Single photon double and triple ionisation of allene

Abstract

Double and triple ionization of allene are investigated using electron-electron, ion-ion, electron-electron-ion and electron-electron-ion-ion (ee, ii, eei, eeii) coincidence spectroscopies at selected photon energies. The results provide supporting evidence for a previously proposed roaming mechanism in H$_3^+$ formation by double ionisation. The lowest vertical double ionization energy is found to be 28.5 eV, while adiabatic double ionisation is not accessed by vertical ionisation at the neutral geometry. The triple ionization energy is found to be close to 50 eV in agreement with theoretical predictions. The doubly charged parent ion is stable up to about 2 eV above threshold, after which dissociations by charge separation and by double charge retention occur with comparable intensities. Fragmentation to H$^+$ + C$_3$H$_3^+$ starts immediately above threshold as a slow (metastable) decay with 130+/-10 ns mean lifetime.