A photoelectron-ion-ion triple coincidence technique for the study of double photoionization and its consequences

A new technique in which two photoions are detected in coincidence with a photoelectron is described, and its advantages in the study of double ionization are explored. The power of the technique to distinguish decay routes is demonstrated by proof that the equal mass fragmentation of SO₂²⁺produces predominantly O₂⁺ + S⁺and not O⁺ + O⁺. The dissociation of CH₃I²⁺ following He(II) ionization is shown to involve at least twelve distinct pathways, instead of the six previously known.A major potential of the new technique is to elucidate the dynamics of three-fragment decays: it is shown that the formation of C⁺ + S⁺ from CS₂²⁺ can be modelled as a sequential, rather than a simultaneous explosion. The new technique gives the first demonstration, in the case of NO₂, of angular anisotropy in the electrons ejected in double photoionization. The counting statistics of the new method are shown to allow absolute calibration of the detection efficiencies for both electrons and ions. Finally, the technique offers a new method for the detection and investigation of slowly dissociating doubly-charged ions.