Low-energy Electron-molecule Collisions Research Articles

Solving Vibronic Dynamics in Electron Continuum.

We present a general two-dimensional model of conical intersection between metastable states that are vibronically coupled not only directly but also indirectly through a virtual electron in the autodetachment continuum. This model is used as a test ground for the design and comparison of iterative solvers for resonance dynamics in low-energy electron-molecule collisions. Two Krylov-subspace methods with various preconditioning schemes are compared. To demonstrate the applicability of the proposed methods on even larger models, we also test the performance of one of the methods on a recent model of vibrational excitation of CO2 by electron impact based on three vibronically coupled discrete states in continuum (Renner-Teller doublet of shape resonances coupled to a sigma virtual state) including four vibrational degrees of freedom. Two-dimensional electron energy-loss spectra resulting from electron-molecule scattering within the models are briefly discussed.

Correlated ion-(ion/neutral) time of flight mass spectrometer

The fragmentation of molecular systems into ions and neutral species is ubiquitous in fundamental and applied science. While the ion fragments are relatively easily detected by mass spectrometry technique, the information on the neutral product that is formed in correlation is challenging. In this contribution, we present a detailed description of the correlated ion-(ion/neutral) time of flight mass spectrometer, which is dedicated to the study of molecular dissociation induced by electrons at low energies (<20 eV). This new mass spectrometer uptakes the challenge to provide the correlation of ion/neural species produced in low energy electron-molecule collision processes.

A new time of flight mass spectrometer for absolute dissociative electron attachment cross-section measurements in gas phase.

A new time of flight mass spectrometer (TOFMS) has been developed to study the absolute dissociative electron attachment (DEA) cross section using a relative flow technique of a wide variety of molecules in gas phase, ranging from simple diatomic to complex biomolecules. Unlike the Wiley-McLaren type TOFMS, here the total ion collection condition has been achieved without compromising the mass resolution by introducing a field free drift region after the lensing arrangement. The field free interaction region is provided for low energy electron molecule collision studies. The spectrometer can be used to study a wide range of masses (H- ion to few hundreds atomic mass unit). The mass resolution capability of the spectrometer has been checked experimentally by measuring the mass spectra of fragment anions arising from DEA to methanol. Overall performance of the spectrometer has been tested by measuring the absolute DEA cross section of the ground state SO2 molecule, and the results are satisfactory.

Complete data acquisition and analysis system for low-energy electron–molecule collision studies

A complete data acquisition system has been developed that can work with any personal computer irrespective of the operating system installed on it. The software can be used in low and intermediate electron-energy collision studies with ground-state molecules in gas phase using a combination of RS-232, GPIB, and USB-interfaced devices. Various tabletop instruments and nuclear instrumentation module (NIM) -based electronics have been interfaced and have communicated with the software, which is based on LabVIEW. This is tested with dissociative electron attachment (DEA) and polar dissociation studies to oxygen molecule and successfully used in a DEA study of carbon monoxide and carbon dioxide.

Isomeric effects on fragmentations of crotonaldehyde and methacrolein in low-energy electron–molecule collisions

Fragmentation behavior upon low energy (10–16eV) electron molecule collision of two isomeric α,β-enones, crotonaldehyde and methacrolein, has been studied by quadrupole mass spectrometry. Three predominant reaction channels identified immediately above the ionization threshold are H, CO and HCO losses from the parent molecular ions, and isomeric effects are vividly manifested with respect to these channels. Signals corresponding to CH3CO+ ion, produced due to methyl migration, are displayed only by methacrolein. The observed isomeric effects are interpreted using the energies of the transition states and thermodynamic stabilities of the products predicted by calculation at DFT/B3LYP/6311++G(d,p) level.

