Measurements Of 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.

Photoelectron imaging of PtI2 - and its PtI- photodissociation product.

The photoelectron imaging of PtI2 - is presented over photon energies ranging from hν = 3.2 to 4.5eV. The electron affinity of PtI2 is found to be 3.4 ± 0.1eV, and the photoelectron spectrum contains three distinct peaks corresponding to three low-lying neutral states. Using a simple d-block model and the measured photoelectron angular distributions, the three states are tentatively assigned. Photodissociation of PtI2 - is also observed, leading to the formation of I- and of PtI-. The latter allows us to determine the electron affinity of PtI to be 2.35 ± 0.10eV. The spectrum of PtI- is similarly structured with three peaks which, again, can be tentatively assigned using a similar model that agrees with the photoelectron angular distributions.

Probing Molecular Frame Wigner Time Delay and Electron Wavepacket Phase Structure of CO Molecule

The time delay of photoelectron emission serves as a fundamental building block to understand the ultrafast electron emission dynamics in strong-field physics. Here, we study the photoelectron angular streaking of CO molecules by using two-color ( 400 + 800 nm) corotating circularly polarized fields. By coincidently measuring photoelectrons with the dissociative ions, we present molecular frame photoelectron angular distributions with respect to the instantaneous driving electric field signatures. We develop a semiclassical nonadiabatic molecular quantum-trajectory Monte Carlo (MO-QTMC) model that fully captures the experimental observations and further ab initio simulations. We disentangle the orientation-resolved contribution of the anisotropic ionic potential and the molecular orbital structure on the measured photoelectron angular distributions. Furthermore, by analyzing the photoelectron interference patterns, we extract the sub-Coulomb-barrier phase distribution of the photoelectron wavepacket and reconstruct the orientation- and energy-resolved Wigner time delay in the molecular frame. Holographic angular streaking with bicircular fields can be used for probing polyatomic molecules in the future.

Probing gaseous molecular structure by molecular-frame photoelectron angular distributions.

Carbon 1s photoelectron angular distributions of an iodomethane molecule were measured relative to the recoil-frame determined by the momentum correlation between I+ and CH3 + at photoelectron energies of 3, 6.1, and 12 eV. The energy dependent behavior of the recoil-frame photoelectron angular distributions is reproduced reasonably well by the time-dependent density functional theory with B-spline methods. We discuss potential applications of the fully differential photoelectron angular distribution measurements in the molecular frame to three-dimensional molecular structural determinations identifying the directions and lengths of the bonds.

Effects of molecular rotation after ionization and prior to fragmentation on observed recoil-frame photoelectron angular distributions in the dissociative photoionization of nonlinear molecules

Experimental angle-resolved photoelectron-photoion coincidence experiments measure photoelectron angular distributions (PADs) in dissociative photoionization (DPI) in the reference frame provided by the momenta of the emitted heavy fragments. By extension of the nomenclature used with DPI of diatomic molecules, we refer to such a PAD as a recoil-frame PAD (RFPAD). When the dissociation is fast compared to molecular rotational and bending motions, the emission directions of the heavy fragments can be used to determine the orientation of the bonds that are broken in the DPI at the time of the ionization, which is known as the axial-recoil approximation (ARA). When the ARA is valid, the RFPADs correspond to molecular-frame photoelectron angular distributions (MFPADs) when the momenta of a sufficient number of the heavy fragments are determined. When only two fragments are formed, the experiment cannot measure the orientation of the fragments about the recoil axes so that the resulting measured PAD is an azimuthally averaged RFPAD (AA-RFPAD). In this study we consider how the breakdown of the ARA due to rotation will modify the observed RFPADs for DPI processes in nonlinear molecules for ionization by light of arbitrary polarization. This model is applied to the core C $1s$ DPI of ${\mathrm{CH}}_{4}$, with the results compared to experimental measurements and previous theoretical calculations done within the ARA. The published results indicate that there is a breakdown in the ARA for two-fragment events where the heavy-fragment kinetic energy release was less than 9 eV. Including the breakdown of the ARA due to rotation in our calculations gives very good agreement with the experimental AA-RFPAD, leading to an estimate of upper bounds on the predissociative lifetimes as a function of the kinetic energy release of the intermediate ion states formed in the DPI process.

Selecting core-hole localization or delocalization in CS2 by photofragmentation dynamics.

