Session M05: Electron-molecule and atom-molecule collisions

Live

We report theoretical molecular 3-body distorted wave (M3DW) fully differential cross sections for electron impact ionization of several DNA analogue molecules.~These results are useful to model the role of electrons in causing damage to DNA in biological systems [1]. Since experimental measurements do not determine the orientation of the molecule at the time of ionization, theoretical calculations must average over all possible orientations.~ Owing to computational limitations, our earlier work introduced the orientation averaged molecular orbital (OAMO) approximation, but the overall agreement with experiment was not very good [2]. While OAMO seemed to work better for larger molecules, it did not predict the detailed structures seen in the data. Our current treatment, however, improves the method where the cross section is calculated for a number of different molecular orientations and then numerically averaged. This proper averaging technique yields much better agreement with experiment [3].~ We will show results and comparisons for several DNA analogue molecules in the conference. [1] Sanz \textit{et al.}, \textit{Int. J. Radiat. Biol.} \textbf{88}, 71 (2012); [2] Gao \textit{et al.}, \textit{J. Chem. Phys.} \textbf{123}, 204302 (2005); [3] Esam Ali, H. S. Chakraborty, and D. H. Madison (submitted).   

Live

Electron collisions with molecules are vital for modelling planetary atmospheres as well as interstellar and industrial plasmas. Reliable data for molecules of interest is extremely sparse and, to date, very few of these molecules have been benchmarked. We present benchmark integrated and differential cross-sections for electron collisions with molecular hydrogen. These calculations are of practical importance, especially for excited electronic states that can be intrinsically difficult to resolve experimentally. We compare the well established \emph{ab initio} R-matrix method with the newly developed Molecular Convergent Close-Coupling (MCCC) approach. Owing to new developments in the UKRMol+ code [1], such as the addition of a mixed B-spline and Gaussian-type orbital continuum, we have been able to push the boundaries of our previous R-matrix calculations to produce accurate cross-sections. Our model uses an R-matrix sphere of 100 Bohr -- the largest ever used -- to contain the diffuse excited states of H$_2$, and a large number of outer region channels (1,500 per symmetry). The results show good agreement with the MCCC results [2] and experimental data.\\$[1]$ Z. Ma\v{s}\'{i}n \emph{et al.}, CPC 249, 107092 (2020).\\$[2]$ M. C. Zammit \emph{et al.}, PRA 95, 022708 (2017).   

Live

Despite computational methodologies designed to model atomic and molecular inelastic collisions being widely available nowadays, these are yet restricted to small systems such as atom--diatom and diatom--diatom within the time-independent quantum mechanical approach. Moreover, recent advances in AMO physics brought to attention physical conditions (i.e. ultracold collisions, light--assisted excited colliding partners, high atomic densities etc.) and heavy atomic/molecular species (often also charged) that are nearly numerically intractable. In particular, after steady advances in the early 90's, the atom--triatom case is yet lacking a full quantum mechanical approach. In this work we propose a time--independent formalism, based on a set of arrangement--fixed Jacobi coordinates, to model collisions and scattering of an atom by a triatomic target, accounting for all degrees of freedom. Numerical results are presented for the case study H$_{2}$O + H.   

Live

Vibrational Stark effect (VSE) spectroscopy allows for direct measurement of electric fields in biological systems, such as proteins, by utilizing a carbonyl or nitrile group as a vibrational probe. Because the probe's molecular vibrations are sensitive to noncovalent interactions of the environment, VSE spectroscopy provides an unique way to test the accuracy of electrostatic interactions in computational models. Here, we present research to address the challenges of quantifying electrostatic interactions as a thermodynamic average. Using realistic finite-temperature simulations, we quantify the relative electrostatic contributions of residues surrounding ketosteroid isomerase, and subsequently elucidate how inter-residue charge transfer as well as local and non-local polarization effects influence the electric field at the position of a molecular probe. In particular, we demonstrate that the inclusion of polarization effects and charge transfer are essential for a computational model to capture the correct thermodynamic structural average in comparison to experimental VSE spectroscopy.   

Live

We have measured the sum of center of mass (c.m.) frame kinetic energies of H$+$, H$+$ and H- from the collision induced three-body dissociation of $H_{3}^{+} $with He. This was accomplished by measuring the individual laboratory energies and scattering angles of the fragment ions that originate from a single excitation and dissociation event using triple coincidence techniques [1]. These quantities were then transformed into the frame of the fast-moving projectile. We find the rather startling result that the sum of projectile frame energies of the dissociation fragments, form discrete sets that are associated with the quantized rotational states of $H_{3}^{+} $. We also find that the observed sets are dominated by ortho states with angular momentum quantum numbers J$=$1,3,5, although para states are present. Additionally, our data reveal multiple sets of triple coincidence events that form patterns in the c.m. frame energy interval 4.5 eV to 10eV. We describe physical mechanisms that occur in the intermediate state of doubly excited $H_{3}^{+} $that produce these correlated sets. [1] L.M. Wiese, et al., Phys. Rev. Lett. 79, 1997 (4982).   

