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The electronic structures of actinide systems are extremely complicated because of strong electron correlations and relativistic effects. The atomic actinide anion poses even more challenges for experimental and theoretical investigations due to the extra loosely bound electron. In this work, we report the electronic structure of ${\mathrm{U}}^{\ensuremath{-}}$ using the slow-electron velocity-map imaging method. The electron affinity of U was measured to be 314.97(9) meV. Two excited states of ${\mathrm{U}}^{\ensuremath{-}}$ were observed and their energies were determined to be 84(11) and 160(12) meV above its ground state, respectively. The present study provides convincing evidence that the transition between the first excited state and the ground state of ${\mathrm{U}}^{\ensuremath{-}}$ is electric dipole (E1) allowed.

Despite the fact that the laser-cooling method is a well-established technique to obtain ultracold neutral atoms and atomic cations, it has rarely if ever been applied to atomic anions due to the lack of suitable electric-dipole transitions. Efforts of more than a decade have until recently only resulted in ${\mathrm{La}}^{\ensuremath{-}}$ as a promising anion candidate for laser cooling, but our previous work [Tang et al., Phys. Rev. Lett. 123, 203002 (2019)] showed that ${\mathrm{Th}}^{\ensuremath{-}}$ is also a potential candidate. Here we report on a combination of experimental and theoretical studies to determine the frequencies and rates, as well as branching ratios, for the relevant transitions in ${\mathrm{Th}}^{\ensuremath{-}}$. The resonant frequency of the laser-cooling transition is determined to be $\ensuremath{\nu}=123.455(30)$ THz $[\ensuremath{\lambda}=2428.4(6)\phantom{\rule{0.16em}{0ex}}\mathrm{nm}]$. The transition rate is calculated as $A=1.17\ifmmode\times\else\texttimes\fi{}{10}^{4}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$. Since the branching fraction to dark states is negligible, $1.47\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$, this represents an ideal closed cycle in ${\mathrm{Th}}^{\ensuremath{-}}$ for laser cooling. Furthermore, the zero nuclear spin of $^{232}\mathrm{Th}$ makes the cooling process possible in a Penning trap, which can be used to confine both antiprotons and ${\mathrm{Th}}^{\ensuremath{-}}$ ions. The presented ion dynamics simulations show that the laser-cooled ${\mathrm{Th}}^{\ensuremath{-}}$ anions can effectively cool antiprotons to a temperature around 10 mK.

Electron affinities (EAs) of many lanthanide elements still remain unknown due to their complicated electronic structures and relatively low EA values. In the present work, we utilized the slow-velocity map-imaging method combined with an ion trap to resolve conundrums. The EA values of praseodymium, neodymium, and terbium are determined to be $881.0(37)\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ or 0.109 23(46) eV, $786.3(26)\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ or 0.097 48(31) eV, and $1059.1(64)\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ or 0.131 31(79) eV, respectively. We also observed several excited states of Pr, Nd, and Tb anions.

We report an experimental and theoretical investigation into the dynamics of electron-impact ionization of the unresolved outermost molecular orbitals (6ag + 1b1g) of pyrazine (C4N2H4). Experimental triple differential cross sections (TDCS) are compared with cross sections calculated within a molecular 3-body distorted wave (M3DW) framework. The M3DW theory was able to reproduce the qualitative scattering behaviour in the binary region, but failed to describe the absolute intensity of the TDCS and the recoil scattering behaviour. Momentum profiles were used to identify possible sources where the M3DW may mask the quantum character of the ionized orbitals in describing the ionization dynamics.

Atomic lanthanum anion ${\mathrm{La}}^{\ensuremath{-}}$ is the best candidate for laser cooling of any atomic anions found so far. The bound-bound electric dipole transitions of ${\mathrm{La}}^{\ensuremath{-}}$ have been identified and measured. Theoretical studies have shown that ${\mathrm{La}}^{\ensuremath{-}}$ has a very complicated electronic structure, which cannot be well resolved by the traditional photoelectron spectroscopy. In the present work, we report high-resolution photoelectron spectroscopy of ${\mathrm{La}}^{\ensuremath{-}}$ via the slow-electron velocity-map imaging method in combination with an ion trap. The electron affinity of La was determined to be $4496.97(20)\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ or 0.557 553(25) eV. In addition, the energy levels of ${\mathrm{La}}^{\ensuremath{-}}$, $^{3}F_{3}^{e}$, $^{3}F_{4}^{e}$, $^{1}D_{2}^{e}$, $^{1}D_{2}^{o}$, $^{3}P_{0}^{e}$, $^{3}P_{1}^{e}$, and $^{3}P_{2}^{e}$, were also determined.

