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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.

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