FORMATION AND DISSOCIATION OF THE NITRIC-ACID DICATION

Electron-impact double ionization of nitric acid, HNO3 has been invest igated by using ion-ion coincidence techniques coupled with time-of-fl ight mass spectrometry. The coincidence spectra show that HNO32+ disso ciates via a variety of two- and three-body reactions to form the foll owing pairs of ions: OH+ + NO+, O+ + NO+, OH+ + NO2+, O+ + HNO2+, H+ N+, H+ + O+, H+ + NO+, and H+ + NO2+. The experimental data are consi stent with the HNO32+ ion decaying to form the observed ion pairs via an initial charge-separating bond cleavage followed by the dissociatio n of the resulting singly charged ions. Interpretation of the coincide nce spectra yields a determination of the excitation energy required t o form the lowest energy dicationic state that dissociates to form OH + NO2+. Comparison of this excitation energy with estimated values of the double ionization energy of HNO3 indicate that this state is prob ably the ground electronic state of HNO32+ and, hence, these investiga tions provide a first estimate for the double ionization energy of HNO 3. (Int J Mass Spectrom 177 (1998) 119-129) (C) 1998 Elsevier Science B.V.