Evidence of Core Hole localization in Molecular Frame Photoelectron Distributions in Dichloroethylene

For the past few years, we have conducted theoretical and experimental searches for evidence of core hole localization in X-ray photoionization of molecules by determining molecular frame photoelectron angular dependences (MFPADs) and recoil frame photoelectron angular dependences (RFPADs). Results have ranged from unambiguous, in the case of CF₄ F(1s), to less evident, as in the case of Cl(2p) ionization in CCl₄, where calculations suggest that the observed RFPADs mostly match theory with the electron hole delocalized among the four equivalent Cl atoms. Recent experiments on 1,1 dichloroethylene (H₂C₂Cl₂) show a contrasting result that suggests a second case of the visibility of strong core hole localization on one of a set of equivalent atoms in the MFPAD. Upon Auger decay following Cl(2p) ionization, one dissociation channel produces H⁺ + H⁺ + Cl⁺. Comparisons of the resulting MFPAD for unequal energy sharing between the protons with theoretical calculations suggest that in this case the electron hole is strongly localized on one Cl atom. We will present the results of this investigation that combines ab initio calculations using the Complex Kohn method and COLTRIMS experiments at the Advanced Light Source (ALS) at the Lawrence Berkeley National Laboratory.