Production of metastables in ion-molecule collisions

A second-order process observed in the emission from the A 2Σ states of HCl+ and HBr+ produced by collisions of 2.5 keV He+ ions with HBr and HCl is shown to arise from the formation of fast He metastables in the primary collision. These fast metastables react with the molecules to produce molecule-ions in the A 2Σ state which have a vibrational distribution much different from that expected on the basis of a Franck-Condon excitation. This vibrational distortion has been observed for thermal He metastable reactions with HBr and HCl but has never been observed for fast metastables and is difficult to reconcile with present theory of Penning ionization. The vibrational distribution in HCl+ produced by the fast He metastables is distorted considerably more than that produced by thermal metastables. The cross section for reaction of fast He metastables with HBr is 90× 10−16 cm2. The Ne+ ions reacting with HBr and HCl also produce fast Ne metastables. These react to produce molecule-ions with distorted vibrational distributions. An investigation of Ne+ and He+ reacting with HF shows that no molecular emission is produced. The reaction of Ar+ with HCl to produce Ar metastable has a cross section comparable to the near-resonant charge transfer process producing HCl+ (A 2Σ).