Anomalous branch intensities in the threshold photoionization of HCl

The rotationally resolved threshold photoionization spectrum of HCl has been observed for ionization into both spin-orbit components of the ground electronic state of HCl⁺. The data indicate an extreme asymmetry in the apportioning of angular momentum between the escaping photoelectron and the ion core. Observed transitions in which the core rotation decreases are found to be heavily favored compared to those where the core rotation increases by an equal amount. A mechanism of increased negative branch intensity due to field- or dipole-induced mixing of Rydberg series converging to higher ion rotational levels is proposed as a possible explanation. Direct observation of the transition HCl (X¹Σ⁺,J″ = 0) →HCl ⁺ (X̃²Π_3/2,J⁺ = 3/2) yields an ionization potential of 102 802.8±2 cm^-1 for HCl.