Electronic interaction between valence and dipole-bound states of the cyanoacetylene anion

The electron attachment properties of cyanoacetylene HCCCN are investigated with particular emphasis on the coupling between dipole-bound and valence states. As an initial step both the dipole-bound and the valence state of HCCCN− are studied separately using high level ab initio methods. Predictions for the geometry of the valence anion, the electron binding energy of the dipole-bound state, the energy of the temporary anion associated with vertical attachment into the valence state, the vertical detachment energy of the valence anion, and the adiabatic electron affinity of HCCCN are given. Our results indicate that the electron affinity found in the NIST web-book is not that of HCCCN but of some other C3HN species. The two anionic states interact with each other, and we study their electronic coupling by computing the two electron binding energies along oneand two-dimensional cuts through the potential energy surfaces, and fitting a diabatic model potential to the ab initio data. In particular, the two-dimensional cuts allow us to examine the geometry dependence of the electronic coupling, and to ask the question whether the coupling elements inferred from one-dimensional cuts represent typical values. Moreover, the influence of the theoretical method on the computed coupling elements is investigated, and the possibility of employing the diabatic model potential as a mean to extrapolate bound state binding energies into the metastable domain is pointed out. PACS. 31.50.Gh Surface crossings, non-adiabatic couplings – 31.50.Bc Potential energy surfaces for ground electronic states – 34.70.+e Charge transfer – 34.80.Gs Molecular excitation and ionization by electron impact