High-resolution Electron Momentum Spectroscopy Research Articles

Outer-valence ionization of nitrous oxide: A high-resolution electron momentum spectroscopy investigation

We report a detailed study on the outer-valence ionization of nitrous oxide (N2O) by a high-resolution (e, 2e) spectrometer employing asymmetric non-coplanar kinematics at an impact energy of 2500 eV. Binding energy spectrum in the energy region of 11–30 eV is obtained. Benefiting from the high energy resolution of 0.55 eV, four main peaks and six satellite structures are clearly identified in this region. The measured electron momentum profiles are compared with the theoretical calculations. Renner-Teller-effect induced geometry distortion is used to qualitatively interpret the observed discrepancy at low momentum between experiment and theory for B2Π state. The assignments and the pole strengths for these ionization states are determined by comparing the shape and the intensity of the experimental electron momentum profiles with those of the theoretical ones. In particular, two low-lying satellites at 21.0 eV and 27.1 eV are confirmed for the first time and attributed to the 2Π and 2Σ shake-up states. The present results show the evidence of strong electron correlation effects in the final states for the outer-valence ionization of N2O.

The electronic structure of bicyclo[2.2.2]octa-2,5-dione

Understanding the structure of strained hydrocarbons is important for understanding bonding and controlling reactivity and selectivity. Here we report an experimental and theoretical investigation into the valence electronic structure of bicyclo[2.2.2]octa-2,5-dione (BCOD). A high-resolution electron momentum spectroscopy (EMS) technique was employed to investigate the momentum dependence of specific ionization channels. These experiments were supplemented by an analysis of a measured photoelectron spectrum. Computational chemistry was used to explain the complex electronic structure of BCOD. We analyzed the influence of strain in bridged cyclic diketones by comparison of BCOD with corresponding results from structurally similar molecules to elucidate π/σ through-bond interactions.

High-resolution electron-momentum spectroscopy of the valence orbitals of the benzene molecule

The binding-energy spectrum and electron-momentum spectra of valence orbitals of benzene were measured via a high-resolution electron-momentum spectrometer at incident energies of 600 and 1200 eV plus the binding energy. The experimental momentum profiles were compared with the results from a plane-wave impulse approximation (PWIA) calculation using Dyson orbitals obtained from symmetry-adapted-cluster configuration-interaction calculations. For the ${1e}_{1\mathrm{g}}, {3e}_{2\mathrm{g}}, {3e}_{1\mathrm{u}}$, and ${2b}_{1\mathrm{u}}$ orbitals, the discrepancy between the experimental distributions and the PWIA calculations was tentatively assigned to the distorted wave effects. The current high-resolution electron-momentum spectroscopy results provide the experimental benchmark for rigorous distorted-wave calculations about benzene in the future.

Electron-impact ionization-excitation of the neon valence shell studied by high-resolution electron-momentum spectroscopy

We present experimental and theoretical results for electron-impact-induced ionization and excitation of the neon valence shell by using a highly sensitive and high-resolution ($e, 2e$) spectrometer. The controversies in previous electron momentum spectroscopic results have been resolved. The satellites at 55.83 and 71.50 eV are mainly attributed to a ${\phantom{\rule{4pt}{0ex}}}^{2}P$ manifold, and mixed up with some ${}^{2}S$, not a pure ${}^{2}S$ as were assigned by the earlier electron-momentum spectroscopy. Our experimental momentum distribution for the satellite at 55.83 eV is consistent with that reported by Watanabe et al. [J. Electron Spectrosc. Relat. Phenom 142, 325 (2005)], but not consistent with Samardzic et al.'s results [Phys. Rev. A 48, 4390 (1993)]. The difference between the observed momentum distribution of the satellite at 55.83 eV and the calculations with the plane-wave impulse approximation suggests the need for further improvement of the theoretical methods.

Observing interference effect in binary (e, 2e) of molecules

We present investigations on the two-center and multi-center interference effects of molecules in binary (e, 2e) experiments. The high energy resolution electron momentum spectroscopy (EMS) measurements on H2 are reported with final vibrational states resolved. The experimental momentum profiles for ionization transitions to the individual final vibrational states of the ion are obtained. The measured and calculated vibrational ratios of the cross sections deviate from Franck-Condon principle, which can be ascribed to the Young-type two-center interference. Furthermore, with the help of our latest version of EMS spectrometer which has considerably higher sensitivity and much wider momentum range from 0 to 8 a.u., we are able to extend our observations to multi-center interference effect in high symmetry molecules like NF3 and CF4 with several oscillation periods included.

