Electron attachment to CO2 clusters

Vertical and adiabatic electron attachment to carbon dioxide clusters (CO2)(N) (N=2-5) is studied at high ab initio levels of theory. As a first step the geometries of neutral and anionic CO2 clusters are reexamined. The potential energy surfaces of both neutral and anionic CO2 clusters show many minima, and several isomers have been reported so far. Here we present new lowest-energy isomers for the tetramer and pentamer anion clusters, as well as high-level results for relative and CO2 evaporation energies. Electron correlation is crucial for the computed properties, and since we had to make certain compromises about the theoretical level in order to include larger clusters, a thorough investigation of different ab initio methods is performed for the dimer. The vertical affinities of the investigated (CO2)(N) clusters are found to be clearly negative; i.e., vertical attachment into valence orbitals leads to temporary anion states. The energies of these resonances as well as the associated autodetachment lifetimes are computed using complex absorbing potentials at the frozen-orbital and correlated second-order Green's function levels. Whereas the cluster environment has a surprisingly weak influence on the negative vertical affinities, solvation has a strong effect on the adiabatic electron affinities. The computed trends suggest that the tetramer is at the brink of stability and that the pentamer cluster is the smallest species possessing an appreciable positive electron affinity. Our findings have profound implications for the interpretation of the low-energy electron attachment experiments and provide a basis for the discussion and modeling the attachment process. (C) 2003 American Institute of Physics.