Electron Momentum Spectrometer Research Articles

Vibrational effects on the valence electronic structure of acetaldehyde: An electron momentum spectroscopy investigation

We report an electron momentum spectroscopy investigation on the valence electronic structure of an important interstellar organic molecule acetaldehyde. The electron momentum profiles are measured using a high-sensitivity angle and energy dispersive multichannel electron momentum spectrometer at incident energy of 1200 eV plus the binding energy. The measurements are compared with the theoretical calculations by density functional theory (B3LYP) and Hartree-Fock method with 6-311++G** and aug-cc-pVTZ basis sets carried out at the equilibrium molecular geometry and by considering vibrational effects. The results exhibit significant influence of nuclear vibrational motion on the momentum profiles for valence orbitals of acetaldehyde, especially 2a′′, 8a′ and 1a′′ orbitals. Analysis of contributions of individual vibrational normal modes shows that CH3 d-deform mode v5 and CH3 d-stretch mode ν11 play dominant roles. In addition, a (6a′)−1 satellite state at ∼20.8 eV is observed.

Electron momentum spectroscopy study on inner orbitals of methyl iodide

The binding energy spectrum and electron momentum profiles of the inner orbitals of methyl iodide have been measured using an electron momentum spectrometer at the impact energy of 1200 eV plus binding energy. Two peaks in the binding energy spectrum, arising from the spin-orbit splitting, are observed and the corresponding electron momentum profiles are obtained. Relativistic density functional calculations are performed to elucidate the experimental electron momentum profiles of two spin-orbit splitting components, showing agreement with each other except for the intensity in low momentum region. The measured high intensity in the low momentum region can be further explained by the distorted wave calculation.

Electron-momentum-spectroscopy study on the valence electronic structure of methyl iodide: Electron correlation and relativistic effects

We report an experimental and theoretical study on the electronic structure of methyl iodide. The binding energy spectrum (BES) and electron-momentum profiles (EMPs) of valence orbitals have been measured using a high-sensitivity electron-momentum spectrometer at the impact energy of 1200 eV plus binding energy. Theoretical calculations considering the relativistic and electron correlation effects are performed to illuminate the measured results. For the two outermost orbitals, the relativistic effects are revealed by the experimental and theoretical EMPs for two spin-orbit splitting components ($2{e}_{3/2}$ and $2{e}_{1/2}$) and the C-I bonding orbital ($3{a}_{1}$). In the inner valence region, satellite structures associated with the ionizations from $2{a}_{1}$ and $1{a}_{1}$ orbitals are observed in the BES and the pole strengths for the satellites are determined by comparing the measured EMPs with the calculated ones. Moreover, distinct relativistic and electron correlation effects on the EMPs for the $2{a}_{1}$ orbital and its satellites have been found.

Observation of strong relativistic and distorted-wave effects in (e,2e) electron-momentum spectroscopy of mercury

We report a measurement of the valence orbital-momentum profiles of mercury (Hg) using a high-sensitivity binary $(e,2e)$ electron-momentum spectrometer at incident energies of 600 and 1200 eV plus the binding energy. The $6{s}_{1/2}$ orbital and the spin-orbit components of $5{d}_{5/2}$ and $5{d}_{3/2}$ orbitals are well separated in the measured binding-energy spectra. The experimental momentum distributions for the individual orbitals and the branching ratio of $5{d}_{5/2}$ to $5{d}_{3/2}$ are obtained and compared with predictions from a plane-wave impulse approximation (PWIA) in which the orbital wave functions are calculated using the nonrelativistic (NR) and spin-orbital (SO) relativistic theories. The SO relativistic calculations are in better agreement with experiment than the NR, indicating clearly the importance of relativistic effects in the electronic structure of Hg. We also observe some discrepancies between experiment and PWIA calculations in the high-momentum region of $6{s}_{1/2}$ and the low-momentum region of $5d$ orbitals which display a dynamic dependence on the impact energy. These discrepancies become smaller at a higher energy of about 1200 eV and thus can be qualitatively assigned to the distorted-wave effect in the $(e,2e)$ reaction of Hg.

