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It is not possible to make the chemical by adding OH- to Hg2+ in aqueous solution since this merely precipitates yellow solid HgO, although it is possible that Hg(OH)2 is momentarily produced during the reaction.
The first experimental evidence for the existence of the molecule was reported by Wang and Andrews in 2004. They produced it by irradiating a frozen mixture of mercury, oxygen and hydrogen with light from a mercury arc lamp. The mixture had been produced by evaporating mercury atoms at 50 °C into a gas consisting of neon, argon or deuterium (in separate experiments) plus 2 to 8% hydrogen and 0.2 to 2.0% oxygen. The mixture was then condensed at 5 kelvin onto a caesium iodide window through which it could be irradiated.