A position-sensitive coincidence spectrometer to image the kinematics of the bimolecular reactions of molecular dications

This paper describes the design and operation of a position-sensitive coincidence spectrometer which detects the pairs of ions formed in the bimolecular reactions of dications. In the experiment a pulsed, energy-selected beam of dications interacts with a neutral collision gas in the source region of a time-of-flight mass spectrometer. Product ions formed in these interactions are detected in coincidence at a position-sensitive detector. From the resulting data, we can derive the velocity vectors, in the centre of mass frame, of both of the product ions. From these pairs of vectors, detailed information on the reaction dynamics and kinematics can be extracted. The power of the new technique has been demonstrated by a study of the electron transfer reaction between Ar2+ and He, which shows that the P-3 and D-1 states of Ar2+ react with He, via electron transfer, to form pairs of singly charged ions with markedly different angular distributions.