Rare dimer molecules

The electronic spectra of rare gas dimers have been a subject of interest for many years, mainly because these dimers are model systems for studying van der Waals interactions, and because of their potential as media for VUV and XUV lasers. Yet very little is known about the excited states of these dimers. Two experimental techniques were combined in our laboratory for this investigation four-wave summixing (4-WSM) and a pulsed supersonic jet to produce rotationally and vibrationally cold dimer molecules. In this way it was possible to resolve rovibronic structures in several isotopic band systems of Xe2, Kr2 and Ar2, in the region 150 to 104 nm, to determine the relevant molecular constants, and to calculate potential energy curves for the ground states and the three lowest (stable) excited states, for all three dimers [41,42,43]. 

In the carbonyls, each molecule of carbon monoxide donates two electrons to the central atom. Cobalt has 27 extra nuclear electrons, and if two electrons are contributed by each of four carbon monoxide molecules, the cobalt would have an E.A.N. of 35. One more electron is needed to attain the rare gas structure of krypton and this is secured by the sharing of one electron pair between two cobalt monomers. The existence of a metal to metal bond in dicobalt octacarbonyl has been postulated by Ewens (34) and cryoscopic measurements have established without doubt the dimeric structure [Co(CO)4]2 for dicobalt octacarbonyl. 

One of the most commonly employed procedures has been to simply extrapolate the molecular coupling from the available atomic parameters using the so-called atoms-in-molecules approach (72). Here (r,) is assumed to be constant for electrons with the same n and / quantum numbers. The values of are then assumed to be equal to the spin-orbit coupling constants n/, which are derived from atomic spectral data. This approach has been employed by Wadt (73) in all-electron studies, and by other groups (32,74) in effective potential calculations involving the rare-gas dimers and dimer ions. Ermler and co-workers used this approach coupled with AREP calculations to determine spectroscopic properties for various states of Au2 (42), Hg2, and HgTl (75). 

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