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The authors report experimental measurements on electron-impact ionization of CO${}_{2}$ using the angle- and energy-resolved electron-ion-coincidence technique. The results provide experimental evidence of symmetry breaking in electron-impact dissociative ionization of symmetric molecules.

We report an (e, e+ion) spectroscopy study on electron-induced dissociative ionization of N2. Vector correlation between the scattered electron and N+ ion has been measured for inner-valence ionization of N2 at an incident electron energy of 1.4 keV and scattering angles of 2.2°, 4.2°, and 8.2°. By analyzing the experimental data, partial ion yield spectra have been obtained for transitions to the C 2Σ+u, F 2Σ+g, and 2σ−1g ion states, showing that the individual transitions depend on the momentum transferred to the target, K, in different ways. The molecular-orientation dependence of the ionization cross section has subsequently been examined for the F 2Σ+g ionization. To account for the angular anisotropy of the scattering cross section, a compact analytical form has been developed. It is elucidated that for small K the F 2Σ+g ionization preferentially takes place when the molecule has its axis aligned parallel to the momentum transfer vector due to σu shape resonance, while the angular distribution drastically changes with K, indicating strong influences of non-dipole interaction on the ionization dynamics. It has been shown that the present method provides a powerful means to explore K-dependent stereodynamics in electron-induced dissociative ionization of molecules.

We report electron-atom Compton scattering experiments on the ${\mathrm{H}}_{2}$ and ${\mathrm{D}}_{2}$ molecules. Energy-loss spectra of electrons quasielastically backscattered at an angle of 135\ifmmode^\circ\else\textdegree\fi{} are measured as a function of azimuthal angle at an incident electron energy of 2.0 keV. Momentum distributions of the H and D atoms due to molecular vibration are extracted from the experimental data by using a protocol that we propose here. The results are successfully compared with theoretical ones predicted by the molecular vibrational wave functions. It is shown that electron-atom Compton scattering has a unique ability to provide direct information about intramolecular motion of each atom with different mass numbers.

We report an angle-resolved electron-energy-loss spectroscopy study on the valence excitations of $\mathrm{S}{\mathrm{F}}_{6}$. Momentum-transfer-dependent generalized oscillator strengths (GOSs) or GOS profiles of the low-lying valence excitations have been derived from electron-energy-loss spectra measured at an incident electron energy of 3 keV. We have also performed theoretical calculations of the GOS profiles involving the influence of molecular vibration. Comparisons between experiment and theory have revealed that coupling between electronic states through molecular vibration plays a significant role in the $1\phantom{\rule{0.16em}{0ex}}^{1}T_{1g}$ dipole-forbidden transition.

We report an electron momentum spectroscopy study on methyl formate. A symmetric noncoplanar (e, 2e) experiment has been performed at an incident electron energy of 1.2 keV and electron momentum profiles of the valence orbitals have been obtained. On the basis of the result, assignments of the 10a'-1 and 1a″-1 bands have been made to resolve a contradiction between photoelectron spectroscopy and Penning ionization electron spectroscopy studies. Comparisons between experiment and theory reveal that the influence of the molecular vibration has to be taken into account for a proper understanding of the electron momentum profiles. Contributions of individual vibrational normal modes have also been investigated in detail by means of the harmonic analytical quantum mechanical approach.

