The scattering of positrons from atomic hydrogen is studied by using the momentum space coupled channels optical method at intermediate energy (15-100 eV). Ionization continuum and positronium formation channels are included in the coupled channels calculation via a complex equivalent-local optical potential.

The p-electron dominant contributions to the outer valence shell of 2,6-stelladione (C8H8O2) are analyzed using binding energy spectra and the orbital momentum distributions obtained by experimental and theoretical electron momentum spectroscopy. The binding energy spectra are given for azimuthal angles φ=0° and 10°, respectively, in order to reveal information of the s- and p-electron dominant characteristics in these molecular orbitals. The wavefunctions in configuration space are directly mapped into momentum space using the plane wave impulse approximation. This work focuses on the interpretation of the electronic structural information and bonding mechanism of the molecule in momentum space. In particular, p-electron dominant contributions of the strained organic compound are used to support our findings.

Binary (e,2e) spectroscopy (also known as EMS) is applied to problems of chemical interest including imaging of orbital electron densities, electron transfer processes, investigation of different orbital models, the evaluation of wavefunctions and computational methods for larger molecules, frontier orbital electron densities in pharmaceuticals, and distorted wave effects at low momenta for π* type MOs.

High-resolution electron momentum spectroscopy (EMS) has been used to determine the character of the two outermost π-orbitals of norbornadiene. Definitive evidence, from comparisons of measured and calculated momentum distributions, for the dominance of the through-space interaction is presented. This through-space bond dominance is consistent with previous hypotheses based on molecular orbital considerations [Acc. Chem. Res. 4 (1971) 1; J. Am. Chem. Soc. 92 (1970) 706; J. Am. Chem. Soc. 112 (1990) 1710].

Momentum Distributions (MDs), obtained using high-resolution electron momentum spectroscopy (HREMS), are reported for norbornadiene's 18 valence orbitals. Corresponding theoretical results, using generalized gradient approximation density functional theory (DFT) together with TZVP, DZVP, and DZVP2 basis functions and a plane wave impulse approximation to describe the ionization process, are also detailed. This work represents the first comprehensive HREMS/DFT investigation into the complete valence electronic structure of norbornadiene (NBD), with significant results being obtained. In particular, an exacting comparison between our experimental and theoretical MDs enables us to define the “optimum” basis for NBD, from those we studied. This “optimum” basis is then used to extract a wide range of NBD's important molecular property information, which are subsequently compared with the results of independent measurements and calculations. Agreement between our results and those from independent measurements was generally very good, highlighting the utility of HREMS in a priori basis set evaluation.

It is shown that electrons in molecules behave in an essentially orbital-like fashion and that imaging of the spherically averaged orbital electron density can be achieved in momentum space by electron momentum spectroscopy (EMS). EMS measurements, using the binary (e,2e) reaction under binary encounter collision conditions, are demonstrated to effectively probe valence electron (frontier orbital) electron transfer out of a molecule by providing imaging of the spherically averaged Dyson orbital electron momentum density distribution corresponding to the ionization process. Experimental EMS cross-sections for the outermost valence (frontier) electrons of HF, H2O, NH3, CH4, H2S and a range of other molecules are found to be in excellent agreement with MRSD-CI calculations of the respective Dyson orbital densities using the plane wave impulse approximation. High level Hartree–Fock and density functional theory (DFT) (B3LYP and B3PW91) calculations, with large, saturated and diffuse basis sets, demonstrate that the Dyson orbital densities and thus the EMS experiments are also extremely well described by the respective initial state (i.e. neutral molecule) canonical molecular orbital (CMO) Hartree–Fock independent particle electron density distributions, with as good or sometimes an even better description being given by the respective (correlated) Kohn–Sham orbital (KSO) densities of DFT. In this sense EMS is shown to provide experimental imaging of orbital electron densities, with the emerging electrons having a delocalised orbital character immediately prior to `knock-out'. The present experimental and theoretical findings also lend support to the earlier predictions of Fukui as to the possibility of observing the `orbital pattern' experimentally [Int. J. Quant. Chem. 12 (Suppl. 1) (1977) 277] as well as to the recent views of Stowasser and Hoffmann [J. Am. Chem. Soc. 121 (1999) 3414] concerning the `reality' of KSOs. Further supporting evidence for the present findings is provided by a consideration of the results of frontier orbital theory applied to chemical reactions involving electron transfer, such as electrophilic attack [J. Chem. Phys. 20 (1952) 722]. The present results and interpretation are also strongly supported by scanning tunneling microscopy (STM) theory [Phys. Rev. B 31 (1985) 805] and in particular by recent STM experiments on adsorbed C60 molecules [Chem. Phys. Lett. 321 (2000) 78] which show images which correspond very closely with DFT calculations of the electron density distribution in the HOMO orbital of the neutral molecule. The EMS measurements and associated theoretical calculations, together with the evidence from frontier orbital theory and the STM experiments, strongly suggest that delocalised CMO, or often, even better, the KSO, densities provide an operational definition of orbital electron densities, and thus of orbitals, appropriate for use in discussions of chemical bonding as well as for predicting the outcome of chemical reactions and physical processes involving electron transfer.

