Asymmetry In Angular Distribution Research Articles

Strong Electron-Electron-Nuclei Correlations in Two-Photon Double Ionization of H_{2}.

Two-photon double ionization is a paradigmatic example of how electron correlation manifests. In molecular targets, its coupling with the slower nuclear motion introduces an additional complication and induces electron-electron-nuclei correlations. Experimentally, momentum-coincident measurements can provide a complete kinematical image of the molecular full Coulomb breakup. Previous theoretical studies have described this process by ignoring nuclear motion and the subsequent Coulomb explosion of the dication. Here we show, by means of a full-dimensional treatment of two-photon double ionization of the H_{2} molecule, that nuclear motion plays a decisive role even for pulses as short as 1.5fs, a time during which the nuclei are not expected to move significantly. We find strong correlations between nuclear and electronic degrees of freedom, giving access to different electronic processes as a function of nuclear kinetic energy. In particular, we observe unexpectedly strong back-to-back asymmetry in the photoelectron angular distributions, as well as novel interferences resulting from the coherent contributions from two-photon sequential absorption paths via different molecular cationic states.

Cooper minima in high-Z atoms: effects of correlation and relativity on np photoionization

Photoionization dipole transition matrix elements pass through a zero or attain a minimum that leaves imprints on photoionization parameters like the cross-section, angular distribution asymmetry parameter, phase shift, and photoionization time delay. This minimum is commonly known as the ‘Cooper minimum’ (CM). The CM, in general, is strongly affected by relativistic and correlation effects. Previous works investigated CM in the 6p and 5p subshell photoionization up to Z = 100 using the single-particle Dirac-Slater (DS) method. The present work extends the earlier work to Z up to 120 using more accurate methods; Dirac–Hartree–Fock (DHF) which includes the relativistic effects and exchange correlations, and the relativistic random phase approximation (RRPA) which includes both initial and final state electron-electron correlations along with relativistic effects. In addition to the study of photoionization from the 6p and 5p subshells, the 4p subshell has also been investigated in the present work. To demonstrate the prominent effects in the high-Z atoms, Rn (Z = 86), Ra (Z = 88), No (Z = 102), Cn (Z = 112), Og (Z = 118), and Ubn (Z = 120) are investigated.

A numerical study of the anisotropic distribution of γ-ray emission from oriented 129m,131m,133mXe

Abstract Spin-polarized radioactive nuclei emit radioactive decay products in an anisotropic manner that is characteristic of their degree of nuclear orientation. This property can be utilized for nuclear magnetic resonance spectroscopy and imaging. In this manuscript, we present Python and MATLAB numerical simulations that investigate the angular distribution and measurable asymmetry of γ-ray emission from spin-oriented nuclei of the metastable isotopes 129 m , 131 m , 133 m Xe. We examine several cases that represent different degrees of spin alignment and detection geometries. The results of our simulations provide a benchmark for experiments that use γ-decay anisotropy, such as the GAMMA-MRI project, which aims to develop a novel medical-imaging technique combining the strengths of magnetic resonance and nuclear medicine imaging.

Theoretical study of time-resolved photoelectron circular dichroism in the photodissociation of a chiral molecule.

Photoelectron circular dichroism (PECD), the forward-backward asymmetry of the photoelectron angular distribution when ionizing randomly oriented chiral molecules with circularly polarized light, is an established method to investigate chiral properties of molecules in their electronic ground state. Here, we develop a computational strategy for predicting time-resolved PECD (TRPECD) of chemical reactions and demonstrate the method on the photodissociation of 1-iodo-2-methylbutane. Our approach combines multi-configurational quantum-chemical calculations of the relevant potential-energy surfaces of the neutral and singly ionized molecule with ab initio molecular-dynamics (AIMD) calculations. The PECD parameters along the AIMD trajectories are calculated with the aid of electron-molecule scattering calculations based on the Schwinger variational principle implemented in ePolyScat. Our calculations have been performed for two probe wavelengths (133 and 160 nm) accessible through low-order harmonic generation in gases. Our results show that the TRPECD is a highly sensitive probe of photochemical reaction dynamics. Most interestingly, the TRPECD is found to change sign multiple times along the photodissociation coordinate, in agreement with recent experiments on CHBrFI [Svoboda et al., "Femtosecond photoelectron circular dichroism of chemical reactions," Sci. Adv. 8, eabq2811 (2022)]. The computational protocol introduced in the present work is general and readily applicable to other chiral photochemical processes.

