Session Q04: Electron-Atom/Molecule Collisions

Coherent control of nonradiative electron capture in electron-ion collisions: Exploiting Heisenberg's Uncertainty Principle and the projectile beam coherence.

The capture of an electron by a positively charged ion accompanied by photoemission is a well-established mechanism for radiative recombination in electron-ion collisions: the emitted photons take away the excess energy necessary for electron capture or stabilization. We propose a non-radiative mechanism by which the momentum transferred by the incident electron to the center-of-mass of the target ion results in a recoil of the latter, which then provides the necessary outflow of energy to ensure energy conservation and achieve nonradiative recombination. Motivated by recent experimental advances in tailoring photoelectron wave packets in ultrafast multiphoton ionization [1] as well as the observation of fingerprints of the projectile’s coherence in angle-resolved cross-section measurements [2] , we exploit this mechanism and propose a coherent and selective control scheme over competing collisional reaction channels based on the initial preparation of the projectile electron wave packet. We demonstrate that the coherence and relative phase velocity of amplitude-modulated free-electron wave packets generated in interfering multiphoton ionization channels, together with the position-momentum uncertainty associated with the center-of-mass of the target ion, can be exploited to induce quantum interferences among energetically degenerate asymptotic configuration channels, thereby achieving selective control over the collision outcome.

[1] Kerbstadt et al., Advances in Physics: X 4 (2019) 1672583.

[2] Egodapitiya et al. Rev. Lett. 106 (2011) 153202.   

Scattering Mechanisms in Twisted Electron Ionization

Ionization collisions have important consequences in many physical phenomena, and the mechanism that leads to ionization is not universal.  Understanding how and why electrons are removed from atoms and molecules is crucial to forming a complete picture of the physics.  Triple and double differential cross sections (TDCSs and DDCSs) and have been used for decades to examine the physical mechanisms that lead to ionization.  For ionization into the azimuthal plane, the TDCSs at low and intermediate energies exhibit unique qualitative features that can be used to identify single and double scattering mechanisms.  Similarly, angular DDCSs can be used to distinguish close collisions from grazing collisions.  With the recent development of sculpted electron wave packets, a new opportunity has arisen that allows a re-examination of the mechanisms that lead to ionization. We present theoretical TDCSs and DDCSs for (e,2e) ionization of atomic hydrogen and helium using electron vortex projectiles.  Our results show that the mechanisms leading to electron emission into the azimuthal plane are different for vortex and non-vortex projectiles.  Additionally, our results predict that the vortex projectile’s momentum uncertainty causes noticeable changes to the shape and magnitude of the TDCSs and DDCSs.  Our results lead to several predictions that can be experimentally tested.   

Electrometry of Charged Particles with Rydberg Atoms

Rydberg atoms, with their large polarizability, allow us for measurement of weak external electric fields. We measure the electric field of passing charged particles via the Stark shift in Rydberg Rubidium atoms. In addition, low frequency and DC-electric field strength from an electron beam are detected via electromagnetically induced transparency (EIT). Moreover, by measuring Stark shift, we plan to map the trajectory of electrons passing through a Rubidium vapor in a Rydberg state. Our prototype will be implemented on the beamline at the Jefferson laboratory as a novel technique for tracking charged particles.   

Electron Impact Excitation of Extreme Ultra-Violet Transitions in Xe7+–Xe10+ Ions

We have presented a comprehensive study on electron impact excitation of xenon ions (Xe⁷⁺, Xe⁸⁺, Xe⁹⁺ and Xe¹⁰⁺) for the dipole allowed (E1) transitions in the extreme Ultra-Violet region of 8—19nm. We have implemented the multi-configuration Dirac–Fock (MCDF) method for the atomic structure calculation including the Breit and quantum electrodynamic (QED) corrections along with the relativistic configuration interaction. Present results for energy levels, wavelengths and transition rates are compared with previously reported experimental and theoretical results. Further, we have used the relativistic distorted wave method to calculate the cross sections from the excitation threshold to 3000 eV projectile electron energy. We have obtained the fitting parameters of these calculated cross sections using two different formulae for low and high energy ranges. The rate coefficients are also obtained using the calculated cross sections by considering the Maxwellian electron energy distribution function in the electron temperature range from 5 eV to 100 eV.   

