Valence electronic structure of CH3Br and CH3I: Electron momentum distributions and separation energies

Bromomethane (CH₃Br) and iodomethane (CH₃I) have been studied by binary (e,2e) coincidence spectroscopy at 1200 eV using non-coplanar symmetric kinematics. Separation energy spectra have been determined in the energy range up to 47 eV at azimuthal angles of 0° and 8° for CH₃Br and 0° and 6° for CH₃I. The separation energy spectra and the electron momentum distributions measured for each of the valence orbitals are compared with theoretical predictions employing SCF wavefunctions and outer valence type and extended 2 ph-TDA Green function calculations. Electron density and momentum density maps have been calculated for all the valence orbitals using the SCF wavefunctions, and they are used to explain trends and contrasts in the electronic structure and bonding properties of these halomethanes in both position and momentum space.     

Experimental and theoretical binding energy spectra and momentum distributions for the valence orbitals of H2O

The binding energy spectra (10–46 eV) and momentum distributions of the valence orbitals of H₂O have been measured using a new high-sensitivity binary (e,2e) electron spectrometer employing position-sensitive detectors. The binding energy spectrum shows a previously unreported feature at = 27 eV which is shown to be associated with the (2a₁)^?1 ionization process. The region between 25 and 46 eV is compared with previous (e,2e) and X-ray photoelectron measurements as well as with several existing and new many-body calculations indicating a splitting of the 2a₁ ionization pole strength. In addition the separate momentum distributions of the three outer valence orbitals of H₂O have been obtained from deconvoluted binding energy spectra run at a series of azimuthal angles. The results, which show considerably improved signal-to-noise ratio over earlier measurements using single-channel instrumentation are compared with spherically averaged momentum distributions calculated with a variety of wavefunctions.     

Experimental and calculated momentum densities for the valence orbitals of carbon tetrafluoride

Carbon tetraflouoride has been investigated by binary (e,2e) spectroscopy at 1200 eV impact energy. Binding energy spectra (10–60 eV) at azimuthal angles of 0° and 8° are reported and are found to be in quantitative agreement with a previous Green's function calculated spectrum. Momentum distributions corresponding to individual orbitals are also reported and compared with theoretical momentum distributions evaluated using double-zeta quality SCF wavefunctions. Excellent agreement between experimental and theories is found for the strongly bonding 3t₂ orbital and the antibonding 4a₁ orbital but agreement is less good for the outermost non-bonding orbitals. Intense structure due to molecular density (bond) oscillation is observed experimentally in the region above 1.0 a_o^?1 in the case of the non-bonding 4t₂ orbital. It is also notable that the measured 4a₁ momentum distribution exhibits an extremely well-defined “p” character with clear separation between the s and p components. Contour maps of the position-space and momentum-space orbital densities in the F-C-F plane of the molecule are used to provide a qualitative interpretation of the features observed in the momentum distribution. In order to further extend momentum-space chemical concepts to three-dimensional systems, constant density surface plots are also used to give a more comprehensive view of the density functions of the CF₈ molecule.     

Outer valence orbital momentum distributions of carbon dioxide by binary (e,2e) spectroscopy

The outer valence orbital momentum distributions of CO₂ have been reinvestigated using a high momentum resolution (0.1 a_o^?1 fwhm) binary (e,2e) spectrometer operated at 1200 eV impact energy under the non-coplanar symmetric scattering condition. Generally good agreement of the measured momentum distributions with theoretical momentum distributions calculated using literature SCF double-zeta quality wavefunctions has been obtained for the 1π_g, (1π_u + 3σ_u) and 4σ_g orbitals. Although there is a reasonable agreement of the measured momentum distributions with earlier low momentum resolution (0.4 a_o^?1 fwhm) non-coplanar measurements at 400 eV impact energy reported by Cook and Brion, given the large differences in the momentum resolutions much more definitive results are obtained in the present study. In particular, the significantly higher momentum resolution clearly shows the mixed s-p character of the 4σ_g orbital. The present study also gives a much better agreement with theory in the case of the 4σ_g momentum distribution. For each orbital the calculated and where possible the experimentally determined spherically averaged momentum distributions are compared and contrasted with their respective two-dimensional momentum and position density maps. These together with three-dimensional surface plots at selected constant density values of the four outermost orbitals are used to provide a detailed comparison of momentum-space bonding and orbital properties with their more familiar position-space counterparts in the CO₂ triatomic molecule. The calculated momentum-space density contour maps of the core orbitals exhibit rather large density oscillations and the feasibility of future experiments is discussed.     

