The HCl +(A 2Σ +-X 2II) and HBr +(A 2Σ +-X 2II) emissions produced by the He(2 3S), Ne( 3P 0,2)-HCl, HBr penning ionization in low-pressure flowing afterglows

Abstract

Penning ionization of HCl and HBr by He(2 3S) and Ne( 3P 0,2) metastable atoms leading to HCl + (A 2Σ +) and HBr + (A 2Σ +) has been investigated spectroscopically in low-pressure He and Ne afterglows. The vibrational and rotational distributions of HCl + (A) from the Ne( 3P 0,2)-HCl system were invariant over the Ne bath pressure range of 25–800 mTorr. The vibrational distribution was shifted toward lower vibrational levels compared to the Franck-Condon distribution with rotational temperatures of 900, 550, and 350 K for υ′ = 0–2, respectively. This is explained as a consequence of a short-range final state interaction in the NeHCl + (A) → Ne + HCl + (A) process. A significant vibrational relaxation of HCl + (A) and HBr + (A) by collisions with buffer gas atoms has been found for the He(2 3S)-HCl, HBr and Ne( 3P 0,2)-HBr systems. On the basis of this fact, a discrepancy in the observed vibrational distributions in flowing affterglow and crossed beam experiments is attributed to preferential quenching of the higher vibrational levels by collisions with rare gas atoms in the former high-pressure experiment. In contrast to the significant vibrational relaxation, the produced rotational distributions (350–750 K) of HCl + (A) and HBr + (A) were independent of the rare gas pressure.