Momentum Imaging of Molecular Ions : Momentum Spectroscopic Studies of Atomic and Molecular Ionisation

This thesis concerns the study of fragmentation dynamics of highly charged triatomic and polyatomic molecular ions created by high energy electron impact on neutrals. A momentum spectrometer was designed and assembled to carry out the study. An event-by-event reconstruction of the momentum space spanned by the fragments helped us to study various aspects of dissociation, such as identification of channels, lifetime of metastables, kinetic energy release, angular distribution of fragments and preferential bond breaking. Some deductions are supported by quantum chemical computations. The symmetry of a molecular ion is often different from that of the ground state of the neutral. For a linear triatomic, CO_2, we deduced that geometry of the dication is different from that of the neutral. For polyatomics such as CCl_4 and SF_6, the change is very substantial and involves migration of atoms. In addition, we have identified certain stability patterns in the fragments. The stability depends on the possibility of forming filled hybrid molecular orbitals, which in turn is related to even/odd electron numbers in the fragment ions.