Low-energy electron attachment to SF6. I. Kinetic modeling of nondissociative attachment

Low-energy electron-molecule collisions are analyzed by kinetic modeling within the framework of statistical unimolecular rate theory. Nondissociative electron attachment to SF(6) is used to illustrate the approach. An internally consistent representation is provided for attachment cross sections and rate coefficients in relation to detachment lifetimes, and both thermal and specific rate coefficients for detachment. By inspecting experimental data, the contributions of intramolecular vibrational redistribution and vibrationally inelastic collisions can be characterized quantitatively. This allows for a prediction of attachment rate coefficients as a function of electron and gas temperature as well as gas pressure over wide ranges of conditions. The importance of carefully controlling all experimental parameters, including the carrier gas pressure, is illustrated. The kinetic modeling in Part II of this series is extended to dissociative electron attachment to SF(6).

Low-energy electron attachment to SF6. II. Temperature and pressure dependences of dissociative attachment

Low-energy electron-molecule collisions, leading to dissociative attachment through metastable anionic states, are kinetically modeled within the framework of statistical unimolecular rate theory. The reaction e(-)+SF(6)-->SF(5)(-)+F is used as an illustrative example. The modeling is applied to new measurements of branching fractions for SF(5)(-) formation in the bath gas He between 360 and 670 K at 1 and 2 Torr, and between 490 and 620 K over the range of 0.3-9 Torr. The analysis of the data follows the previous kinetic modeling of the nondissociative electron attachment, e(-)+SF(6)-->SF(6)(-), from Part I of this series. Experimental results from the present work and the literature on branching fractions and total cross sections for anion formation as functions of electron energies, bath gas temperatures and pressures, as well as observation times are analyzed. The assumption of a participation of the electronic ground state of SF(6)(-) alone suffices to model the available experimental data. A value of the dissociation energy of SF(6)(-) into SF(5)(-)+F of E(0,dis)=1.61(+/-0.05) eV is determined, which may be compared to the electron affinity of SF(6), EA=1.20(+/-0.05) eV, such as derived in Part III of this series.

Velocity map imaging for low-energy electron–molecule collisions

The development of an experiment for velocity map imaging (VMI) of fragment ions arising from electron–molecule collisions is discussed. The angular distribution of O − from the dissociative attachment and dipolar dissociation of O 2 is measured and compared with the existing data. The details of the technique and its importance to electron–molecule collisions are discussed.

Correlated complex independent particle potential for calculating electronic resonances

We have formulated and applied an analytic continuation method for the recently formulated correlated independent particle potential [A. Beste and R. J. Bartlett J. Chem. Phys. 120, 8395 (2004)] derived from Fock space multireference coupled cluster theory. The technique developed is an advanced ab initio tool for calculating the properties of resonances in the low-energy electron-molecule collision problem. The proposed method quantitatively describes elastic electron-molecule scattering below the first electronically inelastic threshold. A complex absorbing potential is utilized to define the analytic continuation for the potential. A separate treatment of electron correlation and relaxation effects for the projectile-target system and the analytic continuation using the complex absorbing potential is possible, when an approximated form of the correlated complex independent particle potential is used. The method, which is referred to as complex absorbing potential-based correlated independent particle (CAP-CIP), is tested by application to the well-known (2)Pi(g) shape resonance of e-N(2) and the (2)B(2g) shape resonance of e-C(2)H(4) (ethylene) with highly satisfactory results.

Partitioned R-matrix theory for molecules

R-matrix calculations usually require all the eigenvalues and eigenvectors of the inner region Hamiltonian matrix. For molecular problems, particularly when large configuration interaction expansions are used for the target, the Hamiltonian matrix is often too large to be completely diagonalized. Berrington and Ballance (2002 J. Phys. B: At. Mol. Opt. Phys. 35 2275) proposed a partitioned R-matrix theory which only required a proportion of the solutions of the Hamiltonian matrix. This theory was implemented and tested in the atomic R-matrix code. The theory is adapted to the needs of R-matrix calculations on low-energy electron–molecule collisions. A number of alternative procedures are tested. The best is shown to give reliable results with explicit inclusion of only a fraction of the solutions. It is shown that with this revised theory the number of solutions required does not depend on the complexity of the target wavefunction even though this strongly influences the size of the final Hamiltonian matrix. This method will be implemented as part of the UK molecular R-matrix program suite.

