Enhancement of positron annihilation on molecules due to vibrational Feshbach resonances

The zero-range potential model is used to investigate positron collisions and annihilation with molecules. The Kr2 dimer is considered as an example. It is shown that (i) although positrons do not bind to individual Kr atoms, they do form bound states with Kr2. (ii) A sequence of vibrationally excited states of the positron–molecule complex extends into the positron continuum, where it manifests as vibrational Feshbach resonances. (iii) These resonances give a very large contribution to the positron annihilation rate. Even after averaging over the thermal positron energy distribution, the contribution of the lowest Feshbach resonance exceeds that of the non-resonant background by an order of magnitude. 2002 Elsevier Science B.V. All rights reserved. PACS: 34.50.)s; 78.70.Bj; 71.60.+z; 36.10.)k