Near-threshold vibrational excitation of H₂ by positron impact: A projection-operator approach

We report vibrational excitation (ν_i=0→ν_f=1) and total cross sections for positron scattering by H₂. The Feshbach projection operator formalism was employed to vibrationally resolve the fixed-nuclei phase shifts obtained with the Schwinger multichannel method. The near-threshold behavior of the vibrational excitation cross section is in good agreement with available experimental data [ Sullivan et al. Phys. Rev. Lett. 86 1494 (2001)]. Though our fixed-nuclei calculations do not indicate the existence of a e⁺-H₂ virtual state, a proper description of the T matrix threshold behavior is essential (the adiabatic approximation is inadequate). The projection operator approach has long been a powerful tool for studies of nuclear dynamics in electron-molecule collisions, and its application to positron scattering is timely since couplings to nuclear degrees of freedom are known to be very important at low impact energies.