Electric-field induced dissociation of molecules in Rydberg-like highly vibrationally excited ion-pair states

Abstract

Highly vibrationally excited (high-v) ion-pair states of molecules share many of the same properties as high-n Rydberg states. In high-v ion-pair states, a weakly bound pair - an anion and cation - play the roles of electron and ion core. High-v ion-pair states have an infinite number of vibrational levels below their dissociation threshold, and these follow a Rydberg-like formula. It is demonstrated that high-v ion-pair states can be dissociated by the application of weak pulsed electric fields, in a similar manner to the electric-field induced ionization of high-n Rydberg states. It has been possible to exploit this effect - using a technique similar to pulsed-field ionization zero-kinetic-energy photoelectron spectroscopy (PFI-ZEKE) and mass-analyzed threshold ionization spectroscopy (MAT[) - to determine field-free energetic thresholds for ion-pair formation. The thresholds for three processes were studied using this technique: HCl + hv ➔ tt+ + c1-, 0 2 + hv ➔ o+ + o-, and HF + hv ➔ ff+ + F-. The accurately determined field-free ion-pair formation threshold of HCl, together with the known values of the ionization potential of Hand the electron affinity of Cl, allowed the dissociation energy of HCI to be determined to within ±1 cm-1 - a significant improvement over the literature value. This new technique - threshold ion-pair production spectroscopy (TIPPS) - should be applicable to polyatomics, for which many bond energies are only known thermochemically.