Electron-induced dissociative ionization of ${\mathrm{O}}_{2}$ has been investigated by means of $(e,\phantom{\rule{0.16em}{0ex}}e+\mathrm{ion})$ spectroscopy. The linear momenta of the fragment ions have been measured in coincidence with electron energy-loss spectra for ${\mathrm{O}}_{2}$ at scattering angles of $2.{2}^{\ensuremath{\circ}}, 4.{2}^{\ensuremath{\circ}}$, and $8.{2}^{\ensuremath{\circ}}$ using an incident energy of 1.4 keV. From the analysis of the data, partial ion yield spectra have been obtained for the $B\phantom{\rule{0.16em}{0ex}}^{2}\mathrm{\ensuremath{\Sigma}}, {c\phantom{\rule{0.16em}{0ex}}^{4}\mathrm{\ensuremath{\Sigma}}+2\phantom{\rule{0.16em}{0ex}}^{2}\mathrm{\ensuremath{\Sigma}}+2\phantom{\rule{0.16em}{0ex}}^{4}\mathrm{\ensuremath{\Sigma}}}, {2\phantom{\rule{0.16em}{0ex}}^{2}\mathrm{\ensuremath{\Sigma}}+2\phantom{\rule{0.16em}{0ex}}^{4}\mathrm{\ensuremath{\Sigma}}}$, and $3\phantom{\rule{0.16em}{0ex}}^{2}\mathrm{\ensuremath{\Sigma}}$ ionization, allowing us to assess state-specific momentum-transfer dependence of the relative ionization cross sections. It has been shown that the shape resonance in the $B\phantom{\rule{0.16em}{0ex}}^{2}\mathrm{\ensuremath{\Sigma}}$ channel is substantially suppressed by the contributions of electric nondipole transitions. To get insight into the stereodynamics in the electron-${\mathrm{O}}_{2}$ collision processes, the angular distributions of the fragment ions have been examined. It has been revealed that the $(e,\phantom{\rule{0.16em}{0ex}}e+\mathrm{ion})$ cross section for the $2{\ensuremath{\sigma}}_{\mathrm{g}}\ensuremath{\rightarrow}1{\ensuremath{\pi}}_{\mathrm{g}}$ autoionization band exhibits a characteristic angular distribution reflecting the anisotropic shape of the molecular orbital to which the target electron is excited.

An apparatus has been developed for electron-atom Compton scattering experiments that can employ a pulsed laser and a picosecond pulsed electron beam in a pump-and-probe scheme. The design and technical details of the apparatus are described. Furthermore, experimental results on the Xe atom in its ground state are presented to illustrate the performance of the pulsed electron gun and the detection and spectrometric capabilities for scattered electrons. The scope of future application is also discussed, involving real-time measurement of intramolecular force acting on each constituent atom with different mass numbers, in a transient, evolving system during a molecular reaction.

We report an (e, 2e) study on interference effects in the spherically-averaged electron momentum density distributions of the five outermost molecular orbitals (MOs) of SF6, which are each constructed from the 2p atomic orbitals (AOs) on the F atoms. The (e, 2e) experiment was performed in the symmetric noncoplanar geometry and at an incident electron energy of 1.2 keV. Interference patterns for each MO have then been obtained by dividing the experimental data by distorted-wave-Born-approximation cross section for an isolated F 2p AO. A detailed analysis of the results shows that the interference patterns afford a rare opportunity to experimentally clarify symmetries of the MOs. Furthermore, it is demonstrated that they provide information about not only the molecular geometry but also the spatial orientation of the constituent AOs even for this relatively complicated molecule.

We experimentally investigate the molecular-orientation dependence of high-energy electron-impact ionization of CO. The direction of the molecular-axis with respect to the momentum transfer vector K is deduced from the angular correlation between the fragment ion and the scattered electron. The experimental results on the 3 2Π ionization reveal that at small momentum transfer, the ionization probability near the threshold is higher when K points toward the C atom along the molecular axis than when it is in the opposite direction. Such a forward–backward asymmetry does not appear in single-photon ionization and requires non-dipole contributions. It is also shown that the {4 2Σ+ + 5 2Σ+ + 6 2Σ+} ionization preferentially takes place in the vicinity of the molecular orientation parallel to K at small momentum transfer, while non-dipole contributions cause the decrease in the relative intensity of the parallel direction.

This paper reports a theoretical study on electron-impact excitations of acetylene. Momentum-transfer-dependent generalized oscillator strengths (GOSs) or GOS profiles have been calculated using wavefunctions at the equation-of-motion coupled-cluster singles and doubles level. In the calculation the influence of molecular vibration has been taken into account. The computed results are in overall agreement with experimental GOS profiles reported in the literature. Our theoretical analysis has revealed prominent roles of bending vibrations in the low-lying valence excitations induced by electron impact.

We report details of an electron-ion coincidence apparatus, which has been developed for molecular-frame electron energy loss spectroscopy studies. The apparatus is mainly composed of a pulsed electron gun, an energy-dispersive electron spectrometer, and an ion momentum imaging spectrometer. Molecular-orientation dependence of the high-energy electron scattering cross section can be examined by conducting measurements of vector correlation between the momenta of the scattered electron and fragment ion. Background due to false coincidences is significantly reduced by introducing a pulsed electron beam and pulsing scheme of ion extraction. The experimental setup has been tested by measuring the inner-shell excitation of N2 at an incident electron energy of 1.5 keV and a scattering angle of 10.2°.

