Dissociation of compound ions in a high electric field: Atomic tunneling, orientational, and isotope effects

Dissociation of compound ions, HeRh2+, has been studied with a combined technique of pulsed-laser-stimulated field desorption and time-of-flight spectroscopy. It is found that HeRh2+4 ions can dissociate in a field of a few V/AI by atomic tunneling several hundred femtoseconds after their formation. This dissociation time is believed to be one of the fastest reaction times ever successfully measured. The tunneling effect is confirmed with a strong isotope effect observed when He4 is replaced with He3. The field dissociation can occur most readily when the ions are rotated by 180°from their original field-desorption orientation. This orientation effect produces a secondary peak in the time-of-flight spectral line of Rh2+. The experimental results can be satisfactorily explained by using a realistic interaction U(r) between a He atom and a Rh2+ ion. This potential is constructed from an effective-medium model of the repulsion, an induced dipole attraction, and damped dispersion interaction terms.