Abstract. The dissociative electron attachment cross sections for the methyl halides vary in an enormous range from the virtually unmeasurable 10 −23 cm 2 for CH 3 Cl at room temperature to 10 −14 cm 2 for CH 3 I. In this paper we supplement our previous studies by calculations of dissociative electron attachment to CH 3 Br and compare results for all methyl halides studied so far. The rate as a function of temperature for CH 3 Cl and CH 3 Br exhibits an exponential dependence on 1/T (Arrhenius law) with the activation energy lower for CH 3 Br. CH 3 I does not obey the Arrhenius law since the crossing point of the neutral and anion potential curves occurs near the lowest vibrational levels. The cross section as a function of electron energy for all of the methyl halides studied here exhibits structure at the vibrational excitation thresholds that is associated with a vibrational
Abstract. We calculate elastic scattering of positronium (Ps) by the Xe atom using the recently developed pseudopotential method Phys. Rev. A 90 052717] and review general features of Ps scattering from heavier rare-gas atoms: Ar, Kr and Xe. The total scattering cross section is dominated by two contributions: elastic scattering and Ps ionization (break-up). To calculate the Ps ionization cross sections we use the binary-encounter method for Ps collisions with an atomic target. Our results for the ionization cross section agree well with previous calculations carried out in the impulse approximation. Our total Ps-Xe cross section, when plotted as a function of the projectile velocity, exhibits similarity with the electron-Xe cross section for the collision velocities higher than 0.8 a.u., and agrees very well with the measurements at Ps velocities above 0.5 a.u.
We apply a semiempirical R-matrix theory to calculations of vibrational excitation and dissociative attachment in the CF 3 Cl molecule for electron energies below about 3 eV. We employ two sets of model parameters corresponding to two different forms of the CF 3 Cl − potential curve. We fi nd that our present, ab initio calculated anion curve gives vibrational excitation and dissociative attachment cross sections in good agreement with experimental measurements. We also compare the results of our theory with those of a recently published classical theory.
The absolute cross section for dissociative electron attachment to CH 3 Cl embedded within a solid Kr lattice has been measured and calculated. The experimental peak value of the cross section (5ϫ10 Ϫ17 cm 2 ) is four to six orders of magnitude larger than the corresponding peak value for gaseous CH 3 Cl. The shape and magnitude of the energy dependence of the cross section is very well reproduced by resonance R-matrix theory using the polarization energy as the only adjustable parameter. ͓S0163-1829͑97͒51734-6͔
Positronium (Ps) collisions with molecular hydrogen are investigated theoretically. Elastic and Ps ionization cross sections are calculated. For elastic scattering the pseudopotential method, previously developed for rare-gas atoms, is applied. Ps ionization cross sections are calculated using the binary-encounter approximation. The results agree with swarm measurements at low collision energies and with beam measurements at higher energies. The total Ps-H 2 cross section when plotted as a function of collision velocity is close to the e −-H 2 cross section at velocities above the Ps ionization threshold confirming earlier observations [S. J. Brawley et al., Science 330, 789 (2010)]. However, below the threshold the two sets of cross sections are different because of the different nature of the long-range interaction between the projectile and the target, the polarization interaction in the case of e −-H 2 collisions and the van der Waals interaction in the case of Ps-H 2 collisions.
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