Ion Dissociation and Ion-Molecule Reactions Studied with State-Selected Ions

Abstract

The internal energy dependence is examined in ion dissociation and ion-molecule reactions with state selected ions produced by photoionisation with synchrotron radiation and threshold photoelectron-photoion coincidences. The dissociation mechanism of the methane ion and its deuterated compounds stongly varies with internal energy of the parent ion. Whereas the CH4 + X ground state dissociation is statistical, the A (2a1)-1 state (22.4–25 eV photon energy) and the doubly excited states (22.4–32 eV) show direct dissociation mechanisms. The preparation mode of state selected ions by autoionisation is shown to have an influence on the ion dissociation. Examples are given for the dissociation of CO2 +, CF3I+ and H2O+. In the second part, the ion-molecule reaction mechanisms are also shown to be strongly dependent on the internal energy (electronic, vibrational and spin-orbit) of the reactant ion. A near resonant charge transfer reaction N2+(X,A,v) + Ar --> Ar+ +N2 and a non resonant charge transfer reaction O2 +(X,a,v) + Ar Ar+ + O2 are discussed.