Doubly excited valence states of neutral van der Waals molecules: Mg(3<i>p</i>π,3<i>p</i>π 3<i>P</i> <i>J</i>)⋅Ar(3Σ−)

Abstract

The first of a new class of doubly excited valence states of van der Waals molecules, Mg(3pπ,3pπ 3PJ)⋅Ar(3Σ−), has been discovered and characterized by resonance two-photon ionization spectroscopy. This state has been found to be quite strongly bound (D0=2850±100 cm−1 and to have a very short bond length (R0=2.41±0.02 Å). The analogous singly excited state, Mg(3s3pπ 3PJ)⋅Ar(3Π0−), in stark contrast, has a very much smaller bond strength (D0=160±40 cm−1) and a longer bond length (R0=3.66±0.02 Å). In fact, the new Mg(3pπ3pπ 3PJ)⋅Ar state is more than twice as bound than even the Mg(3s 2S1/2)⋅Ar+ ground-state ion (D0=1240±40 cm−1, R0=2.82 Å). The strong bonding is due both to the transverse alignment of the Mg(3pπ) electrons and particularly to the lack of a Mg(3s) electron. The resulting loss of Mg(3sσ)/Ar(3pσ)2 repulsive forces facilitates close approach of the Ar atom. This is turn allows attractive Mg(3pπ)2/Ar dispersive forces and Mg2+/Ar ion/induced-dipole type forces to continue to much smaller internuclear distances, creating a strong, short van der Waals bond. Possible mechanisms for the parity-dependent predissociation and the apparent ‘‘spin–spin’’ type splittings observed for the Mg(3pπ,3pπ 3PJ)⋅Ar(3Σ−) state are also discussed.