A study of the electronic structure of the complete valence shell of allene (1,2‐propadiene) is reported. New high‐resolution binding‐energy spectra were measured in the energy regime 6–34.5 eV over a range of different target electron momenta, so that momentum distributions (MDs) could be determined for each molecular orbital. These data supersede the low‐resolution work of Braidwood et al. [J Phys B 27 (1994) 2075–2087], and also clarify some ambiguities with the earlier results. Theoretical MDs were calculated using a plane wave impulse approximation (PWIA) model for the reaction mechanism and density functional theory (DFT) for the wave function. Three basis sets, at the local spin density (LSD) approximation level and, additionally, incorporating nonlocal corrections such as the generalized gradient approximation (GGA), were studied. A critical comparison between the experimental and theoretical MDs was made, and it allowed us to determine the “optimum” wave function for allene from the basis sets we considered. This wave function is then used to derive allene's chemically interesting molecular properties. A summary of some of these results and a comparison of them with those of other workers is also presented with the level of agreement typically being good. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1321–1333, 2001

Numerical representations of Dyson orbitals can be extracted from the experimental data of electron momentum spectroscopy (EMS). These orbitals are solutions of the quasiparticle equation for the target electronic system. They closely obey the weak-coupling approximation. This defines a normalized Dyson orbital in terms of the sum of EMS cross sections for an orbital manifold of ion states whose momentum profiles have the same shape. Normalized Dyson orbitals are closely related to the orbitals of density functional theory. They are realistic in the sense that they give an independent-particle approximation to the target structure that reproduces experimental data, including molecular data obtained from experiments independent of EMS.

A study of the electronic structure of the complete valence shell of cubane is reported. Results from our many-body Green's function calculation, to the third-order algebraic diagrammatic construction (ADC(3)) level, for the binding energies and spectroscopic factors of the respective valence orbitals of cubane are presented. Binding-energy spectra were measured in the energy regime 6-35 eV over a range of different target electron momenta, so that momentum distributions (MDs) could be determined for each orbital. The corresponding theoretical MDs were calculated using a plane wave impulse approximation (PWIA) model for the reaction mechanism and density functional theory (DFT) for the wave function. Seven basis sets, at the local density approximation (LDA) level and, additionally, incorporating nonlocal correlation functional corrections, were studied. The sensitivity of the level of agreement between the experimental and theoretical MDs to the nonlocal corrections is considered. A critical comparison between the experimental and theoretical MDs allows us to determine the "optimum" wave function for cubane from the basis sets we studied. This wave function is then used to derive cubane's chemically interesting molecular properties. A summary of these results and a comparison of them with those of other workers is presented with the level of agreement typically being good.

