Analysis of the continuum absorption in water vapor at room temperature within the purely rotational and fundamental ro-vibrational bands shows that a significant part (up to a half) of the observed absorption cannot be explained within the framework of the existing concepts of the continuum. Neither of the two most prominent mechanisms of continuum originating, namely, the far wings of monomer lines and the dimers, cannot reproduce the currently available experimental data adequately. We propose a new approach to developing a physically based model of the continuum. It is demonstrated that water dimers and wings of monomer lines may contribute equally to the continuum within the bands, and their contribution should be taken into account in the continuum model. We propose a physical mechanism giving missing justification for the super-Lorentzian behavior of the intermediate line wing. The qualitative validation of the proposed approach is given on the basis of a simple empirical model. The obtained results are directly indicative of the necessity to reconsider the existing line wing theory and can guide this consideration.
Experimental spectra of the H2O continuum in far infrared [31] range approximated by the model allowing for the contribution from the water dimers and the empirically calculated contribution from the line wings. The thin solid curve is the contributions from the line wings.
Experimental spectra of the H2O continuum in far infrared [31] range approximated by the model allowing for the contribution from the water dimers and the empirically calculated contribution from the line wings. The thin dashed curve is the contributions from themetastable dimers.
Experimental spectra of the H2O continuum in far infrared [31] range approximated by the model allowing for the contribution from the water dimers and the empirically calculated contribution from the line wings. The bold solid curve represents the model.
Experimental spectra of the H2O continuum in far infrared [31] range approximated by the model allowing for the contribution from the water dimers and the empirically calculated contribution from the line wings. The dots are the experimental data. [31] T.A. Odintsova, M.Yu. Tretyakov, O. Pirali, P. Roy, Water vapor continuum in the range of rotational spectrum of H2O molecule: New experimental data and their comparative analysis, Journal of Quantitative Spectroscopy and Radiative Transfer, 2017, Volume 187, Issue 2017, Pages 116–123, DOI: 10.1016/j.jqsrt.2016.09.00, http://dx.doi.org/10.1016/j.jqsrt.2016.09.00
We present laboratory measurements of the water vapor continuum absorption in the far-IR region from 14 to 200 cm−1. These data were obtained using a Fourier Transform spectrometer IFS125 HR associated with a multipass absorption cell and with the synchrotron far-IR broadband radiation extracted by the AILES beamline of SOLEIL facility. The spectra were recorded at room temperature (296 K) and pressures ranging from 2.73 to 15.1 mbar. A comparison with presently available experimental data is presented and the nature of the possible contributors to the continuum is discussed. The new data considerably extend and unify diverging results of previous measurements of the continuum performed in several spots within the range from about 3 cm−1 up to 84 cm−1. The new evidence of significant contribution of the water dimer to the continuum formation is revealed in the range of 14–35 cm−1. Analysis of the possible cause of the observed continuum indicates that its significant part in the range of the maximum intensity of water monomer rotational spectrum cannot be explained within the current understanding of the continuum origin.
