Vinylidene dissociation following the Auger-electron decay of inner-shell ionized acetylene

The acetylene-vinylidene isomerization and the dissociation of the C₂H₂²⁺ dication formed in the inner-shell ionization by electron impact have been studied via Auger-electron--ion and Auger-electron--ion--ion coincidence experiments. The occurrence of the isomerization is proved by measuring the production of the CH₂⁺ fragment ion as well as by detecting the C⁺-CH₂⁺ ion pair in coincidence. The state selectivity of the technique shows that isomerization mainly takes place in the three 1πᵤ^-2 lower states of the acetylene dication.