We report molecular-frame electron-scattering cross sections of ${\mathrm{N}}_{2}$, obtained using an electron-ion coincidence technique. Vector correlation between the scattered electron and the fragment ion has been measured at incident electron energy of 1.4 keV for the dipole-forbidden $2{\ensuremath{\sigma}}_{g}\ensuremath{\rightarrow}1{\ensuremath{\pi}}_{g}$ transition of ${\mathrm{N}}_{2}$ as well as for the inner-valence ionization. The generalized oscillator strength thus obtained exhibits characteristic molecular-orientation dependence due to anisotropic shapes of the molecular orbitals participating in the transition. The present method enables one to explore stereodynamics of dipole-forbidden transitions to autoionization states, which are inaccessible by photoabsorption experiments.

We report an electron momentum spectroscopy study of vibrational effects on the electron momentum distributions of the outer valence orbitals of adamantane (C10H16). The symmetric noncoplanar (e, 2e) experiment has been carried out at an incident electron energy of 1.2 keV. Furthermore, theoretical calculations of the electron momentum distributions with vibrational effects being involved have been performed using the harmonic analytical quantum mechanical and Born-Oppenheimer molecular dynamics approaches. In spite of the complex nature of the vibrational structure of this large molecule, both approaches provide overall quantitative insights into the results of the experiment. Comparisons between experiment and theory have shown that ground state nuclear dynamics appreciably affects the momentum profiles of the 7t2, {2t1+3e}, and {5t2+5a1} orbitals. It has been demonstrated that changes in the momentum profiles are mainly due to the vibrational motions associated with the CH bonds. This work has been supported by the FWO Vlaanderen, the Flemish branch of the Belgian National Science Foundation, the “Bijzonder Onderzoeks Fonds” of Hasselt University, and the Japanese Ministry of Education, Culture, Sports, Science and Technology, Grant-in-Aids for Scientific Research (A) (No. 25248002) and (B) (No. 15H03761) and Challenging Exploratory Research (Nos. 25620006 and 15K13615). The computational resources and services used in this work were provided by the VSC (Flemish Supercomputer Center), funded by the Research Foundation, Flanders (FWO) and the Flemish Government, department EWI.

Interference effects on electron momentum density distributions have been studied using electron momentum spectroscopy (EMS) for the three outermost orbitals of CCl4, which are constructed from the Cl 3p nonbonding atomic orbitals. The EMS experiment was conducted in the symmetric noncoplanar geometry at an incident electron energy of 2.0 keV. Interference pattern has then been obtained by dividing the experimental data by distorted-wave-Born-approximation cross section for an isolated Cl 3p atomic orbital. A comparison with the result of our earlier study on CF4 [N. Watanabe, X.-J. Chen, M. Takahashi, Phys. Rev. Lett. 108, 173201 (2012)] has demonstrated that the period of the interference pattern reflects the internuclear distance between the constituent halogen atoms. Furthermore, the present result strongly suggests that distorted-wave effects lead to partial destruction of the interference for CCl4 at large momentum.

We report an electron momentum spectroscopy study on valence shake-up satellites of neon. A symmetric noncoplanar $(e,2e)$ experiment was performed at an incident electron energy of 1.2 keV, and the recoil-ion-momentum--dependent $(e,2e)$ cross sections or momentum profiles of the shake-up satellites have been obtained. Furthermore, theoretical momentum profiles have been calculated on the basis of the distorted-wave Born approximation using configuration interaction wave functions of the initial neutral and final ionic states. Comparison between experiment and theory has revealed that contrary to a general expectation, electron correlation in the initial neutral state has a significant influence on the shapes of momentum profiles for neon satellites. It has been shown that the Dyson orbitals of transitions to the $2{p}^{4}(^{1}\mathrm{S})3s\phantom{\rule{0.16em}{0ex}}^{2}\mathrm{S}$ and $2{p}^{4}(^{1}\mathrm{D})3p\phantom{\rule{0.16em}{0ex}}^{2}\mathrm{P}$ ionic states are more localized than those of the associated primary ionizations due to electron-correlation effects.

A time-resolved electron momentum spectroscopy which employs ultrashort laser and electron pulses in a pump-probe scheme is developed to look at transient molecular orbitals and the recent progress of it is reported here.