Violation of frozen shell approximation in dissociative electron capture by halogenated anthraquinones.

A series of halogenated anthraquinone (AQ) derivatives has been studied by means of electron capture negative ion (NI) mass spectrometry (ECNI-MS). 1Cl-AQ and 2Br-AQ display dramatically steep positive temperature dependencies of Hal(-) ion abundance in the low electron energy region. Molecular NI intensity decreases rapidly with increasing temperature in the case of 1I-AQ. In the case of 2Br-AQ, a metastable NI peak (m/z 22.9) corresponding to the process BrAQ(-) --> Br(-) + AQ(0) was recorded. This means that the characteristic dissociation lifetime of the molecular NI Br-AQ(-) is at least approximately 25 micros at the energy approximately 0.67 eV in the low-temperature spectrum (T approximately 80 degrees C), and at the energy approximately 0.13 eV in the hot spectrum (T approximately 290 degrees C). Together with the observed temperature dependence of the 2Br-AQ curves of effective yield (CEY), this proves that this anion dissociates according to Coulson's model. The same halogen anion behavior is observed in the case of 1Cl-AQ. There are three consecutive stages in the process of molecular NI dissociation of Cl- and Br-substituted AQ, namely, electron capture into the empty pi-orbital by means of the shape resonance mechanism, followed by a radiationless transition into the ground electronic pi-state of the anion, as predicted by Compton in the case of the parabenzoquinone molecule, and, finally, a fluctuative dissociation of the molecular NI accompanied by the transition from the pi-term into the sigma-term, so-called predissociation. Calculations show reasonable agreement with the experimental data. In the case of 1I-AQ, an effect of inversion of empty levels in the process of electron capture by the molecule takes place, a violation of the so-called frozen shell approximation. The phenomenon found may be of significance not only in the case of ECNI-MS, but also in other experimental investigations using low-energy electron-molecule and ion-molecule collisions.

Frequency variation under constant power conditions in hydrogen radio frequency discharges

The effect of driving frequency (13.56–50 MHz) on the electrical characteristics and the optical properties of hydrogen discharges has been studied, under constant power conditions. The determination of the discharge power and impedance was based on current and voltage wave form measurements, while at the same time spatially resolved Hα emission profiles were recorded. As frequency is increased, the rf voltage required for maintaining a constant power level is reduced, while the discharge current increases and the impedance decreases. Concurrently the overall Hα emission intensity decreases and its spatial distribution becomes more uniform. Further analysis of these measurements through a theoretical model reveals that frequency influences the motion of charged species as well as the electron energy and the electric field, resulting in a modification of their spatial distribution. Moreover, the loss rate of charged species is reduced, leading to an increase of the plasma density and to a decrease of the electric field. Under these conditions, the total power spend for electron acceleration increases with frequency, but combined to the higher electron density, leads to a drop of the average energy gained per electron, a drop of the mean electron energy, and an enhancement of the low-energy electron-molecule collision processes against high energy ones.

Adiabatic treatments of vibrational dynamics in low-energy electron-molecule scattering

Accurate calculations of vibrational excitation cross sections for low-energy electron-molecule collisions require theoretical treatment of dynamical effects due to the vibrational kinetic energy operator. As alternatives to solving the integrodifferential equations that describe coupled electronic and vibrational motion, adiabatic methods, which parametrize the internuclear geometry, offer practical and conceptual simplifications. Here we investigate two such methods: the energy-modified adiabatic phase matrix method, which retains the vibrational kinetic energy in the fixed-nuclei scattering matrix and approximates the continuum energy, and the first-order non-degenerate adiabatic approximation, which evaluates fixed-nuclei scattering matrices off the energy-momentum shell in order to ensure strict conservation of energy. The present implementation of these methods is improved over previous versions. They are assessed against benchmark results from converged vibrational close-coupling calculations. Specifically, we compare integral and differential and cross sections for e- scattering at energies from their respective thresholds to 10 eV.