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.

Development of an electron momentum spectrometer for time-resolved experiments employing nanosecond pulsed electron beam.

The low count rate of (e, 2e) electron momentum spectroscopy (EMS) has long been a major limitation of its application to the investigation of molecular dynamics. Here we report a new EMS apparatus developed for time-resolved experiments in the nanosecond time scale, in which a double toroidal energy analyzer is utilized to improve the sensitivity of the spectrometer and a nanosecond pulsed electron gun with a repetition rate of 10 kHz is used to obtain an average beam current up to nA. Meanwhile, a picosecond ultraviolet laser with a repetition rate of 5 kHz is introduced to pump the sample target. The time zero is determined by photoionizing the target using a pump laser and monitoring the change of the electron beam current with time delay between the laser pulse and electron pulse, which is influenced by the plasma induced by the photoionization. The performance of the spectrometer is demonstrated by the EMS measurement on argon using a pulsed electron beam, illustrating the potential abilities of the apparatus for investigating the molecular dynamics in excited states when employing the pump-probe scheme.

Electron Momentum Spectroscopy Investigation of Molecular Conformations of Ethanol Considering Vibrational Effects.

The interpretation of experimental electron momentum distributions (EMDs) of ethanol, one of the simplest molecules having conformers, has confused researchers for years. High-level calculations of Dyson orbital EMDs by thermally averaging the gauche and trans conformers as well as molecular dynamical simulations failed to quantitatively reproduce the experiments for some of the outer valence orbitals. In this work, the valence shell electron binding energy spectrum and EMDs of ethanol are revisited by the high-sensitivity electron momentum spectrometer employing symmetric noncoplanar geometry at an incident energy of 1200 eV plus binding energy, together with a detailed analysis of the influence of vibrational motions on the EMDs for the two conformers employing a harmonic analytical quantum mechanical (HAQM) approach by taking into account all of the vibrational modes. The significant discrepancies between theories and experiments in previous works have now been interpreted quantitatively, indicating that the vibrational effect plays a significant role in reproducing the experimental results, not only through the low-frequency OH and CH3 torsion modes but also through other high-frequency ones. Rational explanation of experimental momentum profiles provides solid evidence that the trans conformer is slightly more stable than the gauche conformer, in accordance with thermodynamic predictions and other experiments. The case of ethanol demonstrates the significance of considering vibrational effects when performing a conformational study on flexible molecules using electron momentum spectroscopy.

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.

Design and Implementation of a High Resolution DAQ System for an (e, 2e+ion) Electron Momentum Spectrometer

An (e, 2e+ion) Electron momentum spectrometer (EMS) upgraded from the third-generation (e, 2e) spectrometer is constructed in the University of Science and Technology of China (USTC). In the EMS, the energy-momentum density of electrons is derived through the charge measurement using the signals of the position sensitive detector (PSD). The information of ion fragments is derived through the time measurement and that information will be used for more research on collision dynamics. A PXI-based data acquisition (DAQ) system was designed. The system mainly consists of a 6-channel charge digital module (CDM) and a 9-channel time digital module (TDM). Shaping amplifiers (SAs) are also designed for charge measurement with CDM. According to the design specification, the CDM with SAs needs to measure charge from 100 to 900 fC with an accuracy of better than 1%. The TDM needs to measure the flight time from 0 to 15 us with a resolution of better than 100 ps. The TDM and CDM can be combined to run synchronously using a coincidence unit which also rejects the background events based on the tracking information of the electrons and the ions. Test results indicate that, within the dynamic range, the accuracy of the CDM is better than 0.4%. The RMS time resolution of better than 30 ps is achieved by using the method of time-to-digital converter (TDC) implemented in a field-programmable gate arrays (FPGA). The DAQ system has been assembled and works well with the EMS detectors.

