Spectroscopic characterization of the Δ3(4<i>d</i>), Π3(4<i>d</i>), Σ+3(4<i>d</i>), and Π3(5<i>p</i>) Rydberg states of the MgAr van der Waals molecule

Abstract

The Mg(3s4dσ 3DJ)⋅Ar(3Σ+), Mg(3s4dπ 3DJ)⋅Ar(3Π), Mg(3s4dδ 3DJ)⋅Ar(3Δ), and Mg(3s5pπ 3PJ)⋅Ar(3Π) Rydberg 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. The 4dπ, 4dδ, and 5pπ states are all strongly bound (D0=1230±50, 1200±40, 1270±50 cm−1, respectively). These bond energies are very similar to that of the ground-state MgAr+ “core’’ ion (D0=1240±40 cm−1), indicating very effective penetration of transversely aligned, diffuse Rydberg electron clouds on the Mg atom by the Ar atom, even for the low-lying n=4,5 Rydberg states. The Mg(3s4dσ 3DJ) ⋅Ar(3Σ+) state was substantially less bound, D0=800±40 cm−1, showing there is still some residual Mg(4dσ)/Ar(3pσ)2 electron–electron repulsion preventing penetration of the axially aligned Mg(4dσ) Rydberg electron cloud by the Ar atom. Successful computer simulations of the rotational structure of several of the vibrational transitions to the 4dΔ and 4dΣ states, assuming Hund’s case “b’’ upper-state character, resulted in R0′ values of 2.80±0.04 Å and 2.90±0.05 Å, respectively (compared to R0=2.82±0.01 Å determined previously by others for the MgAr+ ion). Because the 4dπ and 5pπ states have similarly large bond strengths as well as similar asymptotic Mg(4d 3DJ) and Mg(5d 3PJ) atomic energies, they have “mixed’’ 4dπ/5pπ character.