Annihilation cross section of protonium by electron impact

Cross sections for annihilation of protonium (Pn) induced by electron impact are calculated in a quantal close-coupling method, where the scattering wavefunction is expanded in terms of the atomic orbitals of protonium. The energy dependence of the annihilation cross sections and the decay mechanisms differ considerably for each initial n, l state of Pn(nl), where n and l are the principal and angular momentum quantum numbers. A similar study of the process but with heavy-particle impact by Sakimoto (2005 At. Mol. Opt. Phys. 38 3447) concluded that the annihilation cross section of Pn(nl) scales with n for each l. This scaling rule, except for the initial p states at high energies, cannot be confirmed in the present study.