Spectroscopic catalogues, such as GEISA and HITRAN, do not yet include information on the water vapour continuum that pervades visible, infrared and microwave spectral regions. This is partly because, in some spectral regions, there are rather few laboratory measurements in conditions close to those in the Earth’s atmosphere; hence understanding of the characteristics of the continuum absorption is still emerging. This is particularly so in the near-infrared and visible, where there has been renewed interest and activity in recent years. In this paper we present a critical review focusing on recent laboratory measurements in two near-infrared window regions (centred on 4700 and 6300 cm−1) and include reference to the window centred on 2600 cm−1 where more measurements have been reported. The rather few available measurements, have used Fourier transform spectroscopy (FTS), cavity ring down spectroscopy, optical-feedback – cavity enhanced laser spectroscopy and, in very narrow regions, calorimetric interferometry. These systems have different advantages and disadvantages. Fourier Transform Spectroscopy can measure the continuum across both these and neighbouring windows; by contrast, the cavity laser techniques are limited to fewer wavenumbers, but have a much higher inherent sensitivity. The available results present a diverse view of the characteristics of continuum absorption, with differences in continuum strength exceeding a factor of 10 in the cores of these windows. In individual windows, the temperature dependence of the water vapour self-continuum differs significantly in the few sets of measurements that allow an analysis. The available data also indicate that the temperature dependence differs significantly between different near-infrared windows. These pioneering measurements provide an impetus for further measurements. Improvements and/or extensions in existing techniques would aid progress to a full characterisation of the continuum – as an example, we report pilot measurements of the water vapour self-continuum using a supercontinuum laser source coupled to an FTS. Such improvements, as well as additional measurements and analyses in other laboratories, would enable the inclusion of the water vapour continuum in future spectroscopic databases, and therefore allow for a more reliable forward modelling of the radiative properties of the atmosphere. It would also allow a more confident assessment of different theoretical descriptions of the underlying cause or causes of continuum absorption.
The Journal of Molecular Spectroscopy presents experimental and theoretical articles on all subjects relevant to molecular spectroscopy and its modern applications. An international medium for the publication of some of the most significant research in the field, the Journal of Molecular Spectroscopy is an invaluable resource for astrophysicists, chemists, physicists, engineers, and others involved in molecular spectroscopy research and practice. Submit your Article online The 'Elsevier Editorial System' (or EES) is a web-based system with full online submission, review and status update capabilities. EES allows you to upload files directly from your computer. This is part of our on-going efforts to improve the efficiency and accuracy of our editorial procedures and the quality and timeliness of the manuscripts published.
former Academic Press
As the world’s leading publisher of science and health information, Elsevier serves more than 30 million scientists, students, and health and information professionals worldwide. We are proud to play an essential role in the global science and health communities and to contribute to the advancement of these critical fields. By delivering world-class information and innovative tools to researchers, students, educators and practitioners worldwide, we help them increase their productivity and effectiveness. We continuously make substantial investments that serve the needs of the global science and health communities.
The very weak water vapor self-continuum has been investigated by high sensitivity Cavity Ring Down Spectroscopy in the 1.6 µm window at five temperatures between 302 K and 340 K. The absorption cross-sections, Cs(ν, T), were retrieved for ten selected wavenumbers from a fit of the absorption coefficients measured in real time during pressure ramps, after subtraction of the contributions of the local water monomer lines and of water adsorbed on the CRDS mirrors. The values measured between 5875 and 6665 cm-1 range between 1.5 × 10-25 and 2 × 10-24 cm2 molec-1 atm-1 with a minimum around 6300 cm-1. At 302 K, an agreement within 50% is observed over the whole window with the cross-sections provided by the MT_CKD V2.5 model. Nevertheless, while our measurements show that the Cs(ν, T) decrease from 302 K to 340 K is no more than 50% for all our selected wavenumbers, the MT_CKD V2.5 model predicts a much more pronounced temperature dependence in the centre of the window, the agreement being better on the edges of the window. The obtained results are discussed in relation with theoretical modeling of the water vapor self-continuum as far-wings of monomer lines or water dimer absorption. For potential atmospheric applications, cross-sections are provided at each temperature with a sampling step of 10 cm-1 for the entire 5850 – 6700 cm-1 range.
