Spectroscopic characterization of the excited valence MgXe(3dδ 3Δ1), MgXe(3dπ 3Π), and MgXe(“3dσ” 3Σ+) van der Waals states

Abstract

The Mg(3s3d 3 D J )•Xe( 3 ⌺ ϩ ), Mg(3s3d 3 D J )•Xe( 3 ⌸ 0 ϩ ,0 Ϫ) , and Mg(3s3d␦ 3 D J )•Xe( 3 ⌬ 1 ) excited states have been characterized via resonance enhanced two-photon ionization ͑R2PI͒ spectroscopy of transitions from the long-lived Mg(3s3 p 3 P J )•Xe( 3 ⌸ 0 ϩ ,0 Ϫ) , metastable states of the MgXe van der Waals molecule. Because the excited Mg(3d) orbital is quite diffuse and the Xe atom can approach along the nodal axis of the aligned 3d orbital, minimizing repulsion, the MgXe(3s3d␦ 3 ⌬ 1 ) state (D o ϭ3160Ϯ150 cm Ϫ1 ) is even more strongly bound than the MgXe ϩ core ion ͑for which D o ϭ2848Ϯ150 cm Ϫ1 ͒. The MgXe(3d 3 ⌺ ϩ ) state (D o ϭ1262Ϯ150 cm Ϫ1 ) and the MgXe(3d 3 ⌸ 0 Ϫ) state (D o ϭ1229Ϯ150 cm Ϫ1 ) are much less bound. However, the potential curves of these two states are quite different, and it is suggested that the MgXe(''3d'' 3 ⌺ ϩ ) state is bound only because of substantial mixing of Mg(4 p) Rydberg character into the wave function. Interesting spin-orbit and spin-spin effects, detected and analyzed from the rotational structure of the vibrational bands, are attributed to mixing of some Xe character into molecular orbitals nominally of Mg* excited state character. © 1997 American Institute of Physics. ͓S0021-9606͑97͒00440-6͔

Theoretical Study of M+−RG Complexes (M = Ga, In; RG = He−Rn)

“…Similar D 0 trends to those of Tables and have been observed for the 3d 3 Δ 1 and 3d 3 Π 0 excited valence states of Mg−Ar, Mg−Kr, and Mg−Xe by Breckenridge and co-workers, − who also determined D 0 accurately for Mg + −Ar, Mg + −Kr, and Mg + −Xe by two-color photoionization threshold measurements , in the same apparatus. The D 0 values for the 3 Δ states are all much greater than those of the 3 Π states, and approach (or exceed, for Xe; see below) the D 0 values of the corresponding Mg + −RG ground state ions.…”

“…A reliable value was possible because low v ′ values for the Δ state were directly accessed spectroscopically down to v ′ = 1 from the same Mg−Xe ( 3 Π; v = 0) metastable state from which the two-color photoionization threshold to produce Mg + −Xe was measured accurately. This unusual result was rationalized tentatively by the authors as being due to an “extra” dispersive C 6 attraction between the diffuse 3dδ orbital, beyond the mostly “Mg + /Xe”-type core interaction in the 3 Δ state.…”

Spectroscopic characterization of excited Ca(4s4dδ 3DJ)RG(3Δ1,2) states (RG=Ar, Kr, Xe): No “heavy-atom” mixing of RG(ndδ) character into the wave functions

“…The bond strengths of the CaKr( 3 ⌬ 1 ) and CaXe( 3 ⌬ 1 ) states ͑see Table III͒ are much less, [5][6][7]12 and the bond lengths much longer, [5][6][7]12 than for the MgKr(3d␦ 3 ⌬) and MgXe(3d␦ 3 ⌬) analog, consistent with this idea.…”

“…The apparatus is described in more detail elsewhere. [5][6][7] Briefly, calcium vapor was produced by focusing the second harmonic of a Molectron MY-32/10 Q-switched Nd:YAG laser onto a calcium target rod ͑1/4 in. diameter machined from Alpha/Aesar 99% pure ingot͒.…”

“…Spin-orbit ͑SO͒ effects in such complexes have also been extensively discussed with regard to mixing of orbital alignment bonding character, 1 predissociation, 1 and increases in the spin-orbit interaction by addition of ''heavy-atom'' RG character into nominally metal atom molecular orbitals. [1][2][3][4][5][6][7][8] It has been observed in several cases [1][2][3][4][5][6][7][8] of low-lying excited ''p '' and ''d '' states of M*•RG (M*ϭexcited metal atom or ion; RGϭrare-gas atom͒ van der Waals complexes that the molecular spin-orbit coupling constant, A SO , is sometimes much larger than that predicted from the atomic spin-orbit coupling constant of the M* excited multiplet states to which the M*•RG states correlate at large internuclear distances R. It now appears, at least for low-lying excited states M*, that such increases in A SO values usually result from direct heavy-atom mixing of RG(np ) valence character into the M*•RG wave functions at small internuclear distances R. This is consistent both with ͑i͒ the regular ͑rather than inverted͒ nature of the ⍀ multiplets, even with large increases in A SO , and ͑ii͒ the increase of A SO as vЈ decreases, consistent with greater mixing of RG(np ) valence character at smaller ''effective'' distances R. The direct mixing was first demonstrated by Buenker 8,9 for the LiAr( 2 ⌸) states, and has since been confirmed by others. 10,11 It is interesting to examine analogous cases of ''d␦'' outershell M* configurations in M*(nd␦)•RG(⌬) excited states to see if the same kind of direct ''heavy-atom'' mixing of RG(nd␦) character into the M*RG(⌬) wave functions can occur.…”