Isotope shifts in electron affinities and in binding energies of Pb and hyperfine structure of 207Pb.

The isotope shifts in electron affinities of Pb were measured by Walter et al. [Phys. Rev. A 106, L010801 (2022)] to be -0.002(4) meV for 207-208Pb and -0.003(4) meV for 206-208Pb by scanning the threshold of the photodetachment channel Pb-(S3/2◦4) - Pb (3P0), while Chen and Ning reported 0.015(25) and -0.050(22) meV for the isotope shifts on the binding energies measured relative to 3P2 using the SEVI method [J. Chem. Phys. 145, 084303 (2016)]. Here we revisited these isotope shifts by using our second-generation SEVI spectrometer and obtained -0.001(15) meV for 207-208Pb and -0.001(14) meV for 206-208Pb, respectively. In order to aid the experiment by theory, we performed the first ab initio theoretical calculations of isotope shifts in electron affinities and binding energies of Pb, as well as the hyperfine structure of 207Pb-, by using the MCDHF and RCI methods. The isotope shifts in electron affinities of 207-208Pb and 206-208Pb are -0.0023(8) and -0.0037(13) meV for the 3P0 channel, respectively, in good agreement with Walter et al.'s measurements. The isotope shifts in binding energies relative to 3P1,2, -0.0015(8) and -0.0026(13) meV for 207-208Pb and 206-208Pb, respectively, are compatible with the present measurements. The hyperfine constant for the ground state of 207Pb- obtained by the present calculations, A(S3/2◦4)=-1118 MHz, differs by a factor of 3 from the previous estimation by Bresteau et al. [J. Phys. B: At., Mol. Opt. Phys. 52, 065001 (2019)]. The reliability is supported by the good agreement between the theoretical and experimental hyperfine parameters of 209Bi.

A dynamical (e,2e) investigation into the ionization of the outermost orbitals of R-carvone.

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

Electron impact ionization dynamics of para-benzoquinone.

Triple differential cross sections (TDCSs) for the electron impact ionization of the unresolved combination of the 4 highest occupied molecular orbitals (4b3g, 5b2u, 1b1g, and 2b3u) of para-benzoquinone are reported. These were obtained in an asymmetric coplanar geometry with the scattered electron being observed at the angles -7.5°, -10.0°, -12.5° and -15.0°. The experimental cross sections are compared to theoretical calculations performed at the molecular 3-body distorted wave level, with a marginal level of agreement between them being found. The character of the ionized orbitals, through calculated momentum profiles, provides some qualitative interpretation for the measured angular distributions of the TDCS.

Electron- and photon-impact ionization of furfural.

The He(i) photoelectron spectrum of furfural has been investigated, with its vibrational structure assigned for the first time. The ground and excited ionized states are assigned through ab initio calculations performed at the outer-valence Green's function level. Triple differential cross sections (TDCSs) for electron-impact ionization of the unresolved combination of the 4a″ + 21a' highest and next-highest occupied molecular orbitals have also been obtained. Experimental TDCSs are recorded in a combination of asymmetric coplanar and doubly symmetric coplanar kinematics. The experimental TDCSs are compared to theoretical calculations, obtained within a molecular 3-body distorted wave framework that employed either an orientation average or proper TDCS average. The proper average calculations suggest that they may resolve some of the discrepancies regarding the angular distributions of the TDCS, when compared to calculations employing the orbital average.

Triply differential (e,2e) studies of phenol.

We have measured (e,2e) triple differential cross sections (TDCS) for the electron-impact ionisation of phenol with coplanar asymmetrical kinematics for an incident electron energy of 250 eV. Experimental measurements of the angular distribution of the slow outgoing electrons at 20 eV are obtained when the incident electron scatters through angles of -5°, -10°, and -15°, respectively. The TDCS data are compared with calculations performed within the molecular 3-body distorted wave model. In this case, a mixed level of agreement, that was dependent on the kinematical condition being probed, was observed between the theoretical and experimental results in the binary peak region. The experimental intensity of the recoil features under all kinematical conditions was relatively small, but was still largely underestimated by the theoretical calculations.

A dynamical (e,2e) investigation of the structurally related cyclic ethers tetrahydrofuran, tetrahydropyran, and 1,4-dioxane.

