Non-dissociative and dissociative ionization of nitrogen molecules by positron impact

Abstract

The cross sections for direct and total dissociative and non-dissociative ionization of the nitrogen molecule by positron impact have been measured for projectile energies from threshold to 2000 eV. The results are compared with corresponding data for impact of electrons, protons and antiprotons. The comprehensive model which during recent years has been developed to explain the behaviour of the single-and multiple-ionization cross sections of atoms by charged particle impact was found to also apply for this case of a molecular target. In particular, the interference and factorization models which have been found to explain important features of the cross sections for double ionization of atoms apply also to the dissociative ionization of nitrogen molecules.

Ionization cross-sections for positron collisions with N2

“…A fair shape agreement may be discerned among all experimental determinations and differences in absolute scale may arise from either the inclusion of dissociation (in the case of Ref. [12]) or normalization methods in comparison with [11]. Indeed, the discrepancy of ∼20% between the present data and Fig.…”

“…that of [11] would be essentially removed if the latter were normalized to the more recent electron data of reference [13] rather than of reference [16]. All three experimental determinations broadly agree on the position of the peak at an energy of ∼100 eV.…”

“…The suppression of the latter type of reactions by a positron has been previously observed (e.g. [6]), although it must be noted that a peak value of approximately 0.8 × 10 −16 cm 2 has also been reported for positrons by reference [11].…”

“…3. Measurements of the direct non-dissociative ionization cross-section for positron impact on N2: present measurements (black circles) and [11] (grey circles), compared with those including dissociative ionization [12] (hollow circles). Also shown are the corresponding non-dissociative ionization cross-section by electron impact [13] (solid line) and the theoretical results of reference [15] corresponding to various models (CPE: short dash; ES: dash dot; TS: dash double dot), as discussed in the text.…”

“…The positron crosssection, peaking at 40-50 eV with a magnitude of around 3.8 × 10 −16 cm 2 , exceeds that for electrons, predominantly due to Ps formation, up to approximately 600 eV where they merge. Corresponding determinations of Q t i by [11] display a peak at some 20 eV lower energy with a 50-60% larger magnitude. Also shown are the results of reference [12] which comprise, unlike the present measurement, possible contributions from dissociative (as well as non-dissociative) ionization.…”