Cross sections for vibrational excitation of H2O by electron impact: benchmark system and manual for Quantemol-EC

Cross sections and thermally averaged rate coefficients for vibration (de-)excitation of a water molecule by electron impact are computed. One and two quanta excitations are considered for all three normal modes. The calculations use a theoretical approach that combines the normal mode approximation for vibrational states of water, a vibrational frame transformation employed to evaluate the scattering matrix for vibrational transitions and the UK molecular R-matrix code for electron-scattering calculations. The interval of applicability of the rate coefficients is from 10 to 10,000K. The developed methodology for computing non-resonant vibrational excitation cross-sections employed here has recently been incorporated, with some simplifications, into the QEC (Quantemol Electron Collisions) expert system used to run the new (UKRmol+) UK Molecule R-matrix code. Comparison of results obtained using the complete and QEC approaches are discussed. Recommendations for QEC users are developed.   

An investigation of the radiative electron attachment of C3N through dipole-bound states

Over two hundred molecules have been detected in the interstellar medium (ISM), only six of which are negatively charged.

Although their formation may not yet be well understood, several formation mechanisms have been proposed.

One such pathway is radiative electron attachment (REA), the process by which an electron encounters a neutral molecule and forms a stable anion through photon emission.

Previous calculations have shown that REA rates are too low to explain the observed abundance of CN^-,  C₃N^-, and C₅N^-.

REA through weakly bound dipole states, or dipole-bound states (DBSs), has recently been proposed to enhance REA rates and explain the observed abundance of anions in the case of a neutral counterpart with a supercritical dipole moment, such as C₃N.

Although the effects of the rotation and vibration of polar molecules on DBSs have been studied in the past, no prior fully quantum treatment has considered rotation and short-range electron-molecule interactions.

Doing so in the case of C₃N/C₃N^- is the purpose of this study.

The rotationally selected DBSs of C₃N are computed using Close Coupling calculations and an ab initio local model of the interaction potential.

The results are compared to those theoretical results obtained by the simple point-charge-dipole potential and to experimental data.

The rotationally selected REA cross-section through C₃N^- DBSs are then calculated by a quantum approach and are found to still be too small to explain the abundance of this anion in the ISM.

    

Dissociative Electron Attachment and Fragment Angular Distribution of NO2 molecule

We present the general results of our theoretical study of the dissociative electron attachment to NO2. Our calculated angular distributions are based on the analysis of the molecular-frame entrance amplitude for electron at- tachment obtained from the results of complex Kohn scattering calculations using the axial recoil approximation. We compare our results with available experimentally measured angular distributions.   

An Alternative Theoretical Approach to Ionization and Dissociation in the H2 Gerade System

We present a flexible numerically solvable two-dimensional (with one nuclear and one electronic degree of freedom) model describing the collision of an electron with the H₂⁺ molecule (and its isotopologues) in the gerade symmetry [arXiv:2112.10820 (2021)]. Constructed in a way reminiscent of the previous quantum defect work of Jungen and Ross [Phys. Rev. A 49, 4353 (1994)], the model is comprised of three coupled two-dimensional channels in the space of s, p, d partial waves of the incoming electron. We show how this model reproduces the Born-Oppenheimer properties of the H₂ molecule in the relevant range of internuclear distances and how it can be applied to describe processes such as (ro-)vibrational excitation and dissociative recombination. Previous theoretical studies of H₂⁺ and HD⁺ dissociative recombination employ the frame transformation theory and the MQDT quasidiabatic theory and include various physically motivated approximations. The numerical solution of our model is suitable as an alternative theoretical tool to study these processes without said approximations. We use it to compute rovibrationally inelastic cross sections and dissociative recombination cross sections and compare with the data of previous studies.