A twist on an established imaging method has detected a molecule's short-lived excited state, raising the possibility of movies of chemical reactions.

The influence of thermally induced nuclear dynamics (molecular vibrations) in the initial electronic ground state on the valence orbital momentum profiles of furan has been theoretically investigated using two different approaches. The first of these approaches employs the principles of Born-Oppenheimer molecular dynamics, whereas the so-called harmonic analytical quantum mechanical approach resorts to an analytical decomposition of contributions arising from quantized harmonic vibrational eigenstates. In spite of their intrinsic differences, the two approaches enable consistent insights into the electron momentum distributions inferred from new measurements employing electron momentum spectroscopy and an electron impact energy of 1.2 keV. Both approaches point out in particular an appreciable influence of a few specific molecular vibrations of A1 symmetry on the 9a1 momentum profile, which can be unravelled from considerations on the symmetry characteristics of orbitals and their energy spacing.

We report an electron momentum spectroscopy study of vibrational effects on the electron momentum distributions for the outer valence orbitals of difluoromethane (CH2F2). The symmetric noncoplanar (e,2e) experiment has been performed at an incident electron energy of 1.2 keV. Furthermore, a theoretical calculation of the electron momentum distributions of the CH2F2 molecule has been carried out with vibrational effects being involved. It is shown from comparisons between experiment and theory that it is essential to take into account influences of the CH2 asymmetric stretching and CH2 rocking vibrational modes for a proper understanding of the electron momentum distribution of the 2b1 orbital having the CH-bonding character. The results of CH2F2and additional theoretical calculations for (CH3)2O and H2CO molecules strongly suggest that vibrational effects on electron momentum distributions tend to be appreciable for non-total symmetry molecular orbitals delocalized over some equivalent CH-bond sites.

The role of molecular vibrations has been theoretically investigated in the electronic ground state on the (e,2e) valence orbital momentum profiles of furan by means of two complementary approaches. The first one relies upon the principles of Born-Oppenheimer Molecular Dynamics (BOMD), whereas the second one, referred as Harmonic Analytical Quantum Mechanical (HAQM) approach, includes on quantum-mechanical grounds the effect of vibrations in the structure factors.

A new apparatus for coincidence velocity mapping has been developed for studies on the inner-shell photoionization dynamics of molecules. It enables velocity imaging of electrons and ions simultaneously, and maintains time focusing. The characteristic feature of the apparatus is its applicability to a wide range of photoelectron kinetic energy from a few to 100 eV. To demonstrate the performance of the new apparatus, photoelectron angular distributions from Ne atoms and randomly oriented NO molecules, photoion angular distributions from CO molecules, and photoelectron angular distributions from fixed-in-space CO and NO molecules, have been examined. In the data processing procedures, a new peeling method has been developed to construct a three-dimensional photoelectron angular distribution, which is not cylindrically symmetric, from a two-dimensional one.

Electron momentum profiles of the highest occupied molecular orbital (HOMO) of acetone have been measured under experimental conditions where a use of the plane-wave impulse approximation is justified. A double peak is observed in the experimental profile and it was found that calculated profiles by the use of the standard basis set always underestimate the magnitude of the low-momentum peak. Possible origins of the discrepancies between the calculated and measured profiles are examined, leading to an improved agreement by modifying the exponents of diffuse functions employed.

Photoelectron angular distributions (PADs) for O 1s, C 1s and S 2p1/2, 2p3/2 ionization of OCS molecules have been measured in shape resonance regions. These PAD results are compared with the results for O 1s and C 1s ionization of CO molecules, and multi-scattering Xα (MSXα) calculations. The mechanism of the PAD formation both for parallel and perpendicular transitions differs very significantly in these molecules and a step from a two-centre potential (CO) to a three-centre potential (OCS) plays a principal role in electron scattering and the formation of the resulting PAD. For parallel transitions, it is found that for the S 2p and O 1s ionization the photoelectrons are emitted preferentially in a hemisphere directed to the ionized S and O atom, respectively. In OCS O 1s ionization, the S–C fragment plays the role of a strong 'scatterer' for photoelectrons, and in the shape resonance region most intensities of the PADs are concentrated on the region directed to the O atom. The MSXα calculations for perpendicular transitions reproduce the experimental data, but not so well as in the case of parallel transitions. The results of PAD, calculated with different lmax on different atomic centres, reveal the important role of the d (l = 2) partial wave for the S atom in the partial wave decompositions of photoelectron wavefunctions.

