Session CT13: Collisions Involving Molecules

Student Excellence Award Finalist: Critical review of the associative detachment reaction of molecular nitrogen with the anion of atomic oxygen for accurate modeling of nonthermal gas discharges and transient luminous events

The associative detachment reaction of molecular nitrogen, N₂, with the anion of atomic oxygen, O^-, has been included in several theoretical studies of nonthermal gas discharges, and is considered the basis of some recent findings with regards to transient luminous events [e.g., Luque and Gordillo-Vazquez, Nat. Geosci., vol. 5, no. 1, pp. 22-25, 2012]. This process was introduced to explain the negligible electron attachment observed in current growth experiments in air (as opposed to pure molecular oxygen, O₂) [Moruzzi and Price, J. Phys. D: Appl. Phys., vol. 7, no. 10, pp. 1434-1440, 1974, and references therein]. Specifically, it was suggested that O^- ions produced during a gas discharge in air, release the electron due to subsequent collision with N₂ molecules [Moruzzi and Price, 1974]. Hopper et al., [J. Chem. Phys., vol. 65, no. 12, pp. 5474-5494, 1976] demonstrated that this mechanism should occur only if N₂ is excited to at least the first vibrational level. On the contrary, Rayment and Moruzzi [Int. J. Mass Spectrom. Ion Phys., vol. 26, no. 3, pp. 321-326, 1978] interpreted their experimental results in terms of an associative detachment reaction between O^- and unexcited N₂. Here, we model the experiment in [Rayment and Moruzzi, 1978], identify errors in the theoretical description of that experiment, and provide an alternative interpretation of measurements in [Rayment and Moruzzi, 1978], which includes vibrationally excited N₂, exclusively. Further controlled experiments are required to reconcile the literature on the reaction of O^- with ground state N₂.   

Vibrational Excitation of Molecule Oxygen During Recombination of O Atoms

Vibrationally excited oxygen is of critical importance in nonequilibrium high-enthalpy flows, encountered behind hypersonic shock waves. To date, there are few direct experimental verifications of vibrational state resolved dissociation rates. We present a framework where state specific recombination rates can be inferred from the time-resolved measurements of O₂ vibrational populations, such that the state specific dissociation rates can be obtained from the detailed balance. In the present approach, recombination reactions of atomic oxygen are monitored after a ns pulse discharge burst in an O₂-Ar mixture. The time evolution of O₂(v) populations in the mixture are measured by ps Laser LIF in the Schumann-Runge bands. Levels from v”=7 to 21 have been detected. TALIF at 226 nm has been used to measure the atomic oxygen concentration. Within ∼1 ms after the discharge burst, a rapid decay of O₂(v) is observed, indicating V-V and V-T relaxation of vibrational states populated by electron impact and by quenching of the excited Ar. After the rapid initial decay, the vibrational populations level off and remain nearly constant, or exhibit a transient rise, on the timescale of about 10 ms, suggesting the presence of a persistent source of vibrational excitation from chemical reactions.    

A Semi-Classical Model for Computing Vibrationally-Resolved Electron-Impact Ionization Cross Sections.

A semi-classical model is presented for computing vibrationally-resolved electron-impact ionization cross sections for molecular nitrogen. The model extends the approach used by Wünderlich for molecular hydrogen. The multi-reference configuration interaction (MRCI) method is used with complete active space self-consistent field (CASSCF) reference wavefunctions in Molpro to compute the required electronic wavefunctions and potential energy curves (PECs). The target electron kinetic energy and the transition energy model parameters are calculated directly from the computed electronic wavefunctions. The partial cross sections for a given state-to-state, vibrationally-resolved ionization (p', v') to (p'', v'') are quantified using Franck-Condon Factors (FCFs), which are computed from the nuclear wavefunctions obtained from the Fourier Grid Hamiltonian method using the MRCI PECs. Total cross sections for a given (p', v') state are presented as a summation over the vibrational quantum number v'' of the partial cross sections (p', v') to (p'', v''), where a simplified closure relation for FCFs sets the maximum required v''. The total cross sections for several transitions are compared to literature values.   

