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: N2+ (B 2Σu+-X 2Σg+, A 2Πu−X 2Σg+); CO+(B 2Σ+− X 2Σ+, B 2Σ+−A 2Πi, A 2Πi−X 2Σ+); O2+(A 2Πu−X 2Πg, b 4Σg−−a 4Πu); HCl+(A 2Σ+−X 2Π); HBr+(2Σ+−X 2Π) and Cl2+(A 2Πu−X 2Πg). For the last four cases atomic emissions were observed indicating the presence of reaction channels in competition with Penning ionization. The vibrational populations of O2+(A 2Πu), HCl+(A 2Σ+) and HBr+(A 2Σ+) were not in accord with populations expected for vertical transitions from the molecular ground state. However, the N2+(B 2Σu+), CO(A 2Πi), and O2+(b 4Σg−) vibrational populations were very similar to those expected for a vertical ionization. Rotational populations of N2+(B), CO+(B), HBr+(A) and HCl+(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 N2+(B). The results are discussed in terms of the two state potential model of Penning ionization.