Triple differential cross section measurements for the electron-impact ionization of the highest occupied molecular orbitals of tetrahydropyran and 1,4-dioxane are presented. For each molecule, experimental measurements were performed using the (e,2e) technique in asymmetric coplanar kinematics with an incident electron energy of 250 eV and an ejected electron energy of 20 eV. With the scattered electrons being detected at -5°, the angular distributions of the ejected electrons in the binary and recoil regions were observed. These measurements are compared with calculations performed within the molecular 3-body distorted wave model. Here, reasonable agreement was observed between the theoretical model and the experimental measurements. These measurements are compared with results from a recent study on tetrahydrofuran [D. B. Jones, J. D. Builth-Williams, S. M. Bellm, L. Chiari, C. G. Ning, H. Chaluvadi, B. Lohmann, O. Ingolfsson, D. Madison, and M. J. Brunger, Chem. Phys. Lett. 572, 32 (2013)] in order to evaluate the influence of structure on the dynamics of the ionization process across this series of cyclic ethers.

Vibrational effects on the electron momentum distributions of valence orbitals of formamide.

The ionization energy spectra and electron momentum distributions of formamide were investigated using the high-resolution electron momentum spectrometer in combination with high level calculations. The observed ionization energy spectra and electron momentum distributions were interpreted using symmetry adapted cluster-configuration interaction theory, outer valence Green function, and DFT-B3LYP methods. The ordering of 10a(') and 2a(") orbitals of formamide was assigned unambiguously by comparing the experimental electron momentum distributions with the corresponding theoretical results, i.e., 10a(') has a lower binding energy. In addition, it was found that the low-frequency wagging vibration of the amino group at room temperature has noticeable effects on the electron momentum distributions. The equilibrium-nuclear-positions-approximation, which was widely used in electron momentum spectroscopy, is not accurate for formamide molecule. The calculations based on the thermal average can evidently improve the agreement with the experimental momentum distributions.

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.

Experimental and theoretical investigation of the triple differential cross section for electron impact ionization of pyrimidine molecules.

Cross-section data for electron impact induced ionization of bio-molecules are important for modelling the deposition of energy within a biological medium and for gaining knowledge of electron driven processes at the molecular level. Triply differential cross sections have been measured for the electron impact ionization of the outer valence 7b(2) and 10a(1) orbitals of pyrimidine, using the (e, 2e) technique. The measurements have been performed with coplanar asymmetric kinematics, at an incident electron energy of 250 eV and ejected electron energy of 20 eV, for scattered electron angles of -5°, -10°, and -15°. The ejected electron angular range encompasses both the binary and recoil peaks in the triple differential cross section. Corresponding theoretical calculations have been performed using the molecular 3-body distorted wave model and are in reasonably good agreement with the present experiment.

Dynamical (e,2e) studies of tetrahydrofurfuryl alcohol.

Cross section data for electron scattering from DNA are important for modelling radiation damage in biological systems. Triply differential cross sections for the electron impact ionization of the highest occupied outer valence orbital of tetrahydrofurfuryl alcohol, which can be considered as an analogue to the deoxyribose backbone molecule in DNA, have been measured using the (e,2e) technique. The measurements have been performed with coplanar asymmetric kinematics at an incident electron energy of 250 eV, an ejected electron energy of 20 eV, and at scattered electron angles of -5°, -10°, and -15°. Experimental results are compared with corresponding theoretical calculations performed using the molecular 3-body distorted wave model. Some important differences are observed between the experiment and calculations.

Low energy (e, 2e) study from the 1t(2) orbital of CH4.

Single ionization of the methane (CH(4)) 1t(2) orbital by 54 eV electron impact has been studied experimentally and theoretically. The measured triple differential cross sections cover nearly a 4π solid angle for the emission of low energy electrons and a range of projectile scattering angles. Experimental data are compared with theoretical calculations from the distorted wave Born approximation and the molecular three-body distorted wave models. It is found that theory can give a proper description of the main features of experimental cross section only at smaller scattering angles. For larger scattering angles, significant discrepancies between experiment and theory are observed. The importance of the strength of nuclear scattering from the H-nuclei was theoretically tested by reducing the distance between the carbon nuclei and the hydrogen nuclei and improved agreement with experiment was found for both the scattering plane and the perpendicular plane.

Dyson orbitals of N2O: electron momentum spectroscopy and symmetry adapted cluster-configuration interaction calculations.