Threshold peaks in the vibrational excitation of molecules by electron impact: a time-dependent view

Resonance and threshold effects in low-energy electronmolecule collisions are analyzed in a time-dependent picture. Time-dependent wave packets reflecting the non-Markovian open-system dynamics of electron-molecule collision complexes are explicitly generated for two representative models of pures-wave scattering and dipole-modifieds-wave scattering, respectively. The numerical results demonstrate the relevance of the adiabatic potential-energy curves of resonances, virtual states and bound states for a qualitative understanding of the dynamics and lucidly reveal the existence of nonadiabatic processes in such systems.

Complex Kohn variational method: Application to low-energy electron-molecule collisions.

An algebraic variational method, based on application of the Kohn principle to the T matrix, is formulated for multichannel scattering problems and applied to the case of low-energy electron-molecule collisions. The method requires only Hamiltonian matrix elements and is anomaly-free. The present formulation is noteworthy in that it requires no bound-free nor free-free exchange matrix elements. The new technique is illustrated by application to e/sup -/+H/sub 2/ scattering in both the static-exchange and the two-state approximations. The method is found to be both efficient and accurate.

Electron-molecule scattering treated with the use of separable approximations for the nonlocal part of the interaction: Static exchange calculations for e-+H2 and e-+N2.

We have formulated and applied a method for solving the low-energy electron-molecule collision problem. This method treats the scattering by the local and nonlocal parts of the interaction differently. A differential equation method is used to obtain a solution of the set of coupled equations which describe the scattering by the (nonspherical) local potential. The T-matrix expansion method is used to account for the scattering by the nonlocal part of the interaction. The method is tested by application to low-energy e/sup -/-H/sub 2/ and e/sup -/-N/sub 2/ scattering in the static exchange approximation.

Excitation to symmetry forbidden vibronic levels in the 1B 2u state by controlled electron impact on benzene

The 1B 2u- 1A 1g fluorescence emission of benzene excited by controlled electron impact was measured in the 14 – 100 eV region. The optically forbidden 6° levels were preferentially, excited. This indicates that a non-dipole transition takes place under low-energy electron-molecule collisions.

The Physics of Low-energy Electron-Molecule Collisions: A Guide for the Perplexed and the Uninitiated

The essential physical features of low-energy electron-molecule scattering are described in a qualitative fashion. The context for this discussion is provided by the frame-transformation picture, which entails a 'partitioning' of the quantum scattering problem according to the relative importance of various physical interactions. This picture is then used as the basis for a qualitative overview of several contemporary theoretical techniques for solving the quantum scattering problem that are based on eigenfunction expansions of the system wavefunction and for representing the electronmolecule interaction potential. Finally, progress in three specific problem areas of recent interest is surveyed. The emphasis throughout is on non-resonant elastic scattering and ro-vibrational excitation.

Rotational and Vibrational Transitions in Low-Energy Electron-Molecule Collisions

Rotational and Vibrational Transitions in Low-Energy Electron-Molecule Collisions

Multichannel variational calculations of low-energy electron-molecule collisions: static exchange approximation

Kohn and inverse Kohn variational procedures are used to study the scattering of electrons by closed-shell diatomic molecules in the static exchange approximation. The trial wavefunctions are constructed from free particle, continuum static and bound orbitals. Variational corrections are obtained to the wavefunction as well as the reactance matrix. Various forms of the trial wavefunction are applied to collisions of electrons with H2, N2, LiH and CO, and the resulting eigenphases compares with exact static exchange results. For non-resonant symmetries, the method produces results within 10% of the exact results. Resonance widths and positions are predicted within 25% or better.