Problem of water vapor absorption continuum in atmospheric windows. Return of dimer hypothesis.

Problem of water vapor absorption continuum in atmospheric windows. Return of dimer hypothesis. Abstract Two alternative hypotheses try to explain the origin of the continuum. The present communication gives new argument in favor of the dimer hypothesis. This argument is based on the existence of the wide component of line in absorption spectrum of polyatomic molecules. The continuum of water vapor absorption in the region of atmospheric windows (8–14 and 3–5 µm) was found in 1938 [1]. The first explanation of its nature was that continuum is the sum of far wings of collisionally broadened lines of rotational and vibrational-rotational transitions in water molecules. Afterwards an anomalously strong square dependence of absorption on water vapor pressure was observed along with its exponential negative temperature dependence [2,3]. The slope of the exponent is 5 kcal/mole and is thus equal to the binding energy of molecules in a dimer of water. These are strong arguments in favor of the dimer hypothesis which assumes that the continuum is caused by the presence of water dimers in the gas. But these are indirect proofs. In succeeding years no new facts in favor of the dimer hypothesis were found. But it was definitely determined that no intensive absorption bands of water dimer molecules are present in the region of atmospheric windows. This is considered to be the main argument against the dimer hypothesis. In addition, the shape of the continuum corresponds better to far wings of the lines than to some local bands of molecules absorption. As a result, in succeeding years most scientists again gave preference to the monomer hypothesis [4]. Thus, it became a deadlock situation. There existed explicit indirect indications of the dimer hypothesis. Direct spectroscopic studies seem contradict it and show the validity of the monomer hypothesis, which fails to explain satisfactorily the temperature dependence. The present communication is aimed at drawing the attention of scientists to a new argument in favor of the dimer hypothesis. A rather elegant way to end the deadlock now exists: the continuum is actually formed by far wings of molecule absorption lines. But these are mainly the wings of absorption lines of water dimer molecules. The lines of dimer molecules have intensive Lorentzian wings, whose absorption cross section can be several orders of magnitude higher than for monomers. Such unconventional supposition has come from another field of scientific investigations , namely, from those of IR …