We report a measurement of the valence orbital momentum profiles of tetrabromomethane (${\mathrm{CBr}}_{4}$) using symmetric noncoplanar $(e,2e)$ experiments at the impact energies of about 600 and 1200 eV. The experimental momentum profiles for the individual orbitals $5{t}_{1}, 13{t}_{2}, 5e, 12{t}_{2}$, and $9{a}_{1}$ and the branching ratio of $5{t}_{1}$ to $13{t}_{2}$ and $5e$ to $13{t}_{2}$ are obtained and compared with two kinds of calculations under the plane-wave impulse approximation. One is theoretical momentum profiles that have been calculated at the equilibrium geometry, the other is those that involve vibrational effects using a thermal sampling molecular dynamics method. The calculations considering molecular vibrations are in better agreement with experiment than the equilibrium geometry calculations, indicating the important role of nuclear motions on the valence orbital electronic structures of ${\mathrm{CBr}}_{4}$. The distorted-wave effects are observed in the experimental momentum profiles of $5{t}_{1}, 5e$, and $12{t}_{2}$ which display dynamic dependencies on the impact energies. A multicenter interference or bond oscillation effect has been observed from the momentum profile ratios of $5{t}_{1}$ to $13{t}_{2}$ and $5e$ to $13{t}_{2}$ which has direct information about the bond length of a molecule.

We report an experimental and theoretical investigation of electron-impact single ionization of the highest occupied molecular orbital 1t2 and the next highest occupied molecular orbital 2a1 states of CH4 at an incident electron energy of 250 eV. Triple differential cross sections measured in two different laboratories were compared with results calculated within the molecular 3-body distorted wave and generalized Sturmian function theoretical models. For ionization of the 1t2 state, the binary peak was observed to have a single maximum near the momentum transfer direction that evolved into a double peak for increasing projectile scattering angles, as has been seen for ionization of atomic p-states. A detailed investigation of this evolution was performed. As expected because of its s-type character, for ionization of the 2a1 state, only a single binary peak was observed. Overall, good agreement was found between experiment and theory.

We report a measurement of the valence orbital-momentum profiles of mercury (Hg) using a high-sensitivity binary $(e,2e)$ electron-momentum spectrometer at incident energies of 600 and 1200 eV plus the binding energy. The $6{s}_{1/2}$ orbital and the spin-orbit components of $5{d}_{5/2}$ and $5{d}_{3/2}$ orbitals are well separated in the measured binding-energy spectra. The experimental momentum distributions for the individual orbitals and the branching ratio of $5{d}_{5/2}$ to $5{d}_{3/2}$ are obtained and compared with predictions from a plane-wave impulse approximation (PWIA) in which the orbital wave functions are calculated using the nonrelativistic (NR) and spin-orbital (SO) relativistic theories. The SO relativistic calculations are in better agreement with experiment than the NR, indicating clearly the importance of relativistic effects in the electronic structure of Hg. We also observe some discrepancies between experiment and PWIA calculations in the high-momentum region of $6{s}_{1/2}$ and the low-momentum region of $5d$ orbitals which display a dynamic dependence on the impact energy. These discrepancies become smaller at a higher energy of about 1200 eV and thus can be qualitatively assigned to the distorted-wave effect in the $(e,2e)$ reaction of Hg.

Hafnium is the last transition-metal element with an undetermined electron affinity. Using a slow-electron velocity-map imaging spectrometer equipped with a cold ion trap, the electron affinity of hafnium atoms is measured to be $1436(5)\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ or 0.1780(6) eV. The cold ion trap employed in the present measurement is found to be crucial for reducing the background noise from the hafnium hydride anions.

