Spectroscopic characterization of the excited Mg(3<i>s</i>3<i>d</i> 3<i>D</i> <i>J</i>)⋅Ar(3Π), Mg(3<i>s</i>3<i>d</i> 3<i>D</i> <i>J</i>)⋅Ar(3Δ), and Mg(3<i>s</i>4<i>p</i> 3<i>P</i> <i>J</i>)⋅Ar(3Π) van der Waals states

Abstract

The Mg(3s3dπ 3DJ)⋅Ar(3Π), Mg(3s3dδ 3DJ)⋅Ar(3Δ), and Mg(3s4pπ 3PJ)⋅Ar(3Π) excited states have been characterized via resonance enhanced two-photon ionization (R2PI) spectroscopy of transitions from the long-lived Mg(3s3pπ 3PJ)⋅Ar(3Π0+,0−) metastable states of the MgAr van der Waals molecule. Because the outer excited Mg(3d) and Mg(4p) orbitals are (similarly) quite diffuse and the Ar atom can approach along the nodal axes of each of the aligned orbitals, minimizing repulsion, the Mg(3s3dδ 3DJ)⋅Ar(3Δ) and Mg(3s4pπ 3PJ)⋅Ar(3Π) states are both very strongly bound (D0=1140±40, 1250±60 cm−1, respectively), approaching the bond energy of the MgAr+ ‘‘core’’ ion (for which D0=1240±40 cm−1). In contrast, the Mg(3s3dπ 3DJ)⋅Ar state is more weakly bound (D0=290±40 cm−1), although it has a greater bond strength than the Mg(3s3pπ 3PJ)⋅Ar(3Π0−) lower state (for which D0 is 160±40 cm−1). The effective spin–orbit constant of the Mg(3s3dπ 3DJ)⋅Ar(3Π) state is much larger than expected from the Mg(3s3d 3DJ) multiplet splittings, and also increases in magnitude as v′ decreases; possible reasons for this are discussed. It is suggested that the predissociation of the Mg(3s3dπ 3DJ)⋅Ar(3Π0+) state [the Mg(3s3dπ 3DJ)⋅Ar(3Π0−) state does not predissociate] is caused by a curve-crossing with the repulsive Mg(3s3dσ 1D2)⋅Ar(1Σ0++) state, which lies at a lower asymptotic energy because of substantial mixing of Mg(3p3p 1D2) character into its wave function.