A spectroscopic characterization of the ã 3Π−, A 3Π+, and E 3∑+ states of the MgAr van der Waals molecule

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Abstract
The metastable ã(3Π0−) and A(3Π0+) states of the Mg(3s3p)Ar molecule were synthesized in a free-jet expansion of metastable Mg(3PJ) atoms, produced by the laser vaporization of a magnesium rod, in pure Ar. The ã(3Π0−), A(3Π0+), and E(3∑+) states were characterized spectroscopically by laser-induced fluorescence. The 3Π(v=0) states were found to be very near the Hund’s case (a) limit, with only 0.31 cm−1 separating the 3Π0+ and 3Π0− levels. The internuclear separations, re, were found to be 3.63 Å and the dissociation energies 316 and 296 cm−1, respectively. The E(3∑+) state is quite deeply bound (1128 cm−1) with a much smaller re, 2.83 Å, and it is postulated that the Ar atom has substantially penetrated the outer lobe of the 4s Rydberg orbital of the Mg(3s4s 3S1) atomic state to which this state correlates. Despite large populations of both Mg(3P1) and Mg(3P2), neither the 3Π1 nor 3Π2 states of MgAr were observed, apparently because the molecular 3Π1,2 states are collisionally quenched to the lower-energy 3Π0 state much faster than the corresponding atomic 3P1,2 multiplets are deactivated to the 3P0 level.