Electron momentum spectroscopy (EMS) study of the neon valence satellites is reported. The experiments were performed at impact energies of 1250, 1450 and 1670 eV using a multichannel spectrometer that features high sensitivity. Binding energy spectra up to 100 eV and momentum profiles for the 2p −1 and 2s −1 primary transitions as well as the satellites are presented. The results are used to examine impact energy dependence of the relative intensities and shapes of the satellite momentum profiles. The results are also used to determine symmetries and spectroscopic factors of the satellites, and are compared with the previous experiments by EMS and photoelectron spectroscopy and sophisticated theoretical calculations. The present study has largely resolved controversies in the previous studies.

The photoelectron angular distributions (PADs) for the OCS O 1s, C 1s and S 2p1/2, 2p3/2 ionization have been measured in the shape resonance region. The experimental results have been compared with multiple scattering Xα calculations. It is found that the position of an ionized atom plays a significant role. For a central position (C 1s ionization) the PAD is relatively symmetric. In the case of S 2p and O 1s ionization, electrons are emitted highly preferentially in a solid cone directed from the molecular centre to the respective S or O atom. For the OCS O 1s ionization the S–C fragment plays a very effective role as a 'scatterer' and in the shape resonance region nearly all intensity in PAD is concentrated in the lobe between θ = 50° and 80°, but not along the molecular axis.

An apparatus for electron–electron-fragment ion triple coincidence experiments has been developed to examine binary (e,2e) scattering reaction in the molecular frame. In the axial recoil limit of fragmentation of the residual ion, measurements of vector correlations among the three charged particles are equivalent to (e,2e) experiments with fixed-in-space molecules. Details and performance of the apparatus are reported, together with preliminary result of collision dynamics study on ionisation–excitation processes of fixed-in-space H2 molecules. We believe that this is the first observation of molecular frame (e,2e) cross sections.

We have measured vibrationally-resolved molecular-frame photoelectron angular distributions (MF-PAD) from the C 1s level of CO molecules in the σ shape resonance region. The MF-PAD’s for vibrational levels ( v f =0, 1, and 2) in the C 1s→ e lσ channel are apparently different one from another at each incident photon energy. These experimental MF-PAD’s agree with the present theoretical results from averaging the nuclear distance dependent dipole matrix elements with the relaxed core Hartree–Fock calculations. The results show that the MF-PAD’s sensitively depend on the vibrational levels. This means that the internuclear distance dependences of phases and of magnitudes of dipole matrix elements play a crucial role in the C 1s photoionization of CO.

Electron momentum spectroscopy (EMS) makes it possible to examine orbital patterns of individual molecular orbitals in momentum space. A new spectrometer for electron–electron coincidence experiments for EMS has been developed to obtain orbital patterns quantitatively. Using a spherical analyzer and position-sensitive two-dimensional detectors combined with fast electronics, simultaneous measurements of energy and angular correlations between the two outgoing electrons can be made. This spectrometer features high sensitivity and an ease of changing impact energies. Details of the apparatus are described and impact energy dependence of electron momentum distributions of the HOMO of H2 and biacetyl are compared.

Photoelectron–photoion–photoion coincidence momentum imaging was applied to study 2p photoemission from Ar dimers. We present measurements of the kinetic energy released in fragmentation of Ar++2, angular distributions of energetic fragments, angular distributions of photoelectrons in the laboratory frame and in the molecular frame. The mean kinetic energy of fragment Ar+ ions, 2.2 eV, is larger than the value estimated from the Coulomb explosion model with the equilibrium Ar–Ar distance. No significant differences between the photoelectron angular distributions of monomers and dimers can be found in the laboratory frame. The photoelectron angular distributions of dimers in the molecular frame show a minimum for electron emission along the dimer axis at low energies (1.2 and 3.4 eV) and become isotropic at higher kinetic energies.

Angular distributions of multiplet-specific N 1s photoelectrons from the fixed-in-space NO molecules have been measured for the first time. The dynamics of the σ* shape resonance appearing in the channel leading to the 3Π and 1Π ionic states has been made clear from the analyses of the angular distributions. Multiplet-specific multichannel calculations have reproduced the observed angular distributions fairly well.

