Modeling enhancement and suppression of vibrational Feshbach resonances in positron annihilation on molecules

Experiments have shown that positrons can attach to molecules via vibrational Feshbach resonances. This leads to increased annihilation rates, the magnitudes of which depend upon molecular structure. Presented here is a simplified rate-equation model to describe the competition between annihilation while the positron is attached to the molecule, positron ejection from the entrance state, and diffusion of the vibrational energy to multimode states followed by similar ejection due to vibrational deexcitation. The latter ejection process can involve vibrations more strongly coupled to the positron continuum, producing suppression of the annihilation, or those more weakly coupled to the continuum, resulting in enhanced annihilation rates. This model elucidates the role that mode coupling can play in determining resonant annihilation amplitudes. Simple limits are obtained and compared with experimental results for selected molecules.