Interaction of molecular nitrogen with Free-Electron-Laser radiation

We compute molecular continuum orbitals in the single center expansion scheme. We then employ these orbitals to obtain molecular Auger rates and single-photon ionization cross sections to study the interaction of N2 with Free-Electron-Laser (FEL) pulses. The nuclei are kept fixed. We formulate rate equations for the energetically allowed molecular and atomic transitions and we account for dissociation through additional terms in the rate equations. Solving these equations for different parameters of the FEL pulse, allows us to identify the most efficient parameters of the FEL pulse for obtaining the highest contribution of double core hole states (DCH) in the final atomic ion fragments. Finally we identify the contribution of DCH states in the electron spectra and show that the DCH state contribution is more easily identified in the photo-ionization rather than the Auger transitions

Low and intermediate energy electron collisions with the C2−_2^- molecular anion

Calculations are presented which use the molecular R-matrix with pseudo-states (MRMPS) method to treat electron impact electron detachment and electronic excitation of the carbon dimer anion. Resonances are found above the ionisation threshold of C$_2^-$ with $^1\Sigma^+_g$, $^1\Pi_g$ and $^3\Pi_g$ symmetry. These are shape resonances trapped by the effect of an attractive polarisation potential competing with a repulsive Coulomb interaction. The $\Pi_g$ resonances are found to give structure in the detachment cross section similar to that observed experimentally. Both excitation and detachment cross sections are found to be dominated by large impact parameter collisions whose contribution is modelled using the Born approximation.Comment: 18 pages, 5 figures constructed from 8 file

Electron-impact rotational and hyperfine excitation of HCN, HNC, DCN and DNC

Rotational excitation of isotopologues of HCN and HNC by thermal electron-impact is studied using the molecular {\bf R}-matrix method combined with the adiabatic-nuclei-rotation (ANR) approximation. Rate coefficients are obtained for electron temperatures in the range 5$-$6000 K and for transitions among all levels up to J=8. Hyperfine rates are also derived using the infinite-order-sudden (IOS) scaling method. It is shown that the dominant rotational transitions are dipole allowed, that is those for which $\Delta J=1$. The hyperfine propensity rule $\Delta J=\Delta F$ is found to be stronger than in the case of He$-$HCN collisions. For dipole allowed transitions, electron-impact rates are shown to exceed those for excitation of HCN by He atoms by 6 orders of magnitude. As a result, the present rates should be included in any detailed population model of isotopologues of HCN and HNC in sources where the electron fraction is larger than 10$^{-6}$, for example in interstellar shocks and comets.Comment: 12 pages, 4 figures, accepted in MNRAS (2007 september 3

Electro collisions with molecular nitrogen in its ground and electronically excited states using the R-matrix method

A comprehensive study of electron collisions with the X1Σg+ ground state as well as the metastable A3Σu+ and a1Πg excited states of the N2 molecule is reported using the fixed-nucleus R-matrix method. Integral elastic scattering and electronic excitation cross sections from the X1Σg+ ground state to the eight lowest electronic states, A3Σu+, B3Πg, W3Δu, B'3Σu−, a1Πg, a'1Σu−, w1Δu and C3Πu, overall agree well with the available experimental and theoretical results although updates of some recommended values are suggested. Accurate electron impact electronic transition cross sections starting from the A3Σu+ and a1Πg metastable excited states are reported. The total summed electronic transition cross sections from the a1Πg state is dominant: an order of magnitude higher than those of the X1Σg+ ground state. The de-excitation cross sections generally show a downward trend with increasing incident electron energy, which is different from the elastic and electronic excitation cross sections which generally increase with collision energy. There is a prominent 2Πu symmetry resonance peak at 2.8 eV for electronic de-excitation scattering of a1Πg → B3Πg, which significantly contributes to the total summed cross sections from the a1Πg excited state. The present results provide a new insight which will aid understanding of electron spectra in the atmosphere of the Earth and Titan

Low temperature scattering with the R-matrix method: the Morse potential

Experiments are starting to probe collisions and chemical reactions between atoms and molecules at ultra-low temperatures. We have developed a new theoretical procedure for studying these collisions using the R-matrix method. Here this method is tested for the atom -- atom collisions described by a Morse potential. Analytic solutions for continuum states of the Morse potential are derived and compared with numerical results computed using an R-matrix method where the inner region wavefunctions are obtained using a standard nuclear motion algorithm. Results are given for eigenphases and scattering lengths. Excellent agreement is obtained in all cases. Progress in developing a general procedure for treating ultra-low energy reactive and non-reactive collisions is discussed.Comment: 18 pages, 6 figures, 3 tables, conferenc