The N 1s partial photoionization cross sections of nitric oxide leading to the 3Π and 1Π ionic states have been measured in the shape resonance region for the first time. The twin local maxima in the cross sections have been tentatively assigned, on the basis of simple models for the photoabsorption intensities and for the ratios of branching of the 3Π and 1Π ionic states from the σ* shape resonance state.

An electron momentum spectroscopy (EMS) study of the outer valence orbitals of N2O is reported. The experiments were performed at impact energies of 1000, 1200, 1600 and 1800 eV by using a recently developed multichannel (e, 2e) spectrometer. The experimental momentum profiles are compared with each other to examine their impact energy dependence. The results are used for comparisons with Hartree–Fock (HF) and density functional theory (DFT) calculations using various basis sets. The HF and DFT calculations with large basis sets are in very good agreement with the measured electron momentum profiles. Experimental spectroscopic factors for several transitions are reported.

Angular distributions of C 1s photoelectrons from fixed-in-space CO molecules have been measured with vibrational resolution. A strong dependence of the angular distributions on the vibrational states of the residual molecular ion has been found for the first time in the region of the shape resonance. Calculations in the relaxed core Hartree-Fock approximation have reproduced the angular distributions fairly well in the general shapes of the angular distributions due to the correct description of nuclear motion as an average of the internuclear-distance-dependent dipole amplitudes.

We report an electron momentum spectroscopy study of the two outermost orbitals of the dicarbonyls, glyoxal and biacetyl. The experiments were performed at impact energies of 800, 1200 and 1600 eV using a recently developed multichannel (e, 2e) spectrometer. The experimental momentum profiles clearly show remarkable variations in the low-momentum region with increasing impact energy. Furthermore, it has been found that the two molecules reach their high-energy limits at different impact energies, indicating that the range of validity of the plane-wave impulse approximation (PWIA) largely depends on the target in question. The results at 1600 eV are employed for comparisons with PWIA calculations using Hartree–Fock and density functional theory (DFT). While the DFT calculations reproduce well the observations for glyoxal, considerable discrepancies between experiment and theory exist for biacetyl.

Measurements of photoelectron angular distributions for carbon K-shell ionization of fixed-in-space CO2 molecules with the molecular axis oriented along, perpendicular and at 45 degrees to the electric vector of the light are reported. The major features of these measured spectra are fairly well reproduced by calculations employing a relaxed-core Hartree-Fock approach. In contrast to the angular distribution for K-shell ionization of N-2, which exhibits a rich structure dominated by the f-wave (l = 3) at the shape resonance, the angular distribution for carbon K-shell photoionization of CO2 is quite unstructured over the entire observed range across the shape resonance.

A new apparatus for electron–electron coincidence experiments has been developed to examine molecular orbital patterns quantitatively by electron momentum spectroscopy. Using a spherical analyzer and position-sensitive detectors, it enables one to measure energy and angular correlations between the two outgoing electrons simultaneously. The design and performance of the apparatus is reported together with results on Ar to show extensive improvements in coincidence count rates and statistical precision, covering a wide range of binding energies and momenta.

We report molecular-frame electron scattering cross sections of N2, obtained using an electron-ion coincidence technique. The vector correlation between the scattered electron and the fragment ion has been measured at incident electron energy of 1.4 keV for inner-valence ionization of N2. The angular distributions of the electron scattering cross sections thus obtained exhibit clear momentum transfer dependence, indicating the significant change of the ionization dynamics due to contributions from electric multipole transitions.

We report on experimental and theoretical progress in time-resolved (e, 2e) electron momentum spectroscopy of photodissociation dynamics of the deuterated acetone molecule at 195 nm. Experimentally, we are conducting a series of measurements to have improved experimental results, compared to the earlier ones, by using a pump laser with higher intensity. Theoretically, we have examined the predicted minimum energy reaction path to investigate whether associated (e, 2e) calculations meet the experimental results. A noticeable difference between the experiment and the calculations has been found at around binding energy of 10 eV, suggesting that the observed difference may originate, at least partly, in ever-unconsidered non-minimum energy paths.

We have developed multi-channel apparatus for electron-atom Compton scattering to study the momentum distribution of atoms in a molecule. It combines the features of both a spherical electron energy analyzer and a large-area position sensitive detector, thereby having an ability to cover almost completely the azimuthal angle range available for quasi-elastic electron Rutherford backscattering at an angle of 135°. Details and performance of the apparatus are reported, together with experimental results measured for Xe and CH4 at an incident electron energy of 2 keV. In particular, it is shown that the instrumental sensitivity is remarkably high, which has increased the signal count rate by nearly three orders of magnitude compared to existing setups. This technical progress would be useful for advancing atomic momentum spectroscopy studies.