Quadrupole Effects in the Photoionisation of Sodium 3s in the Vicinity of the Dipole Cooper Minimum

A procedure to obtain relativistic expressions for photoionisation angular distribution parameters using the helicity formulation is discussed for open-shell atoms. Electric dipole and quadrupole transition matrix elements were considered in the present work, to study the photoionisation dynamics of the 3s electron of the sodium atom in the vicinity of the dipole Cooper minimum. We studied dipole–quadrupole interference effects on the photoelectron angular distribution in the region of the dipole Cooper minimum. Interference with quadrupole transitions was found to alter the photoelectron angular distribution, even at rather low photon energies. The initial ground and final ionised state discrete wavefunctions of the atom were obtained in the present work using GRASP, and we employed RATIP with discrete wavefunctions, to construct continuum wavefunctions and to calculate transition amplitudes, total cross-sections and angular distribution asymmetry parameters.

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.

Analysis of angular distribution asymmetries and the associated C!P asymmetries in three-body decays of bottom baryons

We introduce a set of observables representing angular distribution asymmetries, which can be viewed as a generalization of the forward-backward asymmetry of angular distributions, and can be used as an effective tool to search for C!P violation in three-body decays of bottom and charmed baryons. We propose to search for such C!P asymmetries (1) in decays with Lambda ^0, Sigma ^pm , or Lambda _c^+ involved, such as Lambda _brightarrow Lambda ^0 D and Lambda _brightarrow Lambda ^0rho (770)^0; and (2) in three-body decays of bottom baryons with opposite parity intermediate resonances involved. Typical examples include Xi _b^-rightarrow p K^- K^-, Lambda _b^0rightarrow p K_Spi ^- and Lambda _b^0rightarrow p pi ^0pi ^-, in which the last decay channel is used as a toy model to illustrate the basic idea.

Investigation of photoelectron elliptical dichroism for chiral analysis.

This paper describes a compact new instrument, conceived specifically for measurements of Photo Electron Elliptical Dichroism (PEELD) and designed for simplicity of use as a prototype for a practical analytical device. PEELD is an asymmetry in the electron angular distribution obtained from resonantly enhanced multi-photon ionisation of a chiral molecule, where there is also a non-linear dependence on the polarization ellipticity. Despite the fact that PEELD can provide a unique signature of molecular structure and dynamics it has only been investigated in a few molecules to date. This is addressed in the present study in a range of measurements of several terpenes and phenyl-alcohols. These show that the PEELD signatures in structural isomers can be dramatically different and can also be influenced by the intensity of the light. A systematic study in phenyl-alcohols containing the same chromophore and chiral centre configuration shows consistent PEELD behaviour across the molecules except that the magnitude reduces as the distance from the chromophore to the chiral centre increases. These achievements demonstrate that this relatively simple set up can be used for scientific studies as well as providing a blueprint for a practical chiral analysis instrument.

Angle-dependent interferences in electron emission accompanying stimulated Compton scattering from molecules

The high brilliance of ultrashort X-ray pulses recently generated in free electron lasers will soon open the way to the investigation of non-linear processes that still remain inaccessible due to the smallness of the corresponding cross sections. One of them is stimulated Compton scattering from molecules. In this work, we investigate stimulated Compton scattering from fixed-in-space H2 molecules in the few-hundred eV photon energy range, where both dipole and non-dipole transitions are important. We show that the interference between dipole and non-dipole transitions leads to pronounced asymmetries in the electron angular distributions. These asymmetries strongly depend on molecular orientation, to the point that they can lead to electron emission in either the forward or the backward directions with respect to the propagation axis, or in both directions, or even in the orthogonal direction. This is in contrast with Compton scattering from free electrons or atomic targets.

Photoelectron angular distribution studies for two spin–orbit-split components of Xe 3d subshell: a critical comparison between theory and experiment

The photoelectron angular distribution asymmetry parameters β of the Xe 3d subshell were investigated using an x-ray free-electron laser (XFEL) at photon energies of 750 and 800 eV. Owing to the perfect polarization of the XFEL and two-dimensional momentum imaging capability of our velocity map imaging spectrometer, we determined the β values with high accuracy. The β values were also investigated based on relativistic time-dependent density functional theory calculations of up to 900 eV of photon energies. By comparing all the available experimental results including our data with the most reliable theories on the photon energy dependence of the β parameters, serious differences are noted between the experiments and theories. Further studies on resolving this difference will provide new insight into the photoionization processes of the deep inner shells.