Electron momentum spectroscopy of NF3

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 Jahn-Teller effect in the electron momentum spectroscopy of ammonia

The 1e and 3a(1) bands of the ammonia molecule have been studied using the high-resolution electron momentum spectroscopy at impact energies of 1200 and 600 eV. Several slices of 1e and 3a(1) bands in the different binding energy ranges were selected, and their electron-momentum distributions were carefully compared. The discernable difference among the distributions of the selected slices of the 1e band shows that the Jahn-Teller effect indeed influences the electron momentum distribution of the 1e orbital of ammonia.

High-resolution electron-momentum spectroscopy of the valence orbitals of the iodine molecule

The valence orbitals of the iodine molecule (I${}_{2}$) 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 $8{J}_{g}^{3/2}$ to $17{J}_{g}^{1/2}$, $8{J}_{u}^{3/2}$ to $16{J}_{u}^{1/2}$, and the electron-momentum profiles of orbital $16{J}_{g}^{1/2}$ 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.

Calculation of Dyson orbitals using a symmetry-adapted-cluster configuration-interaction method for electron momentum spectroscopy:N2andH2O

The symmetry-adapted-cluster (SAC) configuration-interaction (CI) theory was introduced to interpret the non-coplanar symmetric ($e$,$2e$) results. Dyson orbitals derived from the bench-marked SAC CI general-R method were utilized for computing the electron momentum distributions. The corresponding excitation energies and spectroscopic factors can be used to reproduce the ionization spectra. The implementation was demonstrated by examples of ${\mathrm{N}}_{2}$ and ${\mathrm{H}}_{2}$O. The electron momentum distributions calculated using SAC CI method were compared with recent experimental results, as well as the Hartree-Fock and density-functional-theory calculations. The SAC CI method gave the best performance on the description of the experimental momentum distributions. It was found that the electron momentum distributions of Dyson orbitals related to the satellite lines can be notably different from those of their parent orbitals due to the electron correlation in the initial target states. Present work demonstrated that the SAC CI theory is a very useful and accurate tool for interpreting high-resolution electron momentum spectroscopy results.

Evidences from electron momentum spectroscopy for ultra-fast charge transfers and structural reorganizations in a floppy molecule: Ethanol

Calculations of electron momentum distributions employing advanced Dyson orbital theories and statistical thermodynamics beyond the RRHO approximation fail to quantitatively reproduce the outermost momentum profile inferred from experiments on ethanol employing high resolution Electron Momentum Spectroscopy [1]. Study of the influence of nuclear dynamics in the initial ground state and final ionized state indicates that this discrepancy between theory and experiment reflects a charge transfer occurring during an ultra-fast dissociation of the ethanol radical cation into a methyl radical and H2C=O-H+.

Probing Nuclear Dynamics in Momentum Space: A New Interpretation of (e, 2e) Electron Impact Ionization Experiments on Ethanol

Calculations of electron momentum distributions for equilibrium geometries, employing advanced Dyson orbital theories and statistical thermodynamics beyond the RRHO approximation, fail to quantitatively reproduce the outermost momentum distribution profile inferred from (e, 2e) electron impact ionization experiments on ethanol employing high-resolution electron momentum spectroscopy. A very detailed study of the influence on this momentum distribution of nuclear dynamics in the initial ground state and in the final ionized state is presented according to a thermal averaging over exceedingly large sets of model structures as well as Born-Oppenheimer molecular dynamical simulations on the potential energy surface of the radical cation. Our results give very convincing albeit qualitative indications that the strong turn-up of the (e, 2e) ionization intensities characterizing the highest occupied molecular orbital (HOMO) of ethanol at low electron momenta is the combined result of (1) the extraordinarily flat nature of the conformational energy map of ethanol, which enables significant departures from energy minima in the ground electronic state, (2) strong anomeric interactions between an oxygen lone pair and the central C-C bond for the minor but significant fraction of conformers exhibiting a hydroxyl torsion angle (alpha) at around 90 degrees, and, last but not least, (3) the possibility to observe with this minor conformer fraction ultrafast and highly significant extensions of the central C-C bond, resulting, in turn, in an enhanced delocalization of the HOMO from the oxygen lone pair region onto the methyl group, immediately after the sudden removal of an electron. This charge transfer appears to occur at the very first stages, that is, within an effective time scale on the order of approximately 10 fs, of an ultrafast dissociation of the ethanol radical cation into a methyl radical and a protonated form of formaldehyde.

