Intermolecular Coulombic decay of molecular clusters: Identification of the decay mechanism using a new hole-population analysis

Singly ionized states of molecular clusters with an inner-valence vacancy have recently been shown to undergo an efficient electronic decay. The mechanism of the decay, which is absent in the isolated molecules that build up the clusters, is proposed to be of intermolecular Coulombic nature. As explicit example to further investigate this new decay process, the valence ionization spectrum of the HF(H2O)2 cluster is computed with the ADC(3) one-particle Green's function method. In the inner-valence part of the spectrum, characteristic dense line bundles due to the ultrafast electronic decay of the corresponding cationic states are observed. A new hole-population analysis method for the very many computed cationic states is presented. This method allows for a quantitative measure and characterization of the hole localization pattern of the cationic states. The dense line bundles which mimic the continuous decay distributions in our finite basis set approach are analyzed in detail. The resulting intermolecular character of the states confirms the recently proposed intermolecular Coulombic mechanism for the electronic decay in molecular clusters. The decay leads to dicationic states with two vacancies located on neighboring monomer units.