Binary (e,2e) spectroscopic study of carbonyl sulfide and the momentum-space chemistry of CO2, CS2 and OCS

The valence-shell binding energy spectra (8–44 eV) and molecular orbital momentum distributions of OCS have been studied by non-coplanar symmetric binary (e,2e) spectroscopy. Existing theoretical binding energy spectra calculated using the many-body 2ph-TDA Green's function (GF) method and using the symmetry-adapted cluster (SAC) on method are compared with the experiment. Intense many-body structure in the measured and calculated binding energy spectra indicates the general breakdown of the independent particle ionization picture. Experimental momentum distributions are compared with those calculated using ab initio SCF wavefunctions of minimal basis set quality and of near Hartree—Fock quality. Excellent agreement between the experimental momentum distributions and those calculated by the near Hartree—Fock wavefunction is obtained for the three innermost valence orbitals: 8σ, 7σ and 6σ. The correct order of the close lying outer-valence 2π and 9σ orbitals is unambiguously identified from the shapes of the measured momentum distributions. Momentum and position contour density maps computed from theoretical wavefunctions of near Hartree—Fock quality are used to interpret the shapes and atomic characters of the observed momentum distributions. The momentum densities of the outermost-valence antibonding π orbitals and of the outermost-valence bonding σ orbitals of the linear triatomic group: CO₂, CS₂ and OCS are compared respectively with each other. The associated chemical trends are discussed within the existing framework of momentum-space chemical principles.     

Dissociative ionization of O2 and N2 by electron impact — N+ and O+ kinetic energies and angular distributions

An apparatus containing cross molecular and pulsed electron beams has been used to obtain distributions in kinetic energy and angle of fast (? 0.5 eV) positive ions produced through dissociative ionization of N₂ and O₂ by impact of 50 to 2000 eV electrons. Four main O⁺ ion groups are observed with peak energies of 0.8, 2.0, 3.0, and 5.0 eV. Two main N⁺ groups peaking at 2.0 and 3.0 eV are seen. Angular distributions of both N⁺ and O⁺ ions are essentially isotropic for electron-beam-ion detection angles from 30° to 110°.     

Valence-shell momentum distributions and ionization energies of carbon disulfide

Valence-shell binding energy spectra and momentum distributions of CS₂ have been measured using non-coplanar symmetric binary (e,2e) spectroscopy. The present measurements are compared with previously published binding energy spectra calculated using the many body 2ph-TDA Green's function (GF) method and the symmetry-adapted cluster configuration-interaction (SAC CI) method. The measured and the calculated binding energy spectra both show extensive population splittings particularly above 20 eV, confirming a significant breakdown of independent particle ionization picture. A relatively strong-outer valence many-body state at 17.0 eV is shown to be satellite of the (2π₀)^?1 state, in accord with earlier conclusions of photoelectron studies. Momentum distributions measured at several carefully chosen binding energies are compared with the corresponding molecular orbital momentum distributions calculated using small and extended gaussian basis sets. The good qualitative agreement between momentum distributions measured in the inner-valence region wth theoretical 4σ_m and 5σ_g orbital momentum distributions confirms the qualitative predictions of satellite parentages by GF and SAC CI calculations. Momentum and position density contour maps of individual orbitals are used to interpret the shapes and atomic characters of the experimental momentum distributions. Momentum densities of the valence orbitals of CS₂ are compared with those of the respective valence isoelectronic species CO₂     

Non-adiabatic collisions in H+ + O2 system: An ab initio study

An ab initio study on the low-lying potential energy surfaces of H⁺ + O₂ system for different orientations (γ) of H⁺ have been undertaken employing the multi-reference configuration interaction (MRCI) method and Dunning’s cc-pVTZ basis set to examine their role in influencing the collision dynamics. Nonadiabatic interactions have been analysed for the 2 × 2 case in two dimensions for γ = 0°, 45° and 90°, and the corresponding diabatic potential energy surfaces have been obtained using the diabatic wavefunctions and their CI coefficients. The characteristics of the collision dynamics have been analysed in terms of vibrational coupling matrix elements for both inelastic and charge transfer processes in the restricted geometries. The strengths of coupling matrix elements reflect the vibrational excitation patterns observed in the state-to-state beam experiments.     

