Penning ionization optical spectroscopy: Metastable helium (He 2 3S) atoms with nitrogen, carbon monoxide, oxygen, hydrogen chloride, hydrogen bromide, and chlorine

The generation of triplet metastable helium atoms (19.8 eV) by a hollow cathode discharge coupled to a flow apparatus is described. With this apparatus the Penning ionization reactions of He(2 ³S) with six diatomic molecules were investigated at pressures of 1–5 torr of helium and less than 1 mtorr of reagent at 300, 225, and 77°K. The reactions were monitored by analysis of the visible and ultraviolet emission spectra. The following band systems were observed: ${\mathrm{N}}_2^{+}{\mathrm{(B\hspace{0.17em}}}^2{\Sigma }_u^{+}{\mathrm{‐X\hspace{0.17em}}}^2{\Sigma }_g^{+}{\mathrm{,\hspace{0.17em}A\hspace{0.17em}}}^2{\Pi }_u{\mathrm{-X\hspace{0.17em}}}^2{\Sigma }_g^{+}\mathrm{);\hspace{0.17em}}{\mathrm{CO}}^{+}{\mathrm{(B\hspace{0.17em}}}^2{\Sigma }^{+}{\mathrm{-\; X\hspace{0.17em}}}^2{\Sigma }^{+}{\mathrm{,\hspace{0.17em}B\hspace{0.17em}}}^2{\Sigma }^{+}{\mathrm{-A\hspace{0.17em}}}^2{\Pi }_i{\mathrm{,\hspace{0.17em}A\hspace{0.17em}}}^2{\Pi }_i{\mathrm{-X\hspace{0.17em}}}^2{\Sigma }^{+}\mathrm{);\hspace{0.17em}}{\mathrm{O}}_2^{+}{\mathrm{(A\hspace{0.17em}}}^2{\Pi }_u{\mathrm{-X\hspace{0.17em}}}^2{\Pi }_g{\mathrm{,\hspace{0.17em}b\hspace{0.17em}}}^4{\Sigma }_g^{-}{\mathrm{-a\hspace{0.17em}}}^4{\Pi }_u\mathrm{);\hspace{0.17em}}{\mathrm{HCl}}^{+}{\mathrm{(A\hspace{0.17em}}}^2{\Sigma }^{+}{\mathrm{-X\hspace{0.17em}}}^2\mathrm{\Pi );\hspace{0.17em}}{\mathrm{HBr}}^{+}{(}^2{\Sigma }^{+}{\mathrm{-X\hspace{0.17em}}}^2\mathrm{\Pi )}$ and ${\mathrm{Cl}}_2^{+}{\mathrm{(A\hspace{0.17em}}}^2{\Pi }_u{\mathrm{-X\hspace{0.17em}}}^2{\Pi }_g)$ . For the last four cases atomic emissions were observed indicating the presence of reaction channels in competition with Penning ionization. The vibrational populations of ${\mathrm{O}}_2^{+}{\mathrm{(A\hspace{0.17em}}}^2{\Pi }_u\mathrm{),\hspace{0.17em}}{\mathrm{HCl}}^{+}{\mathrm{(A\hspace{0.17em}}}^2{\Sigma }^{+})$ and ${\mathrm{HBr}}^{+}{\mathrm{(A\hspace{0.17em}}}^2{\Sigma }^{+})$ were not in accord with populations expected for vertical transitions from the molecular ground state. However, the ${\mathrm{N}}_2^{+}{\mathrm{(B\hspace{0.17em}}}^2{\Sigma }_u^{+}\mathrm{),\hspace{0.17em}}\mathrm{CO}{\mathrm{(A\hspace{0.17em}}}^2{\Pi }_i)$ , and ${\mathrm{O}}_2^{+}{\mathrm{(b\hspace{0.17em}}}^4{\Sigma }_g^{-})$ vibrational populations were very similar to those expected for a vertical ionization. Rotational populations of ${\mathrm{N}}_2^{+}\mathrm{(B),\hspace{0.17em}}{\mathrm{CO}}^{+}\mathrm{(B),\hspace{0.17em}}{\mathrm{HBr}}^{+}\mathrm{(A)}$ and ${\mathrm{HCl}}^{+}\mathrm{(A)}$ were found to be very similar to those of the neutral precursor molecule, although at 77°K a non‐Boltzmann distribution was observed for high N′ states of ${\mathrm{N}}_2^{+}\mathrm{(B)}$ . The results are discussed in terms of the two state potential model of Penning ionization.