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The acetylene-vinylidene isomerization and the dissociation of the C 2 H 2 2+ 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 CH 2 + fragment ion as well as by detecting the C + -CH 2 + ion pair in coincidence. The state selectivity of the technique shows that isomerization mainly takes place in the three 1 u −2 lower states of the acetylene dication.Acetylene-vinylidene isomerization is an example of hydrogen migration, a process that occurs in several chemical reactions. As such it has attracted a lot of theoretical and experimental interest. In the last decade vinylidene has been studied in the anion ͓1-3͔, cation ͓4-7͔, dication ͓8,9͔, and radical ͓10͔ forms. Studies of the reaction barrier ͓2,11͔ and the possible stable energetic configurations leading to vinylidene ͓9,12,13͔ have been also performed. On the experimental side Thissen et al. ͓14͔ investigated the decay paths of C 2 H 2 2+ and provided information on the C + -CH 2 + dissociation channel. Levin et al. ͓15͔ and Hayakawa et al. ͓16͔ confirmed the long-lived existence of vinylidene using the Coulomb explosion imaging technique and charge inversion mass spectra, respectively. More recently, Osipov et al. ͓17͔ investigated the rearrangement from acetylene to vinylidene following inner-shell photoionization. By measuring the photoelectron angular distribution in coincidence with the CH 2 + ion these authors estimated a time scale of a few tens of fs for the hydrogen migration in C 2 H 2 2+ formed in the Augerelectron decay of the inner-shell ionized molecules. The theoretical analysis by Duflot et al. ͓9͔ showed that from the low-lying states of acetylene C 2 H 2 2+ , it is possible to populate the low-lying state of vinylidene H 2 CC 2+ , which in turn decays into the C + -CH 2 + ion pair. Two paths have been foreseen: one through a planar transition state and the other through a nonplanar one. The recent theoretical work by Zyubina et al. ͓12͔, which takes into account both the ground and excited states of the dication, reported that the isomerization is favored in the excited states due to the lower potential barrier and in these states the process can occur on the time scale estimated by Osipov et al. ͓17͔.In this work we have studied the acetylene-vinylidene isomerization via Auger-electron-ion and Auger-electronion-ion coincidence experiments. These techniques via the selection of the proper ion fragment ͑CH 2 + ͒ or ion pair ͑C + -CH 2 + ͒ can prove the occurrence of the isomerization and the coincidence with the Auger electron enables the identification of the dication state involved. Thus the present study completes the experimental information of the photoionization experiment by Osipov et al. ͓17͔ and can be directly compared with the theoretical predictions.The setup for these experiments and the procedures for the data collection have been recently...

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