The latest electron affinity value of an iridium atom is 1.564 36(15) eV, determined via a method based on the Wigner threshold law by Bilodeau and co-workers. However, they observed a significant deviation from the Wigner threshold law in the threshold photodetachment experiment. To address this dilemma, we conducted high-resolution photoelectron spectroscopy of Ir− via the slow-electron velocity-map imaging method in combination with an ion trap. The electron affinity of Ir was measured to be 12 614.97(9) cm−1 or 1.564 057(11) eV. We find that the Wigner threshold law is still valid for the threshold photodetachment of Ir− through a p-wave fitting of the photodetachment channel Ir−5d86s23F4→Ir5d86sb4F9/2. The photoelectron angular distributions of photodetachment channels Ir−5d86s23F4→Ir5d76s2a4F9/2 and Ir−5d86s23F4→Ir5d86sb4F9/2 were also investigated. The behavior of anisotropy parameter β indicates a strong interaction between the two channels. Moreover, the energy level 3P2 of Ir−, which was not observed in the previous works, was experimentally determined to be 4163.24(16) cm−1 above the ground state.The latest electron affinity value of an iridium atom is 1.564 36(15) eV, determined via a method based on the Wigner threshold law by Bilodeau and co-workers. However, they observed a significant deviation from the Wigner threshold law in the threshold photodetachment experiment. To address this dilemma, we conducted high-resolution photoelectron spectroscopy of Ir− via the slow-electron velocity-map imaging method in combination with an ion trap. The electron affinity of Ir was measured to be 12 614.97(9) cm−1 or 1.564 057(11) eV. We find that the Wigner threshold law is still valid for the threshold photodetachment of Ir− through a p-wave fitting of the photodetachment channel Ir−5d86s23F4→Ir5d86sb4F9/2. The photoelectron angular distributions of photodetachment channels Ir−5d86s23F4→Ir5d76s2a4F9/2 and Ir−5d86s23F4→Ir5d86sb4F9/2 were also investigated. The behavior of anisotropy parameter β indicates a strong interaction between the two channels. Moreover, the energy level 3P2 of Ir−, which was not obs...

We report an experimental and theoretical investigation into the dynamics of electron-impact ionization of R-carvone. Experimental triple differential cross sections are obtained in asymmetric coplanar kinematic conditions for the ionization of the unresolved combination of the three outermost molecular orbitals (41a-39a) of R-carvone. These cross sections are compared with theoretical cross sections calculated within a molecular 3-body distorted wave (M3DW) framework employing either a proper orientation average or orbital average to account for the random orientation of the molecule probed in the experiment. Here, we observe that the overall scattering behavior observed in the experiment is fairly well reproduced within the M3DW framework when implementing the proper average over orientations. The character of the ionized orbitals also provides some qualitative explanation for the observed scattering behavior. This represents substantial progress when trying to describe the scattering dynamics observed for larger molecules under intermediate-impact energy and asymmetric energy sharing scattering conditions.

Experimental and theoretical triple differential cross sections for intermediate-energy (250 eV) electron-impact single ionization of the ${\mathrm{CO}}_{2}$ are presented for three fixed projectile scattering angles. Results are presented for ionization of the outermost $1{\ensuremath{\pi}}_{g}$ molecular orbital of $\mathrm{C}{\mathrm{O}}_{2}$ in a coplanar asymmetric geometry. The experimental data are compared to predictions from the three-center Coulomb continuum approximation for triatomic targets, and the molecular three-body distorted wave (M3DW) model. It is observed that while both theories are in reasonable qualitative agreement with experiment, the M3DW is in the best overall agreement with experiment.

We report an experimental and theoretical study of low-energy electron-impact ionization of tetrahydrofuran, which is a molecule of biological interest. The experiments were performed using an advanced reaction microscope specially built for electron-impact ionization studies. The theoretical calculations were performed within the molecular three-body distorted-wave model. Reasonably good agreement is found between experiment and theory.

Due to the low cross section of $p$-wave threshold photodetachment and the complicated electronic structures, the uncertainty of electron affinities for many transition elements still remains around 10 meV, which has not been improved for three decades. In this study, the electron affinity of Nb is measured as 917.40(6) meV or $7399.35(50)\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ using the slow electron velocity imaging method. The accuracy was improved by a factor of more than 400 with respect to the previous measurement. Furthermore, the fine structures of $\mathrm{N}{\mathrm{b}}^{\ensuremath{-}}$ were successfully resolved: 16.87(12) meV $({}^{5}{D}_{1})$, 46.75(12) meV (${}^{5}{D}_{2}$), 86.85(12) meV (${}^{5}{D}_{3}$), and 156.00(37) meV (${}^{5}{D}_{4}$) above the ground ${}^{5}{D}_{0}$, respectively.