Bond oscillation, a phenomenon characteristic of the molecular electron-density distribution in momentum space, is demonstrated for the $1{\ensuremath{\sigma}}_{g}$ molecular orbital of ${\mathrm{H}}_{2}$ with an ($e$, $2e$) experiment at large momentum transfer. Analysis of the experimental data in terms of two-center interference effects has revealed that different oscillatory structures can be observed, depending on the model for describing ($e$, $2e$) ionization from the constituent H $1s$ atomic orbitals. It is shown that bond oscillation is highly sensitive to the spatial pattern and chemical bonding nature of the molecular orbital.

An angle-resolved electron energy loss apparatus has been constructed for studying electron scattering by gaseous molecules. Generalized oscillator strength distribution of H 2 O has been determined by the use of the Bethe sum rule and is presented as a form of Bethe surface, covering the momentum transfer range from 0.52 to 3.36 a.u. and the energy loss range up to 400 eV. X-ray incoherent scattering factor or static structure factor S ( K ) has been subsequently derived and compared with those by theoretical calculations as well as by inelastic X-ray scattering studies. S ( K )s by the three different methods show remarkable agreement.

A new technique, based on velocity imaging, has been developed to examine correlations in energy and angle between the several particles formed in dissociative single and double photoionisation at VUV wavelengths. Electrons and positive fragment ions are imaged in coincidence at position-sensitive detectors; the resulting multidimensional data sets contain separable energy distributions, angular distributions and correlations in energy and angle between the particles. In some cases internal or external evidence indicates that pure axial recoil occurs, without molecular rotation. In such cases fixed-molecule photoelectron angular distributions can be extracted and parametrised. Results for H2, N2, NO, CO and O2 are presented. Effects of shape resonances in the ionisation channels are apparent in several cases.

Energy-analysed photoelectrons have been measured in coincidence with product ions from photoionisation of carbonyl sulphide at 21.21 and 40.8 eV photon energy. Yields of the different products and kinetic energy release distributions in fragment ion formation are reported. Results on the states below 20 eV agree with previous work, but our interpretation differs; new data are presented on inner valence states. In dissociation from states in the band at 23 eV there is forward/backward asymmetry in fragment ion direction relative to the electrons, indicating a fast dissociation and possibly identifying the band as a 7σ satellite.

Electron momentum spectroscopy is applied to determine the character of the two outermost π orbitals of norbornadiene and 1,4-cyclohexadiene. From comparisons of observed and calculated electron momentum distributions, the highest occupied molecular orbital is experimentally identified as π − in the former and as π + in the latter.

The electronic structure of the pyrrole (C4H5N) molecule has been studied using electron momentum spectroscopy at an impact energy of 800 eV plus binding energy. Binding energy spectra have been measured at selected electron momenta. An attempt to clarify the band assignments, which have been in controversy, is made with the aid of theoretical electron momentum profiles calculated using the 6-31++G** basis set. A set of assignments which reproduces the observations is proposed, and it is concluded that electron correlation effects are significant in the innermost π electron ionization.

We have constructed an experimental apparatus which allows us to perform various types of non-coincidence experiments such as angle-resolved photo-electron, photoion and resonance Auger electron spectroscopy, as well as coincidence experiments for investigating the angular correlation between two energy-selected charged particles (electrons and/or ions). Their design and performance are described in this paper.

The furan (C4H4O) molecule has been studied using electron momentum spectroscopy at an impact energy of 800 eV. Binding energy spectra ranging up to 31 eV have been measured at target electron momenta from about 0.1 to 3.6 au. Electron momentum profiles of individual orbitals are obtained by deconvolution from a series of the spectra measured at selected electron momenta. The experimental momentum profiles are compared with theoretical ones calculated using the 6-31++G** basis set. Band assignments and the electron correlation effects are investigated through the orbital-specific momentum profiles. The results are compared with Green's function calculations by G. Bieri, L. Asbrink, and W. von Niessen (J. Electron Spectrosc. Relat. Phenom. 27 (1982) 129) in terms of the ionization energies and their corresponding pole strengths.