Energetics and dynamics of threshold photoion-pair formation in $HF∕DF$

Threshold ion-pair production spectroscopy (TIPPS) has been applied to two isotopomers, HF and DF. From the high resolution $(∼0.3cm−1)$ TIPP spectra, the ion-pair thresholds of $HF∕DF$ have been precisely measured. Combined with the ionization energy of H(D), the electron affinity of F, and the zero point energies of $HF∕DF$⁠, the difference between their classical bond dissociation energies was obtained as $De(H–F)−De(D–F)=12.4±0.5cm−1$⁠. Our result provides an experimental estimate of the Born-Oppenheimer breakdown in the ground electronic state. The present work also measured the total ion-pair yield spectra of HF and DF in the threshold region, and the ion-pair formation mechanisms of these two molecules were discussed in light of the high resolution results.