Imaging three-body breakup involving two identical fragments

We study the strong-field fragmentation of CO$_2$ and CO$_2$$^+$ into C$^+$+O$^+$+O$^+$ as examples of three-body breakup involving two identical fragments. This process can happen through concerted- or sequential-breakup mechanisms. In concerted breakup, the two O$^+$ fragments play indistinguishable roles. In sequential breakup, however, one of the O$^+$ fragments comes from the first fragmentation step of CO$_2$$^{3+}$, and the other one comes from unimolecular dissociation of CO$^{2+}$ in the second step. Therefore, in sequential breakup the two O$^+$ fragments may be distinguished. A method is proposed that allows us to separate the concerted and sequential processes when the lifetime of the intermediate molecule is much longer than its rotational period. As a result, it is possible to experimentally distinguish the two O$^+$ fragments in the sequential process.