Ab initio calculation of the vertical excitation energies of small helium cluster ions

Quantum chemical ab initio calculations have been performed for the vertical excitation energies and oscillator strengths of all low-lying electronically excited states of small helium cluster ions, Hen+, n=2, ..., 7. The geometrical structures of the ions were fixed at the equilibrium geometries of the respective ground states, for He4+ and He5+ also one alternative structure was considered. The low-lying excited states can be classified into two categories: The electronic transition can occur either within the central He2+ or He3+ unit or from the peripheral weakly bound He atoms to this unit. The latter transitions are very weak (f≅0.001), closely spaced, with vertical excitation energies of about 5.7 eV. The He2+ and He3+ units have strong transitions at 9.93 and 5.55 eV, respectively; these transitions are only slightly blue-shifted if He2+ or He3+ are placed as 'chromophores' into the centre of a larger Hen+ cluster. The large difference in the vertical excitation energy of the strong transition sould enable an experimental decision of the question whether the cluster ions have He2+ or He3+ cores. (orig.)