Electron momentum spectroscopy of sulphurhexafluoride

The SF₆ molecule has been studied using high-resolution electron momentum spectroscopy [EMS], at a total energy of 1200 eV and using non-coplanar symmetric kinematics. Binding-energy spectra ranging up to 62 eV were measured at out of plane azimuthal angles from 0° to 28°, and in the outer-valence region from 0° to 34°, corresponding to target electron momenta from about 0.1–2.8 au. The binding-energy spectra and electron momentum distributions obtained for the valence orbitals are compared with the results of Green function calculations for the ionization energies and their corresponding pole strengths and the spherically averaged momentum distributions obtained from the SCF wavefunction on which the Green function calculations are based. The SCF basis includes d components on both S and F atoms. In the outer-valence region, where the one-particle picture holds for the ionization process, there is very good agreement between the theoretical energies and pole strengths and the measured ones, but the orbital momentum distributions are given poorly by the SCF wavefunctions. The measured momentum distributions are significantly higher at low momentum (< 1 au), particularly for the 1t_2u and 3e_g orbitals. In the inner-valence region a substantial splitting of the lines occurs, which is only predicted in a qualitative way. The SCF momentum distribution for the 2e_g orbital is in poor agreement with the data, whereas that of the 3t_1u orbital is in very good agreement with the measurements.     

An investigation of the valence orbitals of water by high momentum resolution electron momentum spectroscopy

The momentum distributions of the valence orbitals for water well as the binding energy spectra in the region 10–45 eV have been reinvestigated with a high momentum resolution (≈0.1 a₀^?1 fwhm) binary (e.2e) spectrometer. The binding energy spectra show considerable satellite structure in the region > 25 eV which is consistent with theoretical predictions of final state configuration interaction (many-body effects) involving the (2a₁)^?1 hole state. An investigation of the momentum distribution in the satellite region confirms this assignment. This is in accord with a variety of recent theoretical studies and also consistent with earlier experiments. Differences suggested in earlier comparisons between theory and low momentum resolution experiments for the momentum distributions of the 1b₁ and 3a₁ orbitals have been verified. Several possible theoretical studies are suggested to investigate further this discrepancy between experiment and theory. Bonding effects and thenature of the molecular orbitals of H₂O in momentum space are also discussed.     

Characterisation of 13C implantations in silicon by NRA [13C(p,γ)14N] and RBS

SiC_X layers close to the surface have been produced by implanting 40 keV ¹³C ions into silicon with a fluence of 6×10¹⁷ at./cm² (j=12 μA/cm²) at room temperature (RT). Depth distributions and areal densities (doses) of the implanted carbon have been analysed by the nuclear reaction ¹³C(p,γ)¹⁴N (NRA) which shows a sharp resonance in the excitation function at a proton energy of 1748 keV (Γ=75 eV FWHM). The depth resolution at the surface amounts to 31 nm due to energy spread of the proton beam (1.2 keV FWHM) and resonance width. The surface resolution of the NRA can be increased up to 8 nm when tilting the sample (surface normal) to an angle of 75° with respect to the proton beam direction. Using a NaI detector the detection limit of ¹³C in silicon is approximately 1 at.%. Comparative elastic backscattering measurements with ⁴He⁺ projectiles were performed at 2 MeV (Rutherford backscattering spectroscopy, RBS) and 3.45 MeV (high energy backscattering, HEBS) at a backscattering angle of 171°. The measured ¹³C depth distributions have been compared with a distribution calculated by the Monte Carlo algorithm T-DYN.     

Translational energy distribution of the excited hydrogen atom produced by controlled electron impact on HCl

The translational energy distribution of an atom can be calculated by differentiating the Doppler line shape of its emission line taken at a high optical resolution. The Balmer-β line of the excited hydrogen atom (n = 4) produced by electron impact on HCl has been measured at a high resolution (0.033Å) and at two angles (55° and 90°) with respect to the electron beam. The translation energy distribution depends on the electron energies and has almost two groups of components: ≈ 5 eV (fast) and ≈ eV (slow). Anisotropy is imporant for the slow component. The excitation function shows the corresponding structures. It is concluded that Rydberg states converging to the ²Π state of HCl⁺ produce the fast component and Rydberg states converging to the repulsive HCl⁺ states which cross the ²Σ⁺ state produce the slow component.     

Subnanometre depth resolution in sputter depth profiling of Si layers using grazing‐incident low‐energy O2+ and Ar+ ions

The sputter damage profiles of Si(100) by low‐energy O₂⁺ and Ar⁺ ion bombardment at various angles of incidence were measured using medium‐energy ion scattering spectroscopy. It was observed that the damaged Si surface layer can be minimized down to 0.5–0.6 nm with grazing‐incident 500 eV Ar⁺ and O₂⁺ ions at 80°. To illustrate how the damaged layer thickness can be decreased down to 0.5 nm, molecular dynamics simulations were used. The SIMS depth resolution estimated with trailing‐edge decay length for a Ga delta‐layer in Si with grazing‐incident 650 eV O₂⁺ was 0.9 nm, which is in good agreement with the measured damaged layer thickness. Copyright © 2004 John Wiley & Sons, Ltd.     