Relaxation effects on the magnetic cross section and fluorescence radiation polarization after 4d inner-shell photoionization of Ba-like ions

On the basis of the multi-configuration Dirac–Fock method and the density matrix theory treatment of photoionization (PI) dynamics, a systematic investigation for 4d inner-shell PI of exemplary Ba atom and Ba-like Nd4+, Gd8+ and Yb14+ ions is performed. Starting from the general framework of the relativistic, the relaxation effects are considered by using a set of relaxed one-electron orbitals in solving the coupled Dirac equations. Our results uncover for the first time that the relaxation effects, arising from the overlap integrals between orbitals of the initial state and the relaxed final state, may play a paramount role in determining the magnetic sublevel cross sections, the photoelectron angular-distribution asymmetry parameters, and the fluorescence polarizations of the subsequent x-ray radiation, particularly for neutral atoms close to the threshold regime. Yet the importance of inclusion of the orbital relaxation weakens as the incoming photon energy and/or the degree of atom ionization increases. The nature of these influences is explored, which allows us to get more insight on the physical process and mechanism involved. Our results are consistent with those obtained by the experiments and other theoretical predictions.

Photoelectron satellite spectroscopy and angular distribution of argon atoms using a monochromatic EUV source based on high-order harmonic generation

Photoelectron satellite spectra and angular distribution of argon atoms were measured in the energy range of 34--43 eV, using a well-developed reaction microscope mounted on a newly built high-order harmonic generation (HHG) extreme ultraviolet (EUV) source. Various satellites were resolved and the angular distribution asymmetry parameters $\ensuremath{\beta}$ were determined for both the main lines and the satellites. It is found that our measured $\ensuremath{\beta}$ for the $3p$ and $3s$ main lines are in an excellent agreement with previous results. The $\ensuremath{\beta}$ parameters measured for most of the satellites are first reported in this energy range, providing benchmark data for testing different theoretical models.

Multiphoton photoelectron circular dichroism of limonene with independent polarization state control of the bound-bound and bound-continuum transitions.

Photoionization of randomly oriented chiral molecules with circularly polarized light leads to a strong forward/backward asymmetry in the photoelectron angular distribution. This chiroptical effect, referred to as Photoelectron Circular Dichroism (PECD), was shown to take place in all ionization regimes, from single photon to tunnel ionization. In the Resonance Enhanced Multiphoton Ionisation (REMPI) regime, where most of the table-top PECD experiments have been performed, understanding the role of the intermediate resonances is currently the subject of experimental and theoretical investigations. In an attempt to decouple the role of bound-bound and bound-continuum transitions in REMPI-PECD, we photoionized the (+)-limonene enantiomer using two-color laser fields in [1 + 1'] and [2 + 2'] ionization schemes, where the polarization state of each color can be controlled independently. We demonstrate that the main effect of the bound-bound transition is to break the sample isotropy by orientation-dependent photoexcitation, in agreement with recent theoretical predictions. We show that the angular distribution of PECD strongly depends on the anisotropy of photoexcitation to the intermediate state, which is different for circularly and linearly polarized laser pulses. On the contrary, the helicity of the pulse that drives the bound-bound transition is shown to have a negligible effect on the PECD.

Measurement of the absolute neutron beam polarization from a supermirror polarizer and the absolute efficiency of a neutron spin rotator for the NPDGamma experiment using a polarized 3He neutron spin-filter

Accurately measuring the neutron beam polarization of a high flux, large area neutron beam is necessary for many neutron physics experiments. The Fundamental Neutron Physics Beamline (FnPB) at the Spallation Neutron Source (SNS) is a pulsed neutron beam that was polarized with a supermirror polarizer for the NPDGamma experiment. The polarized neutron beam had a flux of ∼109 neutrons per second per cm2 and a cross sectional area of 10 × 12 cm2. The polarization of this neutron beam and the efficiency of a RF neutron spin rotator installed downstream on this beam were measured by neutron transmission through a polarized 3He neutron spin-filter. The pulsed nature of the SNS enabled us to employ an absolute measurement technique for both quantities which does not depend on accurate knowledge of the phase space of the neutron beam or the 3He polarization in the spin filter and is therefore of interest for any experiments on slow neutron beams from pulsed neutron sources which require knowledge of the absolute value of the neutron polarization. The polarization and spin-reversal efficiency measured in this work were done for the NPDGamma experiment, which measures the parity violating γ-ray angular distribution asymmetry with respect to the neutron spin direction in the capture of polarized neutrons on protons. The experimental technique, results, systematic effects, and applications to neutron capture targets are discussed.