We report the experimental and theoretical investigation of the complete valence shell binding energy spectra and momentum profiles of naphthalene (C10H8), using our high resolution electron momentum spectrometer, at impact energies of 1500 eV and 600 eV. The observed momentum profiles were compared with the Hartree-Fock (HF) and density functional theory (DFT) calculations, and the binding energy spectrum was compared with the Outer valence Green’s function (OVGF) calculations. The impact energy dependent discrepancy between observed momentum distributions and calculations under the plane wave impulse approximation was ascribed to the distorted wave effects.

The development of a third-generation electron momentum spectrometer with significantly improved energy and momentum resolutions at Tsinghua University (ΔE = 0.45–0.68 eV, Δθ = ±0.53° and Δϕ = ±0.84°) has enabled a reinvestigation of the valence orbital electron momentum distributions of H2O with improved statistical accuracy. The measurements have been conducted at impact energies of 1200 eV and 2400 eV in order to check the validity of the plane wave impulse approximation. The obtained ionization spectra and electron momentum distributions have been compared with the results of computations carried out with Hartree Fock [HF] theory, density functional theory in conjunction with the standard B3LYP functional, one-particle Green’s function [1p-GF] theory along with the third-order algebraic diagrammatic construction scheme [ADC(3)], symmetry adapted cluster configuration interaction [SAC-CI] theory, and a variety of multi-reference [MR-SDCI, MR-RSPT2, MR-RSPT3] theories. The influence of the basis set on the computed momentum distributions has been investigated further, using a variety of basis sets ranging from 6-31G to the almost complete d-aug-cc-pV6Z basis set. A main issue in the present work pertains to a shake-up band of very weak intensity at 27.1 eV, of which the related momentum distribution was analyzed for the first time. The experimental evidences and the most thorough theoretical calculations demonstrate that this band borrows its ionization intensity from the 2a1 orbital.

We have recently examined electron-impact ionization of molecules that have one large atom at the center, surrounded by H nuclei (${\mathrm{H}}_{2}\mathrm{O}, \mathrm{N}{\mathrm{H}}_{3}, \mathrm{C}{\mathrm{H}}_{4}$). All of these molecules have ten electrons; however, they vary in their molecular symmetry. We found that the triple-differential cross sections (TDCSs) for the highest occupied molecular orbitals (HOMOs) were similar, as was the character of the HOMO orbitals which had a $p$-type ``peanut'' shape. In this work, we examine ethane (${\mathrm{C}}_{2}{\mathrm{H}}_{6}$) which is a molecule that has two large atoms surrounded by H nuclei, so that its HOMO has a double-peanut shape. The experiment was performed using a coplanar symmetric geometry (equal final-state energies and angles). We find the TDCS for ethane is similar to the single-center molecules at higher energies, and is similar to a diatomic molecule at lower energies.

Results of an experimental study of the valence electronic structure of difluoromethane employing high-resolution Electron Momentum Spectroscopy with various impact energies are reported. One-particle Green’s Function theory is utilized, for the first time, for computing accurate spherically averaged electron momentum distributions. These are derived from Dyson orbitals obtained using the third-order Algebraic Diagrammatic Construction (ADC(3)) scheme. The corresponding eigen-energies also accurately reproduce the (e, 2e) ionization spectrum. Shortcomings of empirical analyses of (e, 2e) experiments based on Kohn–Sham orbitals and eigen-energies are comparatively discussed. A failure of the target Hartree-Fock approximation is noted for the momentum distribution pertaining to the 1b 1 + 3b 2 + 5a 1 levels.

Electron momentum spectroscopy studies of the ethylene highest occupied 1b3u orbital are reported. The experiments were performed at impact energies of 400, 600, 800, 1000, 1200 and 1600 eV using a high resolution and high sensitivity (e, 2e) spectrometer. The orbital electron momentum distributions of 1b3u are presented at a wide range of impact energies, which are used to examine the validity of plane wave impulse approximation and provide an experimental analysis of distorted wave effects in the higher momentum region of molecule with the variation of impact energy.

