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Title: An upper limit for water dimer absorption in the 750 nm spectral region and a revised water line list
Authors: Shillings, A.J.L.
Ball, S.M.
Barber, M.J.
Tennyson, J.
Jones, R.L.
First Published: 2011
Publisher: Copernicus GmbH (Copernicus Publications) on behalf of the European Geosciences Union (EGU).
Citation: Atmospheric Chemistry and Physics, 2011, 11 (9), pp. 4273-4287
Abstract: Absorption of solar radiation by water dimer molecules in the Earth's atmosphere has the potential to act as a positive feedback effect for climate change. There seems little doubt from the results of previous laboratory and theoretical studies that significant concentrations of the water dimer should be present in the atmosphere, yet attempts to detect water dimer absorption signatures in atmospheric field studies have so far yielded inconclusive results. Here we report spectral measurements in the near-infrared around 750 nm in the expected region of the | 0⟩[subscript f] | 4⟩ [subscript b]|0 ⟩ overtone of the water dimer's hydrogen-bonded OH stretching vibration. The results were obtained using broadband cavity ringdown spectroscopy (BBCRDS), a methodology that allows absorption measurements to be made under controlled laboratory conditions but over absorption path lengths representative of atmospheric conditions. In order to account correctly and completely for the overlapping absorption of monomer molecules in the same spectral region, we have also constructed a new list of spectral data (UCL08) for the water monomer in the 750–20 000 cm[superscript −1] (13 μm–500 nm) range. Our results show that the additional lines included in the UCL08 spectral database provide an improved representation of the measured water monomer absorption in the 750 nm region. No absorption features other than those attributable to the water monomer were detected in BBCRDS experiments performed on water vapour samples containing dimer concentrations up to an order of magnitude greater than expected in the ambient atmosphere. The absence of detectable water dimer features leads us to conclude that, in the absence of significant errors in calculated dimer oscillator strengths or monomer/dimer equilibrium constants, the widths of any water dimer absorption features present around 750 nm are of the order of 100 cm[superscript −1] HWHM, and certainly greater than the 25–30 cm[superscript −1] HWHM reported in the literature for lower energy water dimer transitions up to 8000 cm[superscript −1].
DOI Link: 10.5194/acp-11-4273-2011
ISSN: 1680-7316
eISSN: 1680-7324
Version: Publisher Version
Status: Peer-reviewed
Type: Journal Article
Rights: © Author(s) 2011. This is an open-access article distributed under the terms of the Creative Commons Attribution License (, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Appears in Collections:Published Articles, Dept. of Chemistry

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