Coherent control of the photoelectron angular distribution in photoionization of neon

SynopsisWe investigate trichromatic photoionization of neon induced by linearly polarized XUV femtosecond pulses. The fundamental frequency is tuned near an optically allowed transition to create an interference between one-photon and two-photon ionization pathways. The asymmetry in the photoelectron angular distribution is analyzed for a variety of pulse characteristics.

Probing the W tb vertex structure in t-channel single-top-quark production and decay in pp collisions at sqrt{s}=8 TeV with the ATLAS detector

To probe the W tb vertex structure, top-quark and W -boson polarisation observables are measured from t-channel single-top-quark events produced in proton-proton collisions at a centre-of-mass energy of 8 TeV. The dataset corresponds to an integrated luminosity of 20.2 fb−1, recorded with the ATLAS detector at the LHC. Selected events contain one isolated electron or muon, large missing transverse momentum and exactly two jets, with one of them identified as likely to contain a b-hadron. Stringent selection requirements are applied to discriminate t-channel single-top-quark events from background. The polarisation observables are extracted from asymmetries in angular distributions measured with respect to spin quantisation axes appropriately chosen for the top quark and the W boson. The asymmetry measurements are performed at parton level by correcting the observed angular distributions for detector effects and hadronisation after subtracting the background contributions. The measured top-quark and W -boson polarisation values are in agreement with the Standard Model predictions. Limits on the imaginary part of the anomalous coupling gR are also set from model-independent measurements.

Theoretical description of circular dichroism in photoelectron angular distributions of randomly oriented chiral molecules after multi-photon photoionization.

Photoelectron circular dichroism refers to the forward/backward asymmetry in the photoelectron angular distribution with respect to the propagation axis of circularly polarized light. It has recently been demonstrated in femtosecond multi-photon photoionization experiments with randomly oriented camphor and fenchone molecules [C. Lux et al., Angew. Chem., Int. Ed. 51, 4755 (2012) and C. S. Lehmann et al., J. Chem. Phys. 139, 234307 (2013)]. A theoretical framework describing this process as (2+1) resonantly enhanced multi-photon ionization is constructed, which consists of two-photon photoselection from randomly oriented molecules and successive one-photon ionization of the photoselected molecules. It combines perturbation theory for the light-matter interaction with ab initio calculations for the two-photon absorption and a single-center expansion of the photoelectron wavefunction in terms of hydrogenic continuum functions. It is verified that the model correctly reproduces the basic symmetry behavior expected under exchange of handedness and light helicity. When applied to fenchone and camphor, semi-quantitative agreement with the experimental data is found, for which a sufficient d wave character of the electronically excited intermediate state is crucial.

Universality of photoelectron circular dichroism in the photoionization of chiral molecules

Photoionization of chiral molecules by circularly polarized radiation gives rise to a strong forward/backward asymmetry in the photoelectron angular distribution, referred to as photoelectron circular dichroism (PECD). Here we show that PECD is a universal effect that reveals the inherent chirality of the target in all ionization regimes: single photon, multiphoton, above-threshold and tunnel ionization. These different regimes provide complementary spectroscopic information at electronic and vibrational levels. The universality of the PECD can be understood in terms of a classical picture of the ionizing process, in which electron scattering on the chiral potential under the influence of a circularly polarized electric field results in a strong forward/backward asymmetry.

Wavelength dependent photoelectron circular dichroism of limonene studied by femtosecond multiphoton laser ionization and electron-ion coincidence imaging.

Enantiomers of the monoterpene limonene have been investigated by (2 + 1) resonance enhanced multiphoton ionization and photoelectron circular dichroism employing tuneable, circularly polarized femtosecond laser pulses. Electron imaging detection provides 3D momentum measurement while electron-ion coincidence detection can be used to mass-tag individual electrons. Additional filtering, by accepting only parent ion tagged electrons, can be then used to provide discrimination against higher energy dissociative ionization mechanisms where more than three photons are absorbed to better delineate the two photon resonant, one photon ionization pathway. The promotion of different vibrational levels and, tentatively, different electronic ion core configurations in the intermediate Rydberg states can be achieved with different laser excitation wavelengths (420 nm, 412 nm, and 392 nm), in turn producing different state distributions in the resulting cations. Strong chiral asymmetries in the lab frame photoelectron angular distributions are quantified, and a comparison made with a single photon (synchrotron radiation) measurement at an equivalent photon energy.