Isotope effect in the predissociation of the c4Σu-state of O+2

The dissociation of the v=0 and v=1 vibrational levels of the c 4 Sigma u - state of O 2 + has been reinvestigated using a photoion-photoelectron coincidence method. A different branching ratio for the fragmentation into O + ( 4 S 0 )+O( 3 P) and O + ( 4 S 0 )+O( 1 D) has been observed for 16 O + 2 and 18 O + 2 in the v=0 level. This isotope effect shows that the predissociation to the O + ( 4 S 0 )+O( 1 D) adiabatic limit is mainly due to the tunnelling through the c-state potential barrier; no spin-orbit or rotational interaction seems to be involved.