Threshold ion-pair production spectroscopy of HCl/DCl: Born–Oppenheimer breakdown in HCl and $HCl+$ and dynamics of photoion-pair formation

Threshold ion-pair production spectroscopy (TIPPS) has been applied to two isotopomers, HCl and DCl. From the high-resolution TIPP spectra the ion-pair thresholds of the two molecules have been precisely measured. Combined with the known ionization energy of H(D) and the electron affinity of Cl, the difference between their bond dissociation energies is calculated, and therefore an experimental determination of the effect of Born–Oppenheimer breakdown on the dissociation limit of the ground state potential curve has been obtained. The difference in $De$ for the two isotopomers was found to be: $De(H–Cl)−De(D–Cl)=3.2±1.0 cm−1.$ The bond energy for HCl was in agreement with our previous determination, $D0(H–Cl)=35748.2±0.8 cm−1.$ These results are compared to a recent study of Born–Oppenheimer breakdown in HCl by Coxon and Hajigeorgiou, where high resolution spectroscopic data was used to fit Born–Oppenheimer breakdown correction terms for the intramolecular potential function. The present study also measured the high resolution spectra for photoion-pair production for HCl and DCl in the threshold region for ion-pair production (∼86 nm). Although there is qualitative agreement between the current results and previous experimental and theoretical work, there are some important differences. The possible mechanism for ion-pair formation in HCl and DCl is discussed in light of these high resolution results.