MILLIMETRE WAVELENGTH SPECTROSCOPIC OBSERVATIONS OF WATER DIMER IN VAPOUR PHASE

In an attempt to find the cause of an unexpected absorption feature that exists near 8 cm-1 in the millimetre-wave spectrum of the Earth's atmosphere, we have performed laboratory experiments using Fourier Transform spectroscopy, the results of which are presented in this paper. We have duplicated atmospheric conditions as closely as possible and have examined the absorption spectrum of cold water vapour in the wave-number region 5 to 30 cm-1. The new feature was observed in the laboratory and its integrated absorption strength, S, has been measured as a function of both pressure and temperature. S varies as the square of water vapour density, indicating a dimeric effect: in addition we have obtained a value for the association energy, ΔE, of the dimer from the temperature variation of S, and find ΔE = 6±3 kcal mol-1, in agreement with the predicted value of 5 kcal mol-1 for the hydrogen bond energy between two water molecules. Together with these results we present a short discussion of the possible implications that the observation of this new absorbing species may have in sub-millimetre atmospheric physics. © 1969.