Experimental and Theoretical Electron Momentum Spectroscopic Study of the Valence Electronic Structure of Tetrahydrofuran under Pseudorotation

The most populated structure of tetrahydrofuran (THF) has been investigated in our previous study using electron momentum spectroscopy (EMS). Because of the relatively low impact energy (600 eV) and low energy resolution (DeltaE = 1.20 eV) in the previous experiment, only the highest occupied molecular orbital (HOMO) of THF was investigated. The present study reports the most recent high-resolution EMS of THF in the valence space for the first time. The binding energy spectra of THF are measured at 1200 and 2400 eV plus the binding energies, respectively, for a series of azimuthal angles. The experimentally obtained binding energy spectra and orbital momentum distributions (MDs) are employed to study the orbital responses of the pseudorotation motion of THF. The outer valence Greens function (OVGF), the OVGF/6-311++G** model, and density function theory (DFT)-based SAOP/et-pVQZ model are employed to simulate the binding energy spectra. The orbital momentum distributions (MDs) are produced using the DFT-based B3LYP/aug-cc-pVTZ model, incorporating thermodynamic population analysis. Good agreement between theory and experiment is achieved. Orbital MDs of valence orbitals exhibit only slight differences with respect to the impact energies at 1200 and 2400 eV, indicating validation of the plane wave impulse approximation (PWIA). The present study has further discovered that the orbital MDs of the HOMO in the low-momentum region (p < 0.70 a.u) change significantly with the pseudorotation angle, phi, giving a v-shaped cross section, whereas the innermost valence orbital of THF does not vary with pseudorotation, revealing a very different bonding mechanism from the HOMO. The present study explores an innovative approach to study pseudorotation of sugar puckering, which sheds a light to study other biological systems with low energy barriers among ring-puckering conformations.

Study of the Valence Wave Function of Thiophene with High Resolution Electron Momentum Spectroscopy and Advanced Dyson Orbital Theories

Results of an exhaustive experimental study of the valence electronic structure of thiophene using high resolution electron momentum spectroscopy at impact energies of 1200 and 2400 eV are presented. The measurements were performed using an electron momentum spectrometer of the third generation at Tsinghua University, which enables energy, polar and azimuthal angular resolutions of the order of DeltaE = 0.8 eV, Deltatheta = +/-0.53 degrees and Deltaphi = +/-0.84 degrees . These measurements were interpreted by comparison with Green's function calculations of one-electron and shake-up ionization energies as well as of the related Dyson orbital electron momentum distributions, using the so-called third-order algebraic diagrammatic construction scheme (ADC(3)). Comparison of spherically averaged theoretical electron momentum distributions with experimental results very convincingly confirms the presence of two rather intense pi-2 pi*+1 shake-up lines at electron binding energies of 13.8 and 15.5 eV, with pole strengths equal to 0.18 and 0.13, respectively. Analysis of the electron momentum distributions associated with the two lowest 2A2 (pi3-1) and 2B1 (pi2-1) cationic states provides indirect evidence for a symmetry lowering and nuclear dynamical effects due to vibronic coupling interactions between these two states. ADC(3) Dyson orbital momentum distributions are systematically compared with distributions derived from Kohn-Sham (B3LYP) orbitals, and found to provide most generally superior insights into experiment.

