Modelling low-energy electron–molecule capture processes

Abstract

Cross sections and rate coefficients for capture of low-energy electrons with polar and polarizable target molecules are calculated in the framework of Fabrikant and Hotop's extended version of the Vogt-Wannier model and an extension of this approach is given in the present article. Analytical approximations are derived in order to facilitate the application to experiments. A comparison with a selection of experimental electron attachment rate coefficients provides insight into the competition between anion formation through electron capture and scattering processes which do not follow this pathway.

Observation of enhanced rate coefficients in the H2++H2→H3++H reaction at low collision energies

“…On the basis of the predictions of Ref. [20] and the results presented in the accompanying article [22], we attribute the observed enhancement to the joint effects of the ion-quadrupole and Coriolis interactions in collisions involving ortho-H 2 molecules in the j = 1 rotational level, which make up 75% of the beam intensity in our experiments.…”

“…In the limit of zero collision energy, which is not probed yet with sufficient precision in our experiments, the rate constant should approach the Bethe-Wigner limit of about 3.6 k L , as given by 1 4 · 2 + 3 4 · 4.18 for a 1 4 -3 4 para-ortho H 2 mixture at low temperature [22]. Several aspects of the low-collision-energy behavior predicted theoretically by Dashevskaya et al [22] remain untested by our experiments, such as the weak oscillations of the reaction rate coefficient at low collision energies, which provide information on the contributions of individual partial waves, and the magnitude of the reaction rate at the lowest energies, which is dominated by the relocking of the angular momentum of the ground-state H 2 molecules as the capture rate approaches the s-wave-scattering limit. We expect that improvements of the signal-to-noise ratio and of the energy resolution of our measurements will make the observation of the predicted oscillations of the rate coefficient possible in the future and will enable us to observe an even larger departure from the classical Langevin-capture model.…”

Dissociative electron attachment to C2F5 radicals

“…The measured rate constants of dissociative electron attachment to C 2 F 5 , reaction (5), are fairly slow; only about 1 in 100 collisions results in dissociative electron attachment. 32 These rate constants depend, of course, on the rates of the individual processes (7)- (10). The collisional stabilization process, (9), can be discounted in the present case as no trace of stable parent ion C 2 F − 5 was observed in the VENDAMS experiment.…”

“…There is a negative temperature dependence, indicative of an efficient s-wave process. 32,33 An unpublished rate constant has been referenced 25 for electron attachment to C 2 F 5 I of 2 × 10 −8 cm 3 s −1 at 300 K due to Sungawa and co-workers 19,[34][35][36][37] That value is incompatible with the present measurements.…”

“…Briefly, collision rate constants are calculated using extended Vogt-Wannier theory. 32,33 Both the attachment rate constant (7) and the resulting energy distribution of C 2 F − * 5 are calculated using an empirical factor to account for inefficiency of capturing higher energy electrons beyond the limit set by extended Vogt-Wannier theory. This increased inefficiency may be due either to the rate of intramolecular vibrational energy redistribution i.e., the incorporation of the electron and its energy into the molecule via electron-phonon coupling) or from competition by electron scattering.…”