Direct spectroscopic determination of the Hg2 bond length and an analysis of the 2540 Å band

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Abstract
The ground state bond length of the gas phase Hg2 was determined by taking high resolution spectra of an isotopically selective vibronic band of the jet‐cooled molecule via the X̃(O+g)→D̃(1u) transition. The rotational structure allows a determination of the ground state equilibrium internuclear separation of Re=3.63±0.04 Å, about 0.4 Å greater than the previous accepted value from gas phase viscosity measurements. Combining this with the previously determined change in bond length between the X̃ and D̃ states (ΔRe=1.1 Å) gives a D̃‐state bond length of only 2.5 Å. A careful analysis of the 2540 Å X̃(O+g)→F̃(O+u) band of the dimer, which has resisted previous analyses, shows that the band is in fact made up of a long progression of sequence bands which are virtually superimposed. Furthermore, the v‘=0–v’=1 transition is about 300 times less intense than the 0–0 transition. Franck–Condon analysis leads us to conclude that the O+u potential energy curve is virtually identical to that of the ground state, with internuclear separations in the two states which differ by less than 0.02 Å.