Photoelectron-photoion-photoion momentum spectroscopy as a direct probe of the core-hole localization in C 1s photoionization of C2H2

Angular distributions of C 1s photoelectrons, relative to a dissociation axis for C2H2, have been measured with a photoelectron - photoion - photoion coincidence technique. The photoelectron angular distribution ( PAD) for three two- body fragmentations ( symmetric, non- symmetric and proton- migration fragmentations) is completely different. The PADs for non- symmetric fragmentation provide direct evidence of a localized core- hole and preferential bond breaking following Auger decays. Moreover, the PAD for symmetric fragmentation has been interpreted as the interference between photoemissions from the two carbon atoms.