Velocity imaging spectrometer for negative fragment ions: application to dynamics of O2 and N2O ion-pair dissociation

A velocity imaging spectrometer has been developed to observe negative ions from molecular ion-pair dissociation. The imaging spectrometer is equipped with a pair of permanent magnets. The resulting magnetic field prevents efficiently the electrons from reaching the detector, without seriously affecting negative ions' trajectories. The performance of the imaging spectrometer is demonstrated by the observations of photoelectrons from O2 and He, and O− from O2. Application to ion-pair dissociation of O2 and N2O is presented, and it proves that the present imaging method provides useful information on the assignments of superexcited states and on the dynamics of ion-pair dissociation.