Structure and properties of Nen+ clusters from a diatomics-in-molecules approach

The diatomics-in-molecules (DIM) approach is used to investigate the structure and properties of Ne-n(+) clusters for 3 less than or equal to n less than or equal to 25. The DIM energies and wavefunctions can be obtained within the representation matrix formalism without the appearance of overlap integrals, and antisymmetry for permutation of electrons between different atoms can also be taken into account implicitly. The results are formally the same as those obtained in the traditional DIM framework. Candidate structures for the lowest energy minimum at each size are located using a basin-hopping technique, and the relative stabilities appear to agree well with intensity anomalies in the mass spectra of Ne cluster cations. Significant differences in the equilibrium configurations of Ne-n(+) compared Ar-n(+) appear for n > 3, corresponding to the relatively flexible ionic core undergoing a transition at n = 15 from symmetric tetraatomic to asymmetric triatomic, with the closely spaced diatomic unit preserved in both structures. The induced dipole-induced dipole interaction is found to affect the global minimum structure and electronic transition intensities significantly. Electronic spectra for Ne-n(+) and associated ionization potentials for neutral Ne-n clusters are predicted. The excitation spectra are found to be dominated by a visible band, resembling the major transition in symmetric Ne-3(+) and determined by the charge transfer from the central diatom to the wing atom(s) of the core. An ultraviolet band, originating from the transition within the diatom, is predicted to be the next most intense even at the equilibrium configuration, in contrast to heavier ran gases.