We report here the first measurements of the frontier highest occupied molecular orbital (HOMO) momentum profile and the complete valence shell binding energy spectra of cyclohexene by a binary (e, 2e) spectrometer using the non-coplanar symmetric kinematics. The experimental orbital electron momentum distribution is well described by the theoretical calculations.

The highest occupied molecular orbital (HOMO)of difluoromethane (CH2F2) has been firstly studied by binary (e, 2e) electron momentum spectroscopy (EMS), at the impact energy of 1200 eV plus binding energy, and using symmetric non-coplanar kinematics. The experimental momentum profile of the HOMO is compared with Hartree–Fock (HF)and density functional theory (DFT) methods with various basis sets. The experimental measurement is well described by the HF and DFT calculations except for the low-momentum region. 2004 Elsevier B.V. All rights reserved.

Triple differential cross section measurements for ionisation of N2 are presented in a coplanar geometry where one of the outgoing electrons was fixed in angle. Data were obtained at incident electron energies 20 and 40 eV above the ionisation potential for the 3σ g and 1π u states, the outgoing electrons carrying equal energies. Six sets of measurements were obtained at each energy, with fixed angles of 45°, 90° and 125° to the incident electron direction. The data are compared to new calculations using distorted wave methods.

We examine fully differential cross sections for 176 eV electron-impact dissociative excitation-ionization of orientated ${D}_{2}$ for transitions to final ion states $2s{\ensuremath{\sigma}}_{g}$, $2p{\ensuremath{\sigma}}_{u}$, and $2p{\ensuremath{\pi}}_{u}$. In previous work [Phys. Rev. A 88, 062705 (2013)], we calculated these cross sections using the molecular four-body distorted wave (M4DW) method with the ground-state ${D}_{2}$ wave function being approximated by a product of two Dyson 1$s$-type orbitals. The theoretical results were compared with experimental measurements for five different orientations of the target molecule (four in the scattering plane and one perpendicular to the scattering plane). For the unresolved $2s{\ensuremath{\sigma}}_{g}+2p{\ensuremath{\pi}}_{u}$ final states, good agreement with experiment was found for two of the five measured orientations, and for the $2p{\ensuremath{\sigma}}_{u}$ final state, good agreement was found for three of the five orientations. However, theory was a factor of 200 smaller than experiment for the $2p{\ensuremath{\sigma}}_{u}$ state. In this paper, we investigate the importance of the approximation for the molecular ground-state wave function by repeating the M4DW calculation using a better variational wave function for the ground state.

For the last few years, our group has calculated cross sections for electron-impact ionization of molecules using the molecular three-body distorted-wave approximation coupled with the orientation-averaged molecular orbital (OAMO) approximation. This approximation was very successful for calculating ionization cross sections for hydrogen molecules and to a lesser extent nitrogen molecules. Recently we used the approximation to calculate single ionization cross sections for the $1{t}_{2}$ state of methane (CH${}_{4}$) and we found major discrepancies with the experimental data. Here we investigate the validity of the OAMO approximation by calculating cross sections that have been properly averaged over all molecular orientations. These calculations with proper averages are in much better agreement with experiment than the OAMO calculations.

We present both experimental and theoretical results for the dissociative ionization of ${\mathrm{D}}_{2}$ molecules induced by electron impact. Cross sections are determined in the molecular frame and are fully differential in the energies and emission angles of the dissociation fragments. Transitions are considered from the $X$${}^{1}{\ensuremath{\Sigma}}_{g}{}^{+}$ electronic ground state of D${}_{2}$ to the $2s{\ensuremath{\sigma}}_{g}$, $2p{\ensuremath{\pi}}_{u}$ and $2p{\ensuremath{\sigma}}_{u}$ excited states of D${}_{2}{}^{+}$. The experimental results are compared to calculations performed within the molecular four-body distorted-wave framework to describe the multicenter nature of the scattering process. The cross sections reveal a dramatic dependence on both the alignment of the internuclear axis with respect to the direction of the projectile momentum and on the symmetry of the excited dissociating state which is energetically resolved.

