DISSOCIATION OF DOUBLY CHARGED CH2=CD2 AND CH2=CF2 IN THE REGION OF VALENCE SHELL PHOTOEXCITATION

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 CX(m)+ + CY(n)+ fragment ions at a threshold below 37 eV photon energy. The decomposition into H+ + C2HD + D+ + C2H2D+, and F+ + C2H2F+ are also appreciable, and (ii) the fragmentation into H+ (D+ or F+) + CX(m)+ takes place at hnu > 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 pi(C=C) electron results in central C=C bond breakage, while that of two electrons of the sigma(CH), sigma(CC), and C 2s orbitals of ethene forms two smaller fragment ions of the H+ + CX(m)+ type.