Elastic electron scattering by thermal mixture of glycine conformers in gas phase

A theoretical study of electron scattering by a thermal mixture of glycine molecules in the energy range from 1 to 10 eV is performed using the UK-RMol codes which are based on the R-Matrix method. The six lowest relative Gibbs free energies glycine conformers considered, Ip, IIp, IIn, IIIp, IIIn and IVn, are significantly populated in thermal mixtures. All these conformers present similar resonance structures in the eigenphase sums: a lower-energy resonance state near 1.8 eV and another at higher-energy above 7 eV. For the six conformers the lowest resonance lies between 1.75 eV and 2.21 eV. The very large dipole moments of 6.32 D and 5.67 D for IIp and IIn, respectively, makes the magnitude of their cross sections significantly larger than other conformers, which increases the average cross sections in thermal mixtures compared with the cross sections of the lowest energy Ip conformer. Three conformer population sets are used to calculate the averaged differential and integral cross sections: two theoretical sets based on the the relative Gibbs free energies and another set that aims to mimic experiment based on the observed populations. The averaged cross sections are similar for all population sets, but differ from the Ip conformer cross section. This suggests that, for large and flexible molecules, the computed average cross sections should be used when comparing with experimental data

Calculation of rate constants for vibrational and rotational excitation of the H+(3) ion by electron impact

We present theoretical thermally averaged rate constants for vibrational and rotational (de-)excitation of the H+(3) ion by electron impact. The constants are calculated using the multichannel quantum-defect approach. The calculation includes processes that involve a change |delta J| \u3c 2 in the rotational angular momentum J of H+(3). The rate constants are calculated for states with J \u3c 5 for rotational transitions of the H+(3) ground vibrational level. The thermal rates for transitions among the lowest eight vibrational levels are also presented, averaged over the rotational structure of the vibrational levels. The conditions for producing non-thermal rotational and vibrational distributions of H+(3) in astrophysical environments are discussed

Uncertainty Estimates for Theoretical Atomic and Molecular Data

Sources of uncertainty are reviewed for calculated atomic and molecular data that are important for plasma modeling: atomic and molecular structure and cross sections for electron-atom, electron-molecule, and heavy particle collisions. We concentrate on model uncertainties due to approximations to the fundamental many-body quantum mechanical equations and we aim to provide guidelines to estimate uncertainties as a routine part of computations of data for structure and scattering.Comment: 65 pages, 18 Figures, 3 Tables. J. Phys. D: Appl. Phys. Final accepted versio

Electron scattering on molecular nitrogen: common gas, uncommon cross sections

We discuss peculiar features of electron scattering on the N2 molecule and the N2+ ion, that are important for modeling plasmas, Earth’s and other planets’ atmospheres. These features are, among others: the resonant enhancement of the vibrational excitation in the region of the shape resonance around 2.4 eV, the resonant character of some of electronic excitation channels (and high values of these cross sections, both for triplet and singlet states), high cross section for the dissociation into neutrals, high cross sections for elastic scattering (and electronic transitions) on metastable states. For the N2+ ion we discuss both dissociation and the dissociative ionization, leading to the formation of atoms in excited states, and dissociative recombination which depends strongly on the initial vibrational state of the ion. We conclude that the theory became an indispensable completion of experiments, predicting many of partial cross sections and their physical features. We hope that the data presented will serve to improve models of nitrogen plasmas and atmospheres. Graphical abstract: [Figure not available: see fulltext.]

Why SF6 eats electrons: identifying high electrical strength molecules from their electron collision properties

The electrical strength of a molecule is a measure of its ability to act as an insulator and to absorb electrons. SF6 is a high electric strength gas. This work tries to explain why molecules like SF6 have a high electrical strength from the perspective of electron molecule scattering. The presence of a very low energy (<<1 eV) totally symmetric state in form of a very low-lying resonance, virtual state or very weakly bound state appears to be crucial. R-matrix calculations performed at the static exchange plus polarisation (SEP) level are performed for a number of molecules that show a range of electrical strengths. SEP calculations suggest that SF6 has a strong low energy 2Ag resonance feature which becomes a weakly bound state as more virtual orbitals are included in the calculation. High electrical strength molecules such as CCl4, CCl3F and CCl2F2 also have a totally-symmetric low-energy resonance, while the low electrical strength molecules such as CH4, CH3F...CF4 and CClF3, do not show any resonance behaviour in the low energy region. It is suggested that this low energy feature can be used as an indicator when searching for new molecules with high electrical strength facilitating searches for new gases which could provide an alternative to SF6