Mechanisms of positron annihilation on molecules

The aim of this work is to identify the mechanisms responsible for very large rates and other peculiarities observed in low-energy positron annihilation on molecules. The two mechanisms considered are: (i) Direct annihilation of the incoming positron with one of the molecular electrons. This mechanism dominates for atoms and small molecules. I show that its contribution to the annihilation rate can be related to the positron elastic scattering cross section. This mechanism is characterized by strong energy dependence of Zeff at small positron energies and high Zeff values (up to 10 3) for room temperature positrons, if a low-lying virtual level or a weakly bound state exists for the positron. (ii) Resonant annihilation, which takes place when the positron undergoes resonant capture into a vibrationally excited quasibound state of the positron-molecule complex. This mechanism dominates for larger molecules capable of forming bound states with the positron. For this mechanism Zeff averaged over some energy interval, e.g., due to thermal positron energy distribution, is proportional to the level density of the positron-molecule complex, which is basically determined by the spectrum of molecular vibrational states populated in the positron capture. The resonant mechanism can produce very large annihilation rates corresponding to Zeff ∼ 108. It is highly sensitive to molecular structure and shows a characteristic ε−1/2 behaviour of Zeff at small positron energies ε. The theory is used to analyse calculated and measured Zeff for a number of atoms and molecules. 34.50.-s, 78.70.Bj, 71.60.+z, 36.10.-k Typeset using REVTEX