Electronic core levels in molecules are highly localized around one atomic site. However, in single-photon ionization of symmetric molecules, the question of core-hole localization versus delocalization over two equivalent atoms has long been debated as the answer lies at the heart of quantum mechanics. Here, using a joint experimental and theoretical study of core-ionized carbon disulfide (CS2), we demonstrate that it is possible to experimentally select distinct molecular-fragmentation pathways in which the core hole can be considered as either localized on one sulfur atom or delocalized between two indistinguishable sulfur atoms. This feat is accomplished by measuring photoelectron angular distributions within the frame of the molecule, directly probing entanglement or disentanglement of quantum pathways as a function of how the molecule dissociates.

Imaging the electronic structure of valence orbitals in the XUV ionization of aligned molecules

We present measurements of photoelectron angular distributions (PADs) for the ionization of aligned CO2 molecules by an extreme ultraviolet laser pulse (17–45 eV) that is produced via high-harmonic generation, and that is polarized perpendicularly to the alignment laser axis. Differential PADs are recorded by taking the difference between two measurements for aligned and anti-aligned molecules. The results are compared with ab initio multi-channel R-matrix calculations, which are extended to include the computation of PADs. The calculations agree very well with the experiment, and are resolved with respect to the internal state of the ion and the polarization of the ionizing laser field relative to the molecular frame. For a sufficiently high, but experimentally achievable degree of alignment, the nodal structure of the molecular orbital becomes visible in the PAD.

Calibration of the initial longitudinal momentum spread of tunneling ionization

Recently, the initial longitudinal momentum at the tunnel exit has attracted considerable interest in the strong-field community. We have measured the photoelectron momentum distribution of Kr atoms at an intensity of $1.2\ifmmode\times\else\texttimes\fi{}{10}^{14}\phantom{\rule{0.28em}{0ex}}{\mathrm{W}/\mathrm{cm}}^{2}$ and the ellipticity-resolved angular distribution of Ne atoms at an intensity of $3.6\ifmmode\times\else\texttimes\fi{}{10}^{14}\phantom{\rule{0.28em}{0ex}}{\mathrm{W}/\mathrm{cm}}^{2}$ (790-nm, 25-fs laser pulses). To shed more light on the debated issue, we have introduced an initial longitudinal momentum spread at the tunnel exit to compare with the measured photoelectron angular distributions using a dedicated semiclassical model. The simulation indicates that the value of this longitudinal momentum spread is in the range 0.0--0.1 a.u., which is less than the initial transverse momentum spread predicted by the tunneling theory.

Imaging molecular structure through femtosecond photoelectron diffraction on aligned and oriented gas-phase molecules.

This paper gives an account of our progress towards performing femtosecond time-resolved photoelectron diffraction on gas-phase molecules in a pump-probe setup combining optical lasers and an X-ray free-electron laser. We present results of two experiments aimed at measuring photoelectron angular distributions of laser-aligned 1-ethynyl-4-fluorobenzene (C(8)H(5)F) and dissociating, laser-aligned 1,4-dibromobenzene (C(6)H(4)Br(2)) molecules and discuss them in the larger context of photoelectron diffraction on gas-phase molecules. We also show how the strong nanosecond laser pulse used for adiabatically laser-aligning the molecules influences the measured electron and ion spectra and angular distributions, and discuss how this may affect the outcome of future time-resolved photoelectron diffraction experiments.

Slow photoelectron velocity-map imaging spectroscopy of the C9H7 (indenyl) and C13H9 (fluorenyl) anions

High-resolution photoelectron spectra are reported of the cryogenically cooled indenyl and fluorenyl anions, C9H7(-) and C13H9(-), obtained with slow electron velocity-map imaging. The spectra show well-resolved transitions to the neutral ground states, giving electron affinities of 1.8019(6) eV for indenyl and 1.8751(3) eV for fluorenyl. Numerous vibrations are observed and assigned for the first time in the radical ground states, including several transitions that are allowed only through vibronic coupling. The fluorenyl spectra can be interpreted with a Franck-Condon simulation, but explaining the indenyl spectra requires careful consideration of vibronic coupling and photodetachment threshold effects. Comparison of high- and low-resolution spectra along with measurements of photoelectron angular distributions provide further insights into the interplay between vibronic coupling and the photodetachment dynamics. Transitions to the neutral first excited states are also seen, with term energies of 0.95(5) eV and 1.257(4) eV for indenyl and fluorenyl, respectively. Those peaks are much wider than the experimental resolution, suggesting that nearby conical intersections must be considered to fully understand the vibronic structure of the neutral radicals.

