Mercury photosensitised luminescence of water vapour

Abstract

The mercury photosensitised luminescence of water vapour is shown to arise from a bimolecular reaction of Hg(6³P₀) with H₂O, by three types of steady-state experiment; no evidence for termolecular reactions has been found.

(1) The emission profile is unchanged as the vapour pressure of H₂O is varied between 2 and 600 Torr.

(2) The quantum yields (∼0.25) of the luminescence show little dependence on the total pressure.

(3) By comparing the intensity of the HgH₂O^* emission to the intensity of the HgNH^*₃ emission from the same gas mixture, the rate of reaction of Hg(6³P₀) with H₂O is shown to be unaffected when the total pressure is varied from 150 to 600 Torr.

The rate coefficient of the reaction, Hg(6³P₀)+ H₂O = Hg(6¹S₀)+ H₂O +hν, was recorded as 1.5(± 0.2)× 10^–15 cm³ molecule^–1s^–1 at 293 K.

The rate of quenching of the 2537 Å line by H₂O was found to vary inversely with temperature; it is concluded that the temperature dependent re-excitation of Hg(6³P₀) to Hg(6³P₁) is an important process, even at 293 K. It is suggested that the main route for radiationless decay is also a bimolecular reaction of Hg(6³P₀) with H₂O.