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 $\left(\Delta E\sim 25\mathrm{meV}\mathrm{FWHM}\right)$ beam of positrons from a Penning-Malmberg 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 threshold and 30 eV. For Ar and $H_2$, comparison can be made with theoretical calculations, and, in the case of $H_2$, the results resolve a significant discrepancy between the only two available calculations.

• Received 27 March 2001

DOI:https://doi.org/10.1103/PhysRevLett.87.073201