Electron momentum spectroscopy can investigate qualitative predictions based on a physical understanding of the one-electron structure of a system. We describe examples of this understanding for molecules and solids.

Electron momentum spectroscopy has involved the verification of an ab initio model for the target-ion overlap, or Dyson orbital, in a calculation of a high-energy (e, 2e) reaction by comparing the resulting cross section with experimental data. We describe an inverse method of extracting the overlap directly from experimental data by using a quantum-mechanically constrained statistical fitting procedure in conjunction with previously verified reaction models.

Electron scattering on the ground state of copper is considered using the convergent close-coupling (CCC) and coupled-channel optical (CCO) methods. In both cases the copper atom is treated by the inert d10 core model, with only the elastic ...3d104s 2S and ...3d104p2P channels considered. Comparison with experiment and other theories show best agreement of the experimental data with the CCC calculations, though some discrepancies are evident. We speculate on the source of the remaining discrepancies.

We employ a numerical inverse method of extracting the target-ion overlap, or normalised Dyson orbital, directly from experimental electron momentum spectroscopy data by using a quantum- mechanically constrained statistical fitting procedure. This method is used in conjunction with the previously verified, for molecular targets, plane wave impulse approximation (PWIA) reaction model. The present procedure was applied to previously measured momentum distributions (MDs) for the 2e′ and 1e′ valence orbitals of cyclopropane, the 7ag orbital of trans 1,3-butadiene, the 2e orbital of 1,2-propadiene and the 3a′1 orbital of [1.1.1]propellane. We note that this is the first extensive application of the present method to organic molecular systems. In each case the derived normalised Dyson orbital provided a superior representation of the experimental MD than did the corresponding Hartree-Fock orbital. The ramifications of this result are discussed in the text.

Differential cross sections, integrated cross sections and ionization cross sections for electron scattering on the metastable level of helium are calculated at intermediate energies and compared with experimental measurements and other theoretical calculations. The method used is the coupled-channels optical method with an ab initio complex polarization potential.

We use the density-matrix formalism to parametrize a spin-polarized $(e,2e)$ experiment where a $p$ electron is ejected from a closed-shell system and the fine structure of the ion is resolved experimentally. This formulation allows a definition from general principles of the recently observed spin up-down asymmetry [X. Guo and co-workers, Phys. Rev. Lett. 76, 1228 (1996); J. Phys. B 30, 4097 (1997)] due to the fine-structure effect [S. Jones et al., Phys. Rev. Lett. 72, 2554 (1994); G. F. Hanne, in Abstracts of Contributed Papers, XVIIth International Conference on the Physics of Electronic and Atomic Collisions, edited by W. R. MacGillivray, I. E. McCarthy, and M. C. Standage (Hilger, Bristol, 1992), p. 199] and the complete definition of potentially observable quantities such as the polarization of the fast- and slow-emitted-electron beams and the orientation of the ion beam after the collision. These parameters are defined for the general case where the incoming-electron beam has an arbitrary polarization $\mathbf{P}{=(P}_{x}{,P}_{y}{,P}_{z})$ and calculated in an asymmetric kinematic for the case where the initial-electron beam is, respectively, unpolarized and polarized perpendicular to the scattering plane.

We demonstrate that electron momentum spectroscopy ~EMS!, based on the high-energy (e ,2e) reaction, can provide clean information on the electronic structure of the target that is not obscured by interchannel coupling in the ionization continuum. This is in sharp contrast to high-energy photoionization in which target states with different angular momentum, which are close in energy, are always mixed through interchannel coupling in the continuum. In atoms, in particular, the photoionization spectra of nl(l.0) subshells always have significant intensity due to admixture from the neighboring ns subshell. This mixing is shown to be negligible in electron momentum spectroscopy, a point we specifically illustrate for the (e ,2e) spectra of the argon M shell. @S1050-2947~98!08409-1#