Ionisation triple differential cross sections have been determined experimentally and theoretically for the neutral molecule N2 over a range of geometries from coplanar to the perpendicular plane. Data were obtained at incident electron energies ∼10 and ∼20 eV above the ionisation potential of the 3σg, 1πu and 2σg states, using both equal and non-equal outgoing electron energies. The data were taken with the incident electron beam in the scattering plane (ψ = 0°), at 45° to this plane and orthogonal to the plane (ψ = 90°). The set of nine measured differential cross sections at a given energy were then inter-normalised to each other. The data are compared to new calculations using various distorted wave methods, and differences between theory and experiment are discussed.

KLN would like to thank the European commission for a Marie Curie International Incoming Fellowship at the University of Manchester. We would like to thank the technicians in the Schuster laboratory for providing excellent support for the experimental apparatus. This work was partly supported by the US National Science Foundation under Grant. No. PHY-1068237 and by the National Natural Science Foundation of China under Grants No. 11174175. Computational work was performed with Institutional Computing resources made available through the Los Alamos National Laboratory and XSEDE resources provided by the Texas Advanced Computing Center (Grant No. TG-MCA07S029). The Los Alamos National Laboratory is operated by Los Alamos National Security, LLC, for the National Nuclear Security Administration of the US Department of Energy under Contract No. DE-AC5206NA25396.

Multiply charged anions possess a repulsive Coulomb barrier (RCB) against electron emission, thus allowing for long-lived metastable species with negative electron binding energies. For the prototypical multianion, bisdisulizole tetra-anion, we demonstrate that electronically excited states supported by the RCB can undergo resonant tunneling. The dynamics of this process was investigated by one-photon photoelectron imaging and femtosecond pump-probe photoelectron spectroscopy and confirmed by theoretical calculations. Efficient resonant tunneling emission of electrons from the excited states of multianions may be common for systems with sufficiently large RCB. This may provide new opportunities to study electron emission dynamics in complex systems.

The valence orbitals of the iodine molecule (I2) have been studied using the high-resolution electron-momentum spectroscopy at impact energies of 1200 and 600 eV. Experimental momentum distributions of outer valence orbitals were compared with the nonrelativistic, scalar relativistic, and spin-orbital relativistic calculations, as well as the relativistic pseudopotential calculation. The experimental cross-section ratios of 8J 3/2 g to 17J 1/2 g , 8J 3/2 u to 16J 1/2 u , and the electron-momentum profiles of orbital 16J 1/2 g clearly manifested the relativistic effects. In the inner valence region, the symmetry-adapted-cluster configuration-interaction theory was used to interpret the ionization spectrum and the electron-momentum distributions.

Low energy experimental and theoretical triple differential cross sections for the highest occupied molecular orbital of methane (1t(2)) and for the 2p atomic orbital of neon are presented and compared. These targets are iso-electronic, each containing 10 electrons and the chosen orbital within each target has p-electron character. Observation of the differences and similarities of the cross sections for these two species hence gives insight into the different scattering mechanisms occurring for atomic and molecular targets. The experiments used perpendicular, symmetric kinematics with outgoing electron energies between 1.5 eV and 30 eV for CH(4) and 2.5 eV and 25 eV for neon. The experimental data from these targets are compared with theoretical predictions using a distorted-wave Born approximation. Reasonably good agreement is seen between the experiment and theory for neon while mixed results are observed for CH(4). This is most likely due to approximations of the target orientation made within the model.

International audience; Experimental and theoretical results are presented for electron impact ionization of water in the energy regime from near threshold to intermediate energies. Results were taken in symmetric coplanar and non-coplanar geometries, with both equal and non-equal outgoing electron energies. The models approximate the random orientation of the target using a spherical averaging of the wave-function prior to the collision, using sophisticated distorted wave Born calculations that include postcollisional interactions in first order and to all orders of perturbation theory. The calculations predict the data most accurately at the lowest energy studied (4eV above threshold) in a coplanar symmetric geometry, whereas the comparison between theory and experiment is generally marginal for higher energies and for non-coplanar geometries.

A number of previous studies have suggested the possibility of two-center interference effects in the single ionization of diatomic molecules such as ${\text{H}}_{2}$ and ${\text{N}}_{2}$. While interference effects have been successfully observed in the ionization of ${\text{H}}_{2}$, to date evidence for interference in ${\text{N}}_{2}$ ionization has yet to be conclusively demonstrated. This study presents triply differential cross sections for electron impact ionization of ${\text{N}}_{2}$, measured using the $(e,2e)$ technique. The data are probed for signatures of two-center interference effects. Evidence for interference manifesting in the cross sections is observed.

