Millimetre wave spectroscopy of water vapour using harmonics from IMPATT diode oscillators

The power in harmonics generated by millimetre wave IMPATT oscillators has been used to study absorption by water vapour in the wavenumber range from 5.6 to 6.3 cm-1 (168 to 189 GHz). The harmonic content of the oscillators was determined by Fourier transform spectroscopy, which gives an unambiguous measure of the power in a given harmonic. When the sample had a value of fractional saturation between 0.94 and unity (which represents vapour in contact with liquid water) new absorption features were observed which had a strength comparable to the 6.12 cm-1 water monomer line. They are attributed to the formation of vapour phase molecular complexes.