Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
5428645 | Journal of Quantitative Spectroscopy and Radiative Transfer | 2013 | 11 Pages |
â¢The water vapour self-continuum is measured by CRDS at room temperature near 1.6 µm.â¢The real time pressure dependence of the continuum was studied during pressure cycles.â¢The contribution of water adsorbed on the mirrors was identified and subtracted.â¢The Cs(T=296 K) values range between 3Ã10â25 and 3Ã10â24 cm2 moleculeâ1 atmâ1.â¢The derived self-continuum cross sections agree reasonably with the MT_CKD 2.5 model.
The water vapour self-continuum has been investigated by high sensitivity Cavity Ring Down Spectroscopy at room temperature in the 1.6 µm window. The real time pressure dependence of the continuum was investigated during pressure cycles up to 12 Torr for fifteen selected wavenumber values. The continuum absorption coefficient measured between 5875 and 6450 cmâ1 shows a minimum value around 6300 cmâ1 and ranges between 1Ã10â9 and 8Ã10â9 cmâ1 for 8 Torr of water vapour. The continuum level is observed to deviate significantly from the expected quadratic dependence versus the pressure. This deviation is interpreted as due to a significant contribution of water adsorbed on the super mirrors to the cavity loss rate. The pressure dependence is well reproduced by a second order polynomial. We interpret the linear and quadratic terms as the adsorbed water and vapour water contribution, respectively.The derived self-continuum cross sections, Cs(T=296 K), ranging between 3Ã10â25 and 3Ã10â24 cm2 moleculeâ1 atmâ1 are found in reasonable agreement with the last version of the MT_CKD 2.5 model but in disagreement with recent FTS measurements. The FTS cross section values are between one and two orders of magnitude higher than our values and mostly frequency independent over the investigated spectral region. The achieved baseline stability of the CRDS spectra (better than 1Ã10â10 cmâ1) level totally rules out water continuum absorption at the FTS level (1.2Ã10â7 cmâ1 at 9 Torr) in the CRDS cell. In order to find the origin of such conflicting results, the differences and possible experimental biases in the two measurement methods are discussed.