect of the forward–backward asymmetry of the angular photoelectron distribution.

Determination of local magnetic moment in L10-FeNi using photoelectron emission microscopy (PEEM)

Magnetic domain structure of L10-type FeNi is observed by angular dependent photoelectron emission microscopy. It reveals the perpendicular magnetic component in magnetic domain structure with Bloch wall. The formation of Bloch wall is ascribed to the large magnetic anisotropy energy of L10-FeNi.

Monitoring the effect of a control pulse on a conical intersection by time-resolved photoelectron spectroscopy

We have previously shown how femtosecond angle- and energy-resolved photoelectron spectroscopy can be used to monitor quantum wavepacket bifurcation at an avoided crossing or conical intersection and also how a symmetry-allowed conical intersection can be effectively morphed into an avoided crossing by photo-induced symmetry breaking. The latter result suggests that varying the parameters of a laser to modify a conical intersection might control the rate of passage of wavepackets through such regions, providing a gating process for different chemical products. In this paper, we show with full quantum mechanical calculations that such optical control of conical intersections can actually be monitored in real time with femtosecond angle- and energy-resolved photoelectron spectroscopy. In turn, this suggests that one can optimally control the gating process at a conical intersection by monitoring the photoelectron velocity map images, which should provide far more efficient and rapid optimal control than measuring the ratio of products. To demonstrate the sensitivity of time-resolved photoelectron spectra for detecting the consequences of such optical control, as well as for monitoring how the wavepacket bifurcation is affected by the control, we report results for quantum wavepackets going through the region of the symmetry-allowed conical intersection between the first two (2)A' states of NO(2) that is transformed to an avoided crossing. Geometry- and energy-dependent photoionization matrix elements are explicitly incorporated in these studies. Time-resolved photoelectron angular distributions and photoelectron images are seen to systematically reflect the effects of the control pulse.

Angular distribution of electrons during two-photon ionization of atoms by elliptically polarized radiation

Two-photon ionization of the hydrogen-like uranium ion by elliptically polarized radiation is investigated in the present work. It is demonstrated that the fundamental effect of interference of the probability amplitudes arises in the angular photoelectron distribution when the degree of ellipticity of ionizing radiation is varied.

Inner shell ionization and relaxation of CO molecule studied by coincidence spectroscopies

A multicoincidence electron–electron spectrometer has been modified by including two ion detectors in order to enable both photoelectron–Auger electron and photoelectron (or Auger electron)-ion coincidence measurements as a substantial step towards a complete experiment. Results of the first use of this set-up to study the photoelectron–Auger electron angular correlations and photoelectron angular distributions of “fixed-in-space” molecules in the case of the C 1s ionization of the CO molecule are presented and compared with theoretical calculations in the Hartree–Fock approximation based on the two-step model.

Angular distribution of X-ray photoelectrons emitted from silver

The no-loss peak of Ag 3d photoelectrons and 3d XPS spectra at different incidence angles of X-rays were measured with a double angular photoelectron intelligent analyzers (DAPHNIA) attached to a wide energy range X-ray beam line BL15XU at SPring-8. Ag 3d X-ray photoelectron spectroscopy (XPS) spectra were simulated by a Monte Carlo (MC) method using an effective energy loss function. Confirming that the MC simulation based on the use of the effective energy loss function describes successfully Ag 3d XPS spectrum, we applied the MC simulation to the angular distribution of Ag 3d X-ray photoelectrons for different incidence angles of X-rays, and for different take-off angles of photoelectrons. The simulation results agree very well the experimental data for small and medium incidence angle; but there is a large discrepancy for X-ray glancing incidence due to stronger surface generations of photoelectrons in this case.

Development of a Novel Instrument for X-Ray Photoelectron Diffraction and Holography

For X-ray photoelectron diffraction (XPED) and holography measurements we developed a novel laboratory instrument with the multienergy high power X-ray source and the high energy and high angular resolution photoelectron spectrometer system. The photon intensities of Al–Kα, Cr–Lα and Cu–Kα were estimated at 4.6 × 1011 cps, 7.5 × 1010 cps and 7.2 × 1010 cps, respectively. Ag3d XPS also revealed that the energy resolutions of Al–Kα and Cr–Lα sources were 0.9 eV and 3.1 eV, respectively. XPS and XPED of h-BN/Ni(111) excited by Al–Kα and Cr–Lα elucidated the potential and the validity of the XPED and holography analysis by using this novel instrument. Cu–Kα excitation XPS and Ti1s XPED measurements of the SrTiO3 (001) surface have also been performed.

Photoelectron signal simulation from textured samples with modified surface composition

AbstractIn this paper we propose a mathematical model for calculating XPS intensity distributions from multi‐component textured samples with a step‐like distribution approximating the depth compositional profiles. The surface roughness is approximated using triangular prisms. Analytical expressions for calculation of angular photoelectron intensity distributions are given. The results obtained concern textured samples where the height of the surface roughness structures are 3–5 times greater than the ejection depth. The values of the altered layer thickness and the values of the emitted photoelectron's ejection depth are of the same order. This model is appropriate when studying the changes of compositional depth profiles in the range of the photoelectron ejection depth. Copyright © 2002 John Wiley & Sons, Ltd.

Photoelectron signal simulation from textured samples covered by a thin film

In this paper we propose a mathematical model for calculating the XPS intensity distribution from textured sample surfaces covered with a thin film. The surface roughness is approximated using triangular prisms. Analytical expressions for calculation of the angular photoelectron intensity distribution are given. The obtained results allow an understanding of the relation between the escape depths of emitted photoelectrons and the angular XPS intensity distribution. The results show that the X-ray photoelectron signals vary significantly with a change in surface overlayer thickness and prism slope angle α. The maximum of the XPS intensity distribution does not correspond to the maximum escape depth of emitted photoelectrons. The ejected photoelectron ejection angle characterizes the maximum location depth of the investigated component or chemical bond. This model is useful when evaluating the surface roughness and the film thickness.