A combined experimental and theoretical study on the terahertz vibrations of water vapors

Water vapor is of significance in global greenhouse effect. Its strong absorption at infrared region characterizes the fingerprint of water structure. We hereby performed THz time-domain spectroscopy (0.2-2 THz) study on water vapors at relative humidity from10% to 50%, in which the intensities of vibrations were found not proportionally increased with relative humidity. Besides rotation of water monomer, the absorption at THz frequency was probably attributed to small water clusters (N < 9) based on quantum mechanics calculations, where the distinct absorption peaks were matched with the vibrations of clusters. Moreover, planar structures were found to prevail in clusters less than eight waters owing to increasingly strong hydrogen bonds while cube structure dominated in eight-water cluster. This study thus promotes our understanding of not only absorptions of water vapor at far-infrared region but also their structures. (C) 2019 Elsevier B.V. All rights reserved.