Probing molecular conformations in momentum space: The case of n-pentane

A comprehensive study, throughout the valence region, of the electronic structure and electron momentum density distributions of the four conformational isomers of n-pentane is presented. Theoretical (e,2e) valence ionization spectra at high electron impact energies (1200 eV+electron binding energy) and at azimuthal angles ranging from 0 degrees to 10 degrees in a noncoplanar symmetric kinematical setup are generated according to the results of large scale one-particle Green's function calculations of Dyson orbitals and related electron binding energies, using the third-order algebraic-diagrammatic construction [ADC(3)] scheme. The results of a focal point analysis (FPA) of relative conformer energies [A. Salam and M. S. Deleuze, J. Chem. Phys. 116, 1296 (2002)] and improved thermodynamical calculations accounting for hindered rotations are also employed in order to quantitatively evaluate the abundance of each conformer in the gas phase at room temperature and reliably predict the outcome of experiments on n-pentane employing high resolution electron momentum spectroscopy. Comparison with available photoelectron measurements confirms the suggestion that, due to entropy effects, the trans-gauche (tg) conformer strongly dominates the conformational mixture characterizing n-pentane at room temperature. Our simulations demonstrate therefore that experimental measurements of (e,2e) valence ionization spectra and electron momentum distributions would very consistently and straightforwardly image the topological changes and energy variations that molecular orbitals undergo due to torsion of the carbon backbone. The strongest fingerprints for the most stable conformer (tt) are found for the electron momentum distributions associated with ionization channels at the top of the inner-valence region, which sensitively image the development of methylenic hyperconjugation in all-staggered n-alkane chains.

High resolution electron momentum spectroscopy of the valence orbitals of water

The development of a third-generation electron momentum spectrometer with significantly improved energy and momentum resolutions at Tsinghua University (Δ E = 0.45–0.68 eV, Δ θ = ±0.53° and Δ ϕ = ±0.84°) has enabled a reinvestigation of the valence orbital electron momentum distributions of H 2O 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 2a 1 orbital.

High resolution electron momentum spectroscopy of dichlorodifluoromethane: Unambiguous assignments of outer valence molecular orbitals

On account of controversial orbital assignment that appeared in previous works, [J. Chem. Phys. 120, 7933 (2004), and references therein] high resolution electron momentum spectroscopy (EMS) measurements on dichlorodifluoromethane has been carried out using a newly developed high resolution energy-momentum dispersive multichannel spectrometer employing asymmetric noncoplanar geometry at an impact energy of 2500 eV plus binding energy. Four resolved structures and two shoulders were obviously observed in high resolution binding energy spectrum in energy range covering eight outermost valence orbitals, whereas only two broad lobes were resolved in previous EMS studies [J. Chem. Phys. 120, 7933 (2004); Chin. Phys. 14, 2467 (2005)]. The ordering of these orbitals was reassigned unambiguously by simple comparison of experimental momentum distributions with theoretical ones.

Probing Dyson orbitals with Green’s Function theory and Electron Momentum Spectroscopy

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.

EMS investigation of inner valance orbital 1a" for butanone

The binding energy and the spherically averaged momentum distributions of orbital 1a″for butanone have been investigated by high resolution electron momentum spectroscopy (ΔE=1.15eV, Δp=0.1a.u.). The impact energy was 1200eV plus binding energy (i.e. 1206—1242eV) and symmetric non-coplanar kinematics was employed. The experimental binding energy spectra have been compared with published photoelectron spectra. The momentum profile of 1a″ orbital has been compared on a quantitative basis with theoretical calculations using Hartree-Fock method and Density Functional Theory. The position density and momentum density for 1a″ orbital are presented.

Spectral properties of quasiparticles in silicon: A test of many-body theory

high-energy high-resolution electron momentum spectroscopy. It is compared with first-principles calculations based on the interacting one-electron Green’s function which is evaluated in the GW and the cumulant expansion approximations. Positions of the quasiparticle peaks ~dispersion!, their widths ~lifetimes!, and the extensive satellite structures are measured over a broad range of energies and momenta. The band dispersions are well described by both calculations, but the satellite predicted by the GW calculation is not observed. Unlike the GW calculation, the cumulant expansion calculation gives a significantly better description of the shape and momentum dependence of the satellite structure, presenting a promising approach for studying high-energy excitations. The vast majority of experimental and theoretical work on the electronic structure of matter is based on the concept of mean field. Electrons are assumed to interact with a potential ~most accurately derived from the density-functional theory! due to the average charge distribution. Theories dealing explicitly with electron-electron correlation are computationally very challenging and few reliable experimental data exist to test the outcome of the calculations. It is therefore very important to provide benchmark tests to validate the manybody calculations and here we show that electron momentum spectroscopy ~EMS! of high-quality crystalline targets can provide such tests. The prototype semiconductor Si has been used as a test

Comprehensive Experimental and Theoretical Study into the Complete Valence Electronic Structure of Norbornadiene

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 ...