Excitation of electronic states of Ar, H-2, and N-2 by positron impact - art. no. 073201

We have measured the first state-resolved, absolute cross sections for posi tron excitation of electronic states of an atom or molecule using a high re solution (DeltaE similar to 25 meV FWHM) beam of positrons from a Penning-M almberg trap. We present cross sections for the excitation of the low-lying levels of Ar, H-2, and N-2 for incident positron energies between threshol d and 30 eV. For Ar and H-2, comparison can be made with theoretical calcul ations, and, in the case of H-2, the results resolve a significant discrepa ncy between the only two available calculations.