Light-scattering experiments on water Vapour at pressures approaching saturation

Abstract

THE existence of dimers in water vapour at pressures approaching saturation has been postulated for some years and a number of observations have been interpreted in terms of them^1–4. The relative dimer concentration is expected to increase sharply with increasing percentage saturation and decreasing temperature^1,3. From speed of sound measurements, a dimer concentration of 8 parts per 10³ has been reported at 50 °C and 90% saturation³; this might be expected to increase to >20‰ at 95% saturation and 20 °C. It may be supposed that the polarisability of the water dimer will be considerably larger than that of the monomer, and therefore, using an assumed value for polarisability, an estimate of the dimer concentration at a given temperature and vapour pressure can be made by comparing the quadratic term in a plot of the light scattered as a function of pressure with the known second virial coefficient. In early experiments, using a large multi-pass scattering cell, a very large quadratic term was observed. This arose, however, largely from a rapid increase in the elastic scattering from water-contaminated surfaces as the vapour pressure in the cell approached saturation. We have now eliminated this spurious scattering by selecting spectroscopically only those photons which exhibit a frequency shift due to Brillouin and Raman scattering.