Electron momentum spectroscopy and symmetry adapted cluster-configuration interaction (SAC-CI) theory were combined to study electron correlation effects in nitrous oxide molecule (N(2)O). The SAC-CI General-R method accurately reproduced the experimental ionization spectrum. This bench-marked method was also introduced for calculating the momentum distributions of N(2)O Dyson orbitals. Several calculated momentum distributions with different theoretical methods were compared with the high resolution experimental results. In the outer-valence region, Hartree-Fock (HF), density functional theory (DFT), and SAC-CI theory can well describe the experimental momentum distributions. SAC-CI presented a best performance among them. In the inner-valence region, HF and DFT cannot work well due to the severe breaking of the molecular orbital picture, while SAC-CI still produced an excellent description of experimental momentum profiles because it can accurately take into account electron correlations. Moreover, the thermally averaged calculation showed that the geometrical changes induced by the vibration at room temperature have no noticeable effects on momentum distribution of valence orbitals of N(2)O.

Dynamical (e, 2e) studies using tetrahydrofuran as a DNA analog.

Triple differential cross sections for the electron-impact ionization of the outer valence orbital of tetrahydrofuran have been measured using the (e, 2e) technique. The measurements have been performed with coplanar asymmetric kinematics, at an incident electron energy of 250 eV and at an ejected electron energy of 10 eV, over a range of momentum transfers. The experimental results are compared with theoretical calculations carried out using the molecular three-body distorted wave model. The results obtained are important for gaining an understanding of electron driven processes at a molecular level and for modeling energy deposition in living tissue.

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.

An investigation of valence shell orbital momentum profiles of difluoromethane by binary (e,2e) spectroscopy.

The electron binding energy spectra and momentum profiles of the valence orbitals of difluoromethane, also known as HFC32 (HFC-hydrofluorocarbon) (CH(2)F(2)), have been studied by using a high resolution (e,2e) electron momentum spectrometer, at an impact energy of 1200 eV plus the binding energy, and by using symmetric noncoplanar kinematics. The experimental momentum profiles of the outer valence orbitals and 4a(1) inner valence orbital are compared with the theoretical momentum distributions calculated using Hartree-Fock and density functional theory (DFT) methods with various basis sets. In general, the shapes of the experimental momentum distributions are well described by both the Hartree-Fock and DFT calculations when large and diffuse basis sets are used. However, the result also shows that it is hard to choose the different calculations for some orbitals, including the methods and the size of the basis sets employed. The pole strength of the ionization peak from the 4a(1) inner valence orbital is estimated.

Investigation of valence orbitals of propene by electron momentum spectroscopy.

The binding energy spectra and momentum distributions of all valence orbitals of propene were studied by electron momentum spectroscopy (EMS) as well as Hartree-Fock and density functional theoretical calculations. The experiment was carried out at impact energies of 1200 eV and 600 eV on the state-of-the-art EMS spectrometer developed at Tsinghua University recently. The experimental momentum profiles of the valence orbitals were obtained and compared with the various theoretical calculations. Moreover, the experiment with a new analysis method presents a strong support for the correct ordering of the orbital 8a' and 1a'', i.e., 9a' < 8a' < 1a'' < 7a'.

Direct observation of distorted wave effects in ethylene using the (e,2e) reaction.

We report here the direct measurements of electron momentum distributions for ethylene using the (e,2e) reaction at different impact energies from 400 to 2400 eV. The "turn up" effects in the (e,2e) cross sections of the 1b(3g) orbital compared with the plane-wave impulse approximation calculations were observed at low and high momentum regions, and such discrepancies become smaller with the increase of the impact electron energies. It is suggested that the observed discrepancies are due to the distorted-wave effects in molecules, while appropriate theoretical calculations using distorted waves in molecules could not be achieved until now.

The outer valance orbital electron densities of cyclopentane by binary (e,2e) spectroscopy.

The binding energy spectra and electron distributions in momentum space of the valence orbitals of cyclopentane (C(5)H(10)) are studied by Electron Momentum Spectroscopy (EMS) in a noncoplanar symmetric geometry. The impact energy was 1200 eV plus binding energy and energy resolution of the EMS spectrometer was 1.2 eV. The experimental momentum profiles of the outer valence orbitals are compared with the theoretical momentum distributions calculated using Hartree-Fock and density functional theory (DFT) methods. The shapes of the experimental momentum distributions are generally quite well described by both the Hartree-Fock and DFT calculations when the large and diffuse basis sets are used.