Dissociation of doubly charged CH2=CD2 and CH2=CF2 in the region of valence shell photoexcitation

Abstract

Dissociative double photoionization of 1,1-dideuteroethene (CH2=CD2) and 1,1-difluoroethene (CH2=CF2) has been studied using the photoion–photoion coincidence (PIPICO) technique and monochromated synchrotron radiation over the photon energy range of 37–85 eV. PIPICO branching ratios and threshold energies for dissociative double photoionization show two dominant exit channels of the doubly charged ions (i) central C=C bond cleavage to form CXm++CYn+ fragment ions at a threshold below 37 eV photon energy. The decomposition into H++C2HD2+, D++C2H2D+, and F++C2H2F+ are also appreciable, and (ii) the fragmentation into H+(D+ or F+)+CXm+ takes place at hν≳37 eV. Comparison of the present PIPICO branching ratios with the recent theoretical studies of the electronic states of the outer valence doubly charged ethene ion [Liegener, Chem. Phys. 92, 97 (1985) and Ohrendorf et al., J. Chem. Phys. 91, 1734 (1989)] indicates that double ionization involving release of one πC=C electron results in central C=C bond breakage, while that of two electrons of the σCH, σCC, and C 2s orbitals of ethene forms two smaller fragment ions of the H++CXm+type.