Recently, the development of photoelectron velocity map imaging makes it much easier to obtain the photoelectron angular distributions (PADs) experimentally. However, explanations of PADs are only qualitative in most cases, and very limited works have been reported on how to calculate PAD of anions. In the present work, we report a method using the density-functional-theory Kohn-Sham orbitals to calculate the photodetachment cross sections and the anisotropy parameter β. The spherical average over all random molecular orientation is calculated analytically. A program which can handle both the Gaussian type orbital and the Slater type orbital has been coded. The testing calculations on Li(-), C(-), O(-), F(-), CH(-), OH(-), NH2 (-), O2 (-), and S2 (-) show that our method is an efficient way to calculate the photodetachment cross section and anisotropy parameter β for anions, thus promising for large systems.

In 1966, Cohen and Fano (1966 Phys. Rev. 150 30) suggested that one should be able to observe the equivalent of Young’s double slit interference if the double slits were replaced by a diatomic molecule. This suggestion inspired many experimental and theoretical studies searching for double slit interference effects both for photon and particle ionization of diatomic molecules. These effects turned out to be so small for particle ionization that this work proceeded slowly and evidence for interference effects were only found by looking at cross section ratios. Most of the early particle work concentrated on double differential cross sections for heavy particle scattering and the first evidence for two-center interference for electron-impact triple differential cross section (TDCS) did not appear until 2006 for ionization of H2. Subsequent work has now firmly established that two-center interference effects can be seen in the TDCS for electron-impact ionization of H2. However, in spite of several experimental and theoretical studies, similar effects have not been found for electron-impact ionization of N2. Here we report the first evidence for two-center interference for electron-impact ionization of N2.

The unexpected higher intensity at low momentum region in (e,2e) ionization cross sections, as called turn-up effect, was observed recently. This work systemically investigated this effect for the highest occupied molecular orbital of oxygen at various impact energies of 400, 600, 800, 1200, 1800, and 2400 eV by electron momentum spectroscopy. This turn-up became higher as the impact energy declined, which can be qualitatively predicted by the distorted wave explanation. However, it may be also due to some other dynamic interaction, such as the dynamic correlation. Since the oxygen molecule is the simplest multicenter system with such effects, this experimental result could be a test-bed for the accurate calculation of this effect in the molecular system.

We report here the measurements of the complete valence shell binding energy spectra and the valenceorbital momentum profiles of butanone using the binary e,2e electron momentum spectroscopy. The impact energy was 1200 eV plus the binding energy and the symmetric noncoplanar kinematics was used. The experimental momentum profiles of the valence orbitals are compared with the theoretical momentum distributions calculated using Hartree-Fock and density functional theory methods with various basis sets. The experimental measurements are generally described by theoretical calculations except for summed 4a ,1 5a, 3a, and 14a orbital and summed 8a ,7 a, and 6a orbitals.

The measurements of electron density distributions and binding-energy spectrum of the complete valence shell of cyclopentene $({\mathrm{C}}_{5}{\mathrm{H}}_{8})$ using a binary $(e,2e)$ electron momentum spectrometer are reported. The experimental momentum profiles of the valence orbitals are compared with the theoretical distributions calculated using Hartree-Fock and density-functional-theory (DFT) methods with various basis sets. The agreement between theory and experiment for the shape and intensity of the orbital electron momentum distributions is generally good. The DFT calculations employing B3LYP hybrid functional with a saturated and diffuse AUG-CC-PVTZ basis set provide the better descriptions of the experimental data. Some ``turn up'' effects in the low momentum region of the measured $(e,2e)$ cross section compared with the calculations of $3{a}^{\ensuremath{''}}$, $2{a}^{\ensuremath{''}}$, and $3{a}^{\ensuremath{'}}$ orbitals could be mainly attributed to distorted-wave effects. The pole strengths of the main ionization peaks from the orbitals in the inner valence are estimated.