Observation of Interference Effects in (e, 2e) Electron Momentum Spectroscopy of SF6

Two-dimensional electron density map (2D map) of binding energy and relative azimuthal angle (i.e., momentum) for the outer-valence molecular orbitals of SF6 has been measured by a highly sensitive electron momentum spectrometer with noncoplanar symmetric geometry at the impact energy of 1.2 keV plus binding energy. The experimental electron momentum profiles for the relevant molecular orbitals have been extracted from the 2D map and interpreted on the basis of the quantitative calculations using the density functional theory with B3LYP hybrid functional. For the outermost F2p nonbonding orbitals of SF6, the interference patterns are clearly observed in the ratios of the electron momentum profiles of molecular orbitals to that of atomic F2p orbital.

Ring-puckering effects on electron momentum distributions of valence orbitals of oxetane.

The binding energy spectra and electron momentum distributions for the outer-valence molecular orbitals of oxetane have been measured utilizing (e, 2e) electron momentum spectrometer with non-coplanar asymmetric geometry at the impact energy of 2500 eV. The experimental momentum distributions were compared with the density functional theory calculations employing B3LYP hybrid functional with aug-cc-pVTZ basis set. It was found that the calculation at planar geometry (C2v) completely fails to interpret the large "turn-up" at low momentum region in electron momentum distribution of the highest occupied molecular orbital (HOMO) 3b1, while the calculations considering the thermal abundances of planar (C2v) and bent (Cs) conformers or the thermally populated vibrational states of ring-puckering motion have significantly improved the agreement. The results indicate that the ring-puckering motion of oxetane has a strong effect on the electron density distribution of HOMO.

Electron Momentum Spectroscopy of Valence Orbitals of n-Propyl Iodide: Spin-Orbit Coupling Effect and Intramolecular Orbital Interaction

The binding energy spectrum and electron momentum distributions for the outer valence orbitals of n-propyl iodide molecule have been measured using the electron momentum spectrometer employing non-coplanar asymmetric geometry at impact energy of 2.5 keV plus binding energy. The ionization bands have been assigned in detail via the high accuracy SAC-CI general-R method calculation and the experimental momentum profiles are compared with the theoretical ones calculated by Hartree-Fock and B3LYP/aug-cc-pVTZ(C,H)6-311G** (I). The spin-orbit coupling effect and intramolecular orbital interaction have been analyzed for the outermost two bands, which are assigned to the iodine 5p lone pairs, using NBO method and non-relativistic as well as relativistic calculations. It is found that both of the interactions will lead to the observed differences in electron momentum distributions. The experimental results agree with the relativistic theoretical momentum profiles, indicating that the spin-orbit coupling effect dominates in n-propyl iodide molecule.

Experimental and theoretical investigation on the outer valence electronic structure of cyclopropylamine by (e, 2e) electron momentum spectroscopy.

The binding energy spectra and electron momentum distributions for the outer-valence molecular orbitals of gaseous cyclopropylamine (CPA) have been measured by (e, 2e) electron momentum spectrometer employing noncoplanar asymmetric geometry at the impact energy of 2500 eV. The experimental results are interpreted on the basis of the quantitative calculations of the ionization energies and the relevant molecular orbitals at benchmark theoretical levels using the outer-valence Green's function method, the symmetry-adapted cluster configuration interaction method, and the density functional theory with B3LYP hybrid functional. The total energies of the trans and gauche conformers of CPA are also calculated by the second-order Møller-Plesset perturbation theory with large basis sets and the derived enthalpy differences (2.02-2.12 kcal/mol) are consistent with the previous experimental data (2.19 kcal/mol). The theoretical binding energy spectra and electron momentum distributions, in which the relative abundances of trans and gauche are taken into account, are generally in accordance with the experimental results except for the ionization band from the trans 8a' and gauche 11a orbitals. The discrepancy is explained qualitatively in view of the picture of molecular geometry change at the instant of ionization.