Subcycle Dynamics of Coulomb Asymmetry in Strong Elliptical Laser Fields

We measure photoelectron angular distributions of noble gases in intense elliptically polarized laser fields, which indicate strong structure-dependent Coulomb asymmetry. Using a dedicated semiclassical model, we have disentangled the contribution of direct ionization and multiple forward scattering on Coulomb asymmetry in elliptical laser fields. Our theory quantifies the roles of the ionic potential and initial transverse momentum on Coulomb asymmetry, proving that the small lobes of asymmetry are induced by direct ionization and the strong asymmetry is induced by multiple forward scattering in the ionic potential. Both processes are distorted by the Coulomb force acting on the electrons after tunneling. Lowering the ionization potential, the relative contribution of direct ionization on Coulomb asymmetry substantially decreases and Coulomb focusing on multiple rescattering is more important. We do not observe evident initial longitudinal momentum spread at the tunnel exit according to our simulation.

Low Yield of Near-Zero-Momentum Electrons and Partial Atomic Stabilization in Strong-Field Tunneling Ionization

We measure photoelectron angular distributions of single ionization of krypton and xenon atoms by laser pulses at 1320 nm, 0.2-1.0×10(14) W/cm(2), and observe that the yield of near-zero-momentum electrons in the strong-field tunneling ionization regime is significantly suppressed. Semiclassical simulations indicate that this local ionization suppression effect can be attributed to a fraction of the tunneled electrons that are released in a certain window of the initial field phase and transverse velocity are ejected into Rydberg elliptical orbits with a frequency much smaller than that of the laser; i.e., the corresponding atoms are stabilized. These electrons with high-lying atomic orbits are thus prevented from ionization, resulting in the substantially reduced near-zero-momentum electron yield. The refined transition between the Rydberg states of the stabilized atoms has implication on the THz radiation from gas targets in strong laser fields.

How to measure the internuclear vector from photoelectron angular distributions

This paper uses a nonperturbative scattering theory to study photoelectron angular distributions of homonuclear diatomic molecules irradiated by circularly polarized laser fields. This study shows that the nonisotropic feature of photoelectron angular distributions is not due to the polarization of the laser field but the internuclear vector of the molecules. It suggests a method to measure the molecular orientation and the internuclear distance of molecules through the measurement of photoelectron angular distributions.

Ultra-low kinetic energy photoelectron angular distribution measurements in He and Ne using a Velocity Map Imaging spectrometer

We present photoelectron angular distributions (PADs) in Helium and Neon for electrons with excess energies between 5 and 100 meV. These ultra-low kinetic energy PAD measurements were obtained with a modified Velocity Map Imaging spectrometer (VMI) and VUV light from the Advanced Light Source (ALS) synchrotron radiation source. The efficiency and reliability of the spectrometer at this ultra-low kinetic energy range has been tested by determining the variation with energy of the asymmetry, β, parameter of photoelectrons from the s-shell direct ionization in Helium. For Neon, we determined the energy dependent asymmetry parameters across the "s" and "d" autoionizing resonances between the P3/2 and P1/2 ionic states. Furthermore, we measured the asymmetry parameter for photoelectrons produced from the n = 2 to n = 6 satellite states of He. These measurements were performed at values of excess kinetic energy previously unexplored.

Coherent strong-field control of multiple states by a single chirped femtosecond laser pulse

We present a joint experimental and theoretical study on strong-field photo-ionization of sodium atoms using chirped femtosecond laser pulses. By tuning the chirp parameter, selectivity among the population in the highly excited states 5p, 6p, 7p and 5f, 6f is achieved. Different excitation pathways enabling control are identified by simultaneous ionization and measurement of photoelectron angular distributions employing the velocity map imaging technique. Free electron wave packets at an energy of around 1 eV are observed. These photoelectrons originate from two channels. The predominant 2 + 1 + 1 resonance enhanced multi-photon ionization (REMPI) proceeds via the strongly driven two-photon transition 4s←←3s, and subsequent ionization from the states 5p, 6p and 7p whereas the second pathway involves 3 + 1 REMPI via the states 5f and 6f. In addition, electron wave packets from two-photon ionization of the non-resonant transiently populated state 3p are observed close to the ionization threshold. A mainly qualitative five-state model for the predominant excitation channel is studied theoretically to provide insights into the physical mechanisms at play. Our analysis shows that by tuning the chirp parameter the dynamics is effectively controlled by dynamic Stark shifts and level crossings. In particular, we show that under the experimental conditions the passage through an uncommon three-state ‘bow-tie’ level crossing allows the preparation of coherent superposition states.

Carrier-envelope phase-dependent quantum interferences in multiphoton ionization

The angular distribution of photoelectrons created by multiphoton ionization of xenon atoms by a few-cycle laser pulse shows a carrier-envelope phase (CEP) dependent asymmetry. A simple perturbative model based on a sum over indistinguishable quantum paths describes the observed asymmetry as a function of photoelectron energy and CEP. Although the individual multiphoton transition rates depend on the intensity profile of the pulse, the experimentally measured photoelectron angular distributions are sensitive to the absolute spectral phase of the pulse, including both CEP and chirp. We discuss retrieval of the CEP and chirp from the asymmetry pattern, as well as the potential to extract the scattering phase shift.

