Positron spectroscopy of solid N2.

Abstract

We have measured the positron and positronium emission yields from a solid ${\mathrm{N}}_{2}$ target surface bombarded with slow positrons. We determine the inelastic threshold energy for positrons in ${\mathrm{N}}_{2}$, ${\mathit{E}}_{\mathrm{th}}$=7.08\ifmmode\pm\else\textpm\fi{}0.10 eV, and the threshold for forming positronium, ${\mathit{E}}_{\mathrm{Ps}}$=10.0\ifmmode\pm\else\textpm\fi{}0.5 eV. In contrast to the rare-gas solids, the low-lying electronic excitations cause ${\mathit{E}}_{\mathrm{th}}$ to lie below ${\mathit{E}}_{\mathrm{Ps}}$. Unlike electrons, positrons should not excite triplet states at a detectable level, and therefore ${\mathit{E}}_{\mathrm{th}}$ is the energy of the lowest-lying singlet vibrational level less the positron affinity, ${\mathrm{\ensuremath{\varphi}}}_{+}$. We thus determine ${\mathrm{\ensuremath{\varphi}}}_{+}$=1.31\ifmmode\pm\else\textpm\fi{}0.10 eV. From the positronium formation threshold we obtain the positronium binding energy in the solid, ${\mathit{E}}_{\mathit{b}}$=3.8\ifmmode\pm\else\textpm\fi{}0.5 eV. Below the inelastic threshold a positron is found to lose an energy 30\ifmmode\pm\else\textpm\fi{}5 meV per collision in agreement with a model in which the principal energy-loss mechanism is the excitation of optical phonons.