Spectroscopic characterization of the unusually strongly bound, doubly excited van der Waals state, Mg(3pπ3pπ 3PJ)⋅Kr[3Σ−]

The unusual doubly excited van der Waal’s state, Mg(3pπ3pπ 3PJ)⋅Kr[3Σ−], has been characterized using a laser-vaporization, supersonic-jet source and R2PI (Resonance Two-Photon Ionization) spectroscopy. This state is very strongly bound (De=3966 cm−1) and has a short bond length (Re=2.45 Å) compared to its singly excited analogue, Mg(3s3pπ 3PJ)⋅Kr[3Π0−], for which De=267 cm−1 and Re=3.48 Å. In fact, this state is even more than twice as strongly bound as the ground-state Mg(3s)+⋅Kr ion, where De=1949 cm−1 and Re≈2.8 Å. Possible reasons for the strong van der Waal’s bonding are discussed, and it is concluded that the lack of σ-σ repulsion because there is no Mg(3sσ) valence electron must be a major factor; the similar ionic van der Waal’s state Mg+(3pπ)⋅Kr[2Π], which would be obtained by removing one of the Mg(3pπ) electrons, is even more strongly bound, with De≈7200 cm−1 [J. S. Pilgrim, C. S. Yeh, K. R. Berry, and M. A. Duncan, J. Chem. Phys. 100, 7945 (1994)].