Probing the orbital electron momentum distribution for pure gauche conformer in ethanethiol by electron momentum spectroscopy

The orbital electron momentum distribution (EMD) for pure gauche conformer of ethanethiol has been measured by electron momentum spectrometer (EMS) at room temperature. It is clearly proved that with the help of high level CI calculations, it is possible for EMS to probe the orbital EMD for individual conformer in molecules.

Pre-retardation effects on the high-sensitivity angle and energy dispersive multichannel electron momentum spectrometer with 2π angle range

The pre-retardation effects on a newly developed high-sensitivity angle and energy dispersive multichannel electron momentum spectrometer with almost 2π azimuthal angle range has been tested. Experiment on argon 3p shows significant improvement of coincident energy resolution when decelerating analyzed electrons.

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.

Outer- and inner-valence satellites of carbon dioxide: Electron momentum spectroscopy compared with symmetry-adapted-cluster configuration interaction general-R calculations

The extensive study of outer- and inner-valence satellites of carbon dioxide by electron momentum spectroscopy is reported. The experiments have been performed using a high-sensitivity electron momentum spectrometer employing non-coplanar symmetric geometry at impact energy of about 1200 eV. Binding energy spectrum up to 50 eV, above the first double ionization threshold (~37.3 eV), is presented. Four main peaks and twelve satellites have been identified including four embedded in the double ionization continuum, among which the two beyond 42 eV are observed for the first time. High accuracy symmetry-adapted-cluster configuration interaction general-R calculation with aug-cc-pVTZ basis sets has also been performed and the result is in line with the experimental ionization spectrum except the relative intensities for some of the satellites in inner-valence region. The experimental momentum profiles for both the main ionization transitions and satellites have been obtained and compared with theoretical calculations by HF and B3LYP methods with 6-311++G∗ and aug-cc-pVTZ basis sets. Through comparison, the detailed assignments of the satellite bands have been achieved and the pole strengths for the relevant shake-up transitions are determined experimentally for the first time.

An electron momentum spectroscopic study of naphthalene in gas phase

We report the experimental and theoretical investigation of the complete valence shell binding energy spectra and momentum profiles of naphthalene (C10H8), using our high resolution electron momentum spectrometer, at impact energies of 1500 eV and 600 eV. The observed momentum profiles were compared with the Hartree-Fock (HF) and density functional theory (DFT) calculations, and the binding energy spectrum was compared with the Outer valence Green’s function (OVGF) calculations. The impact energy dependent discrepancy between observed momentum distributions and calculations under the plane wave impulse approximation was ascribed to the distorted wave effects.

A highly sensitive electron momentum spectrometer incorporating a multiparticle imaging detector

A multichannel electron momentum spectrometer is reported, which combines the features of both a spherical analyzer and a large-area multiparticle imaging detector. The major advantage of the apparatus over the existing setups is its ability to cover almost completely the available azimuthal angle range for the symmetric noncoplanar (e, 2e) reaction. It produces considerable improvement in instrumental sensitivity and approximately three times extension of the coverable momentum range compared with our existing apparatus. A proof-of-performance experiment on Ne is presented and the potential of the developed spectrometer is highlighted for future studies of electron momentum spectroscopy.

Development of a low energy e− gun for studies of e−–molecule interactions using a recoil-ion and electron momentum spectrometer

To study the interaction between low-energy electrons and molecules, a recoil-ion and electron momentum spectrometer (RIMS) has been developed at Shanghai EBIT laboratory. As the low-energy electron source, the ELG-2 electron gun (Kimball Physics) coupled with an ultrafast pulse generator has been tested. The energy of the electron beam ranges from a few eV to 2 keV, and the pulse width of the beam is about 1 ns.