Electronic decay in weakly bound heteroclusters: Energy transfer versus electron transfer

Inner-valence ionized states of weakly bound systems like van der Waals clu sters can efficiently decay by electron emission. The mechanism of the deca y, which does not occur in the isolated monomer units constituting the clus ters has recently been shown to be of intermolecular/interatomic nature. Th is intermolecular/interatomic Coulombic decay (ICD) mechanism prevails in m any systems ranging from hydrogen-bonded molecular clusters to atomic rare gas clusters. In the present paper we extend our previous studies to weakly bound heteroclusters built up of monomer units of largely differing energe tics. It is shown that, as soon as the double ionization potential of a mon omer unit is lower in energy than the ionization potential of the initially created inner-valence vacancy on a neighboring monomer unit, an additional electronic decay process can take place. In contrast to the ICD mechanism, which involves an efficient energy transfer between the monomer units, thi s second process is essentially based on an electron transfer process. It i s therefore termed electron-transfer mediated decay (ETMD). We have analyze d the mechanisms of the electronic decay processes taking place following i nner-valence ionization in weakly bound heteroclusters in an exemplary stud y of the NeAr dimer. The involved electronic states have been calculated us ing ab initio Green's function techniques. The lifetime of the inner-valenc e Ne(2s(-1))Ar vacancy has been estimated and partitioned according to the contributions of the two decay channels based on a perturbation-theoretical description of the decay process. As a result, the lifetime of the inner-v alence resonance state is estimated to be of the order of 10-100 fs, the sp ecific value strongly depending on the internuclear separation of the monom ers. The ICD process is shown to be by far the dominant decay channel at di stances corresponding to bound states of the dimer. With decreasing internu clear separation the ratio of the ETMD and ICD decay widths quickly increas es over several orders of magnitude. (C) 2001 American Institute of Physics .