The (e,2e) cross sections for transitions to the n=1, 2, and 3 final state of He{sup +} have been measured at impact energies of 1000 and 1600 eV by using a newly developed energy and momentum dispersive spectrometer. Binding energy spectra in the range of 8 to 86 eV and momentum profiles for the transitions to the ground and excited ion states of He{sup +} are reported at different impact energies, and the experimental results are compared with plane wave impulse approximation calculations. The impact energy dependence of cross section ratios for the He{sup +} n=1, 2, and 3 final states are obtained. Some discrepancies are observed between experimental data and theoretical calculations.

tal problem has been achieved via our recent development in the high performance of EMS spectrometer. In this new EMS spectrometer a novel double toroidal analyzer equipped with a series of conical deceleration lenses are used for electron energy and angle analyzing, and electron position detection and data acquisition are realized by using a pair of wedge strip anode position sen- sitive detectors with a Universal Serial Bus multiparameter data-acquisition system (9). The typical energy and time resolutions of 1.2 eV and 2 ns, and theandangle resolutions of � 0:7 � and � 1:9 � , are achieved from the EMS measurements of argon and helium. The significant improvement in the coincidence count rate wins the as- cendancy in the experimental periods. Typically, to obtain orbital electron density distributions with enough statistics, the experimental period of time with conventional EMS is about 2 -3 weeks (day and night), however only 2 -3 hours are needed by using this new spectrometer. Furthermore, the measurements within a wide range of impact energies (400 -2400 eV) have been achieved by using this new EMS spectrometer, which has made it possible to convincingly investigate the molecular distorted-wave effects in detail. In this Letter, the turn up effects of the 1b3g orbital EMS cross sections occurring at low and highp region in ethyl- ene (C2H4) were observed using the (e,2e) reaction at different impact energies of 400, 600, 800, 1000, 1200, 1600, 2000, and 2400 eV. The imagings of orbital electron density distributions in ethylene have been directly ob- tained. The 1b3g orbital momentum distributions are pro- vided as a function of impact energy. The theoretical calculations were carried out using PWIA, while the mo- lecular DWIA theory could not be achieved due to the multicenter nature of molecule targets, which is also an- other motivation of the promotion of molecular distorted- wave theory by means of the impact energy dependence of the orbital momentum distributions, such as 1b 3g orbital in

The electronic structure of nitrogen trifluoride was investigated by combining the high-resolution electron momentum spectroscopy with the high-level calculations. The experimental binding energy spectra and the momentum distributions of each orbital were compared with the results of Hartree—Fock, density functional theory (DFT), and symmetry-adapted-cluster configuration-interaction (SAC-CI) methods. SAC-CI and DFT-B3LYP with the aug-cc-pVTZ basis set can well reproduce the binding energy spectra and the observed momentum distributions of the valence orbitals except 1a2 and 4e orbitals. It was found that the calculated momentum distributions using DFT-B3LYP are even better than those using the high-level SAC-CI method.

The Dyson, Hartree-Fock, Kohn-Sham, natural, and natural-bond orbitals of methane (CH4) were compared with the experimental electron momentum distributions. The Dyson, Hartree-Fock, Kohn-Sham theories can present a direct concise interpretation and well reproduce the experimental results, but the natural-bond orbital cannot.

Experimental and theoretical triple differential cross sections (TDCS) from ammonia are presented in the low energy regime with outgoing electron energies from 20 eV down to 1.5 eV. Ionization measurements from the 3a1, 1e1, and 2a1 molecular orbitals were taken in a coplanar geometry. Data from the 3a1 and 1e1 orbitals were also obtained in a perpendicular plane geometry. The data are compared to predictions from the distorted wave Born approximation and molecular-three-body distorted wave models. The cross sections for the 3a1 and 1e1 orbitals that have p-like character were found to be similar, and were different to that of the 2a1 orbital which has s-like character. These observations are not reproduced by theory, which predicts the structure of the TDCS for all orbitals should be similar. Comparisons are also made to results from experiment and theory for the iso-electronic targets neon and methane.

We present triply differential cross sections for the electron impact ionization of the outer valence orbitals of formic acid (CHOOH) by 100 eV and 250 eV incident electrons. The experiments were performed under asymmetric kinematics, in which the outgoing ejected electron had an energy of 10 eV, over a range of momentum transfers. The experimental results are compared with theoretical calculations carried out using the sophisticated M3DW model, both with and without correlation-polarization-exchange terms included.