Strong bonding in a doubly excited valence state of a van der Waals molecule

The results of ab initio calculations, using extended Gaussian basis sets and including correlation energy through a variety of means, provide strong support for the Mg(3pπ,3pπ 3PJ) · Ar(3Σ−) electronic-state assignment made by Massick and Breckenridge to spectral features in the resonant two-photon photoionization spectrum of vibrationally cold Mg(3s,3pπ 3PJ) · Ar(3Π) van der Waals complexes. The doubly excited Mg(3pπ,3pπ 3PJ) · Ar(3Σ−) state is found to be much more strongly bound than the singly excited Mg(3s,3pπ 3PJ) · Ar(3Π) state, which, in turn, is much more bound than the Mg(3s2 1SO) · Ar(3Σ+) ground state.