Photoelectron imaging and theoretical investigation of bimetallic Bi1–2Ga–2− and Pb1–4− cluster anions

We present the results of photoelectron velocity-map imaging experiments for the photodetachment of small negatively charged Bi(m)Ga(n) (m=1-2, n=0-2), and Pb(n) (n=1-4) clusters at 527 nm. The photoelectron images reveal new features along with their angular distributions in the photoelectron spectra of these clusters. We report the vertical detachment energies of the observed multiple electronic bands and their respective anisotropy parameters for the Bi(m)Ga(n) and Pb(n) clusters derived from the photoelectron images. Experiments on the BiGa(n) clusters reveal that the electron affinity increases with the number of Ga atoms from n=0 to 2. The BiGa(2)(-) cluster is found to be stable, both because of its even electron number and the high electron affinity of BiGa(2). The measured photoelectron angular distributions of the Bi(m)Ga(n) and Pb(n) clusters are dependent on both the orbital symmetry and electron kinetic energies. Density-functional theory calculations employing the generalized gradient approximation for the exchange-correlation potential were performed on these clusters to determine their atomic and electronic structures. From the theoretical calculations, we find that the BiGa(2)(-), Bi(2)Ga(3)(-) and Bi(2)Ga(5)(-) (anionic), and BiGa(3), BiGa(5), Bi(2)Ga(4) and Bi(2)Ga(6) (neutral) clusters are unusually stable. The stability of the anionic and neutral Bi(2)Ga(n) clusters is attributed to an even-odd effect, with clusters having an even number of electrons presenting a larger gain in energy through the addition of a Ga atom to the preceding size compared to odd electron systems. The stability of the neutral BiGa(3) cluster is rationalized as being similar to BiAl(3), an all-metal aromatic cluster.

Multihit two-dimensional charged-particle imaging system with real-time image processing at 1000 frames/s

A high-speed imaging system developed for two-dimensional counting of charged particles is presented. Microchannel plates coupled with a phosphor screen of a short emission lifetime (<1 micros) are used to visualize the two-dimensional positions of charged-particle impacts, and the image on the phosphor screen is captured with a 1 kHz complementary metal oxide semiconductor (CMOS) image sensor (512x512 pixels). A multistage image intensifier consisting of the first and second generation devices was used to compensate for the low sensitivity of CMOS. The centers of gravity (COG) of individual light spots in each image frame are calculated in real time by a field programmable gate array circuit. The performance of this system is tested by time-resolved photoelectron imaging (TR-PEI) of NO using (1+1(')) resonance enhanced multiphoton ionization via the A (2)Sigma(+) state with a femtosecond laser operated at 1 kHz. The new system enabled COG detection for more than ten particles in each frame at 1 kHz and achieved an extremely high degree of accuracy in the measurement of photoelectron angular distributions in TR-PEI.

Complete experiments in photoionization of atoms and molecules

The general discussion of possible complete experiments in photoionization of atoms and molecules is presented. In atoms the complete experiment must include measurements of the angular distribution and spin polarization of photoelectrons and the alignment or/and orientation of the residual ion. Two relations connecting the spin polarization parameters and the parameters characterizing the ion polarization state are presented. The complete experiments performed for Xe atom demonstrated a good agreement between theory and experiment. The complete experiments in photoionization of diatomic molecules can be performed by measuring photoelectron angular distributions for fixed-in-space molecules with circularly or elliptically polarized light. The use of additional experimental data like the angular asymmetry parameter β and the ratio of π to σ symmetry resolved cross sections made it possible to extract as many as 16 dipole matrix elements and phase shift differences for the C K-shell of CO molecule. Comparison with the recent calculations in the relaxed core Hartree–Fock approximation revealed discrepancies between theory and experiment.

Determination of theβparameter for atomic Mn and Cr2pphotoemission: A benchmark test for core-electron photoionization theories

We have determined the anisotropy parameter $\ensuremath{\beta}$ for the electron angular distribution in direct $2p$ photoionization of free Mn and Cr atoms. By using a high-resolution electron spectrometer and by rotating the polarization axis of the linearly polarized synchrotron radiation the values for $\ensuremath{\beta}$ could be derived with high precision and separately for the different multiplet components. Combination of the experimental results with Hartree-Fock based single configuration and multiconfiguration ab initio calculations gives new insight into the atomic $3d$-metal $2p$ photoionization process. Configuration interaction in the final ionic states need to be taken into account to adequately describe the measured photoelectron angular distributions.