Identification of Siegert states in molecular scattering calculations and application to analysis of dissociative electron attachment

Formation of scattering resonant and virtual states (Siegert states) are one of the most striking phenomena of quantum physics. Those states appear as poles of the S-matrix and are solutions of the Schrödinger equation for complex energies. Siegert states localized in the vicinity of the real energy axis cause a dramatic modification of the cross section for various processes including impact dissociation. Identification of such narrow resonances can be done straightforwardly, e.g. by inspecting the eigenhpase sums. However, identification of virtual states and broad resonances is typically more difficult, especially in multi-electron ab initio scattering calculations which do not lend themselves easily to calculations for complex energies. Our inability to identify unambiguously all Siegert states in ab initio data has led to disputes in the community concerning the mechanism of dissociative electron attachment in complex molecules such as HCOOH.

In this talk I will describe a general method for localization and analysis of Siegert states in multi-electron molecules. The method is based on the R-matrix division of space [1] which enables a numerically stable and computationally efficient search for the exponentially diverging resonant and virtual states and the very diffuse weakly bound dipole-bound states. We show that all Siegert states in a selected part of the complex plane can be readily localized, including broad resonances. Finally, we study Siegert states as a function of geometry in electron collisions with a series of molecules (N₂, HNCO, HCOOH, pyrrole). We identify near-threshold dipole-bound or virtual states as the source of the so-called ``σ* mechanism" in dissociative electron attachment to polar molecules.

[1] L.A. Morgan and P.G. Burke, J. Phys. B: At. Mol. Opt. Phys. 21, 2091 (1988).   

Fragmentation dynamics of clusters induced by heavy ion impacts

Dimers, consisting of two noble atoms, atom-molecule or two molecules, are weakly bound by Van der Waals bond or hydrogen bond and ideal model systems to understand the damage dynamical processes in bio-environments induced by energetic particles. When a dimer is ionized by ion, photon or electron impact, due to the presence of neighboring molecules, some forbidden relaxation channels in isolated monomer may be open. Interatomic Coulombic decay (ICD) [1] is a typical process which consumes excess energy of the ionized system and has been confirmed by experiments [2]. The fragmentation related to ICD mainly involve the virtual photon or electron exchange between the dimer components. Two new cases will be presented that involve massive ion and charge migration over large distances within a cluster resulting in fragmentations. The first case is hydrogen bonded acetylene dimer (C₂H₂)₂ irradiated by 200 keV alpha particles. The electronic and nuclear relaxation of the doubly charged acetylene dimer is accomplished via a novel mechanism involving intermolecular proton transfer [3] or the ICD. The two mechanisms trigger fast Coulomb explosion of the dimer due to creation of charge-separated states. The second case deals with a Van der Waals cluster (N₂Ar) which is doubly ionized by 1 MeV ions Ne⁸⁺. The dissociation of the dimer ion (N₂Ar)²⁺ proceeds via an exotic process of heavy ion N⁺ transfer and NAr⁺ ion formation [4]. Due to the presence of neighboring Ar atom in the dimer the tunneling of N⁺ ion from molecule ion N₂²⁺ becomes accessible, as a result, the breakup of the covalent N⁺-N⁺ bond, and the tunneling of N⁺ ion and the formation of N⁺-Ar bond occur at the same instant, and consequently, Coulomb explosion of N⁺ and NAr⁺ ion pairs comes after immediately. These newly observed ion transfer mechanisms may be general in biochemical environments and have potential importance in understanding the irradiation damage caused by energetic particles, and furthermore, may directly affect the expression of DNA and protein in vivo.

Collaborators: X L Zhu, S Xu, S C Yan, and S F Zhang, IMPCAS and UCAS; X Q Hu, Y Wu and J G Wang, IAPCM, Beijing, China; L S Cederbaum and N V Kryzhevoi, Universität Heidelberg, Germany   

Determining electron-biomolecule cross sections using data-driven swarm analysis

Accurate modeling of electron transport through biological media requires the attainment of complete and accurate sets of cross sections for all electron interactions with all relevant biomolecules in the soft-condensed phase. Swarm experiments provide a unique way to assess the self-consistency of such cross section sets. However, when swarm transport measurements are limited in number, the inverse problem of unfolding their underlying cross sections becomes ill-posed. To account for the uncertainty inherent to this "inverse swarm problem", we employ a neural network model that is trained upon sets of cross sections from the LXCat project alongside corresponding transport coefficients found by the numerical solution of Boltzmann's equation. We apply this machine learning approach to measurements from the pulsed-Townsend swarm experiments of de Urquijo and co-workers and subsequently derive plausible neutral dissociation and dissociative electron attachment (DEA) cross sections for the biomolecule analogues tetrahydrofuran (THF) and α-tetrahydrofurfuryl alcohol (THFA).