A recently developed form of threshold ionization spectroscopy has been used to determine the bond energy for HCl to spectroscopic accuracy (±0.8 cm−1). This method is based on excitation to highly vibrationally excited ion-pair states using single-photon transitions from the ground state of HCl. These metastable Rydberg-like states were selectively detected using electricfield induced dissociation. By systematically varying the electric fields involved, and scanning the exciting photon energy, it was possible to determine the field-free energetic threshold for H35Cl+hν→H++35Cl. Using this energy, together with the known values of the ionization potential of H and electron affinity of Cl, a new estimate for the dissociation energy of HCl was obtained: D0(H35Cl)=35 748.2±0.8 cm−1.

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