Live

Accurate measurements of the shapes of molecular resonances provide information about molecular dynamics and validate the potential energy surfaces for various collisional systems. This is due to the fact that the collision-perturbed velocity distribution of the optical coherence manifests itself as the perturbation of the shape of such resonance. We present a theoretical description of this process, using state-of-the-art potential energy surfaces and quantum scattering calculations for diatom-atom and diatom-diatom systems. Not only does this approach properly describe the internal and translational motions of the molecules, but also correlations between them. This results in the subpercent agreement between the calculated and measured spectral line profiles. These theoretical developments are important for reducing systematic errors in optical metrology based on molecular spectroscopy (for instance, they allow for more accurate determination of rovibrational splitting in molecular hydrogen and, hence, for accurate tests of quantum electrodynamics for molecules). Accurate theoretical models of the collision-perturbed molecular spectra will be used for populating line-by-line spectroscopic databases and providing reference spectra for the studies of planetary atmospheres.   

Live

Identification of resonances in computed cross sections is one of the main goals of ab initio scattering calculations. A common approach involves fitting of the data computed for real scattering energies to the Breit-Wigner form (in case of eigenphase sums) or the Lorentzian form (in case of time-delays), see e.g. [1-2]. While this approach works well for narrow resonances, it becomes unreliable especially for very wide resonances and core-excited resonances appearing in electronically inelastic calculations. In this contribution we resurrect an approach for localization of resonant S-matrix poles using the diatomic R-matrix method [3]. The exact energy factorization of the R-matrix approach is ideally suited for finding the Siegert solutions of the Schr\"{o}dinger equation in the complex plane. We describe our implementation within the polyatomic UKRmol+ codes, study its numerical properties and apply it to localization of resonances in low-energy electron collisions with pyrrole including their dependence on nuclear geometry.\\ \noindent [1] Z. Ma\v{s}\'{\i}n and J.D. Gorfinkiel, J. Chem. Phys. 137, 204312 (2012).\\ \noindent [2] D.A. Little, et al, Comp. Phys. Comm. 215, 137 (2017).\\ \noindent [3] L.A. Morgan and P.G. Burke, J. Phys. B: At. Mol. Opt. Phys. 21, 2091 (1988).   

On Demand

Electron collisions with molecular hydrogen and its isotopologues are ubiquitous throughout the Universe and are particularly important in the modeling and analysis of fusion plasmas. To model such plasmas, applications require a complete set of cross sections that are accurate, and resolve the initial and final states of the molecule's electronic and vibrational state. While the need for such electron collision data exists, there are at present no available sets of collision data which include rovibrational sublevels, transitions between excited states, and isotope effects. In addition, widely-used sets of collision data for H$_2$ have many transitions that are in significant disagreement, in some cases differing by an order of magnitude. Recently the convergent close-coupling (CCC) method has been applied to electron scattering from H$_2$, demonstrating convergence of the elastic, excitation and ionization cross sections over a wide-range of electron impact energies. An important development of this approach was the spheroidal formulation, which allows the calculation of accurate collision data over a large range of internuclear separations. Here we present a complete set of CCC cross sections for electron scattering from the vibrationally excited states of H$_2$.   

On Demand

The exact treatment of dissociative electron attachment (DEA)$^1$ to a polyatomic molecule is computationally challenging when the target molecule has more than one vibrational mode. The local approximation (or the “boomerang model”) has been applied to a few polyatomic molecules in the past. However, in many situations, particularly at low electron energies, the local approximation breaks down, and DEA cross sections should be calculated using the non-local complex potential (NLCP)$^2$ theory. In this report we develop the NLCP theory with inclusion of more than one mode of vibrations in the target molecule. In addition to the full quantum treatment, we also develop a semiclassical approach to computing multimode DEA cross section by expressing the electron capture amplitudes and the matrix elements of the Green's function by their semiclassical approximations. We then apply these theories to a generic molecule of the type CY$_3$X and include only the symmetric C-X and CY$_3$ deform (''umbrella") vibrations. Finally, we present and compare the DEA cross sections for CF$_3$Cl obtained via both non-local and semiclassical approaches. $^1$I. I. Fabrikant \emph{et al}., Adv. At., Mol., Opt. Phys. 66, 545 (2017). $^2$ J. N. Bardsley, J. Phys. B: At. Mol. Phys. 1, 349 (1968).   

On Demand

A theoretical approach for the electron-impact vibronic excitation and dissociative recombination of molecular ions with low-lying excited electronic states is described. In this approach, the fixed-nuclear R-matrix method is employed to compute electron-ion scattering matrices and a vibronic frame transformation combined to the closed-channel elimination procedure are employed to construct an energy-dependent scattering matrix describing interactions between vibronic channels of the target ion induced by the incident electron. The approach is applied to CH$^{\mathrm{+}}$ ion of an astrophysical and technological interest. Cross sections for vibronic excitation for different combinations of initial and final vibronic states and dissociative recombination are computed, accounting for Rydberg series of vibronic resonances in the collisional spectrum. A good agreement between electronic-excitation cross sections, obtained using the quantum defect theory and in a direct R-matrix calculation, demonstrates that the present approach provides a reliable tool for determination of vibronic (de-)excitation and dissociative recombination cross sections for targets with low-energy electronic resonances.