Excitation of Electronic States of Ar, H2, and N2 by Positron Impact

We have measured the first state-resolved, absolute cross sections for positron excitation of electronic states of an atom or molecule using a high resolution (ΔE∼25 meV FWHM) beam of positrons from a Penning-Malmberg trap. We present cross sections for the excitation of the low-lying levels of Ar, H2, and N2 for incident positron energies between threshold and 30eV. For Ar and H2, comparison can be made with theoretical calculations, and, in the case of H2, the results resolve a significant discrepancy between the only two available calculations