Energetics and dynamics of threshold photoion-pair formation in HFDF.

Threshold ion-pair production spectroscopy (TIPPS) has been applied to two isotopomers, HF and DF. From the high resolution (approximately 0.3 cm(-1)) TIPP spectra, the ion-pair thresholds of HFDF have been precisely measured. Combined with the ionization energy of H(D), the electron affinity of F, and the zero point energies of HFDF, the difference between their classical bond dissociation energies was obtained as D(e)(H-F)-D(e)(D-F) = 12.4 +/- 0.5 cm(-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.