环戊烷分子的电子动量谱

报导了在高分辨率电子动量谱仪上获得的环戊烷分子的结合能谱和动量谱的实验结果,并用Hartree-Fock方法和密度泛函方法做了理论计算.实验得到的环戊烷分子各电子轨道的电离能值与光电子谱得到的数据一致,动量分布的实验结果也与理论计算基本吻合.     

Potential curves and molecular properties of Na2+

A model potential method in which a molecule is described as a single electron moving in the field of two polarizable cores is used to calculate the potential energy curves and the wavefunctions of the lowest six electronic states of the molecular ion Na₂⁺. The ground X²Σ_g state has a dissociation energy of 0.98 eV at an equilibrium separation of 3.3 Å and the excited ²Π_u state has a dissociation energy of 0.23 eV at an equilibrium separation of 5.2 Å. Various molecular properties of these two bound states are calculated. An analysis of the long range behaviour of all the six states is presented.     

The valence orbitals of NH3 by electron momentum spectroscopy: Quantitative comparisons using Hartree-Fock limit and correlated wavefunctions

The complete valence shell binding energy spectra and valence orbital electron momentum distributions for NH₃ have been measured by high-momentum-resolution electron momentum spectroscopy (EMS). The results are quantitatively compared with theoretical calculations using SCF wavefunctions ranging from DZ quality to a newly developed 126-GTO wavefunction essentially at the Hartree-Fock limit. The 3a₁ and to a lesser extent the 2a₁ valence orbital are not adequately described even at the Hartree-Fock limit with basis set saturation including diffuse functions. The differences between theory and experiment are largely resolved by ion-neutral overlap calculations using CI wavefunctions to incorporate the effects of electron correlation. The 126-G (CI) wavefunctions provide accurate calculation of a wide range of electronic properties of NH₃ and also give good quantitative prediction of the three valence orbital momentum distributions as well as a reasonable prediction of the many-body pole strength distribution observed in the (2a₁)⁻¹ inner valence binding energy spectrum. The present EMS results are compared with recent investigations of wavefunction tails by exterior electron distribution calculations and Penning ionization electron spectroscopy measurements reported by Ohno et al.     

High Resolution Electron Momentum Spectroscopy Study on Ethanol: Orbital Electron Momentum Distributions for Individual Conformers

The outer-valence binding energy spectra of ethanol in the energy range of 9-21 eV are measured by a high-resolution electron momentum spectrometer at an impact energy of 2.5 keV plus the binding energy. The electron momentum distributions for the ionization peaks corresponding to the outer-valence orbitals are obtained by deconvoluting a series of azimuthal angular correlated binding energy spectra. Comparison is made with the theoretical calculations for two conformers, trans and gauche, coexisting in the gas phase of ethanol at the level of B3LYP density functional theory with aug-cc-pVTZ basis sets. It is found that the measured electron momentum distributions for the peaks at 14.5 and 15.2 eV are in good agreement with the theoretical electron momentum distributions for the molecular orbitals of individual conformers (i.e., 8a' of trans and 9a of gauche), but not in accordance with the thermally averaged ones. It demonstrates that the high-resolution electron momentum spectrometer, by inspecting the molecular electronic structure, is a promising technique to identify different conformers in a mixed sample.     

Recoil-ion momentum distribution for He(e,2e)He+-and He(e,3e)He++-reactions

Using Recoil-Ion Momentum Spectroscopy (RIMS) we have measured the momenta of recoiling target ions in He(e,2e)He⁺- and He(e,3e)He⁺⁺-reactions at impact energies between 270 ev and 3200 eV. The recoil-ion momentum reflecting the sum momentum of all outgoing electrons was determined for the first time in all three spatial dimensions separately for single and double ionization by electron impact. The data are compared to results of a nCTMC-calculation.     

(e,2e) Spectroscopy of methane

Measurements are reported on the spectroscopy of methane using the symmetric (e,2e) technique at energies of 600 eV and 1200 eV. The angular correlations of the states with separation energies of 14.2 and 23.1 eV have been measured and compared with the orbital wavefunctions of Snyder and Basch and with some earlier data at 400eV. The angular correlation of the configuration interaction state at 31 eV shows that this state definetely results from the removal of an electron in the 2a₁ orbital. Other structure at high separation energy is also identified with this orbital. Relative strengths of the It₂ and 2a₁ states are compared and found to be in agreement with the theory at 1200eV.