Evidence of Positron Annihilation at Electronic Excitation Threshold for N$_{2}$ $^{\ast }$

We are investigating Positronium (Ps) formation for $<$ 20 eV positrons interacting with N$_{2}$ in a gas scattering cell. The technique [1] of Ps annihilation ratio spectroscopy (PsARS) is used to obtain the ratios of coincidence signals for two of the three gamma rays (S$_{3\gamma })$ in the photon energy window 300 to 460 keV resulting from ortho-Ps decay to that for two 511 keV gamma rays (S$_{2\gamma })$ arising from para-Ps decay and other processes. By comparing these ratios of S$_{3 \gamma }$/S$_{2\gamma }$ for N$_{2}$ to those for Ar it is found that N$_{2}$ exhibits strikingly anomalous behavior near and below the Ps formation threshold. Typically, this ratio remains constant within 2 eV above the Ps threshold. For N$_{2} $, this ratio decreases to zero at the threshold and an S$_{2 \gamma }$ signal remains for an energy of $\sim $0.3 eV below. Since N$_{2}$ has an electronic excitation threshold for positron impact that opens up at $\sim $0.3 eV below the Ps threshold, the present results strongly suggest that the incident positron is electronically exciting N$_{2}$ and then binding to the excited N$_{2}$ in a temporary resonance-like state from which the bound positron annihilates with a molecular electron. $^{\ast }$Research supported by NSF Grant PHY 99-88093.\\ $[1]$ W.E. Kauppila, E.G. Miller, H. F.M. Mohamed, K. Pipinos, T. S. Stein, and E. Surdutovich, Phys. Rev. Lett. 93, 113401 (2004).\\