Water vapour self-continuum in near-visible IR absorption bands: Measurements and semiempirical model of water dimer absorption

The water vapor self-continuum absorption at room temperature in the 1.25 µm window

“…As mentioned in the introduction, water dimers contribute to the self-continuum absorption and have been evidenced as broad spectral signatures in the near infrared in-band regions and as partially resolved rotational spectrum in the mm-submm wave range [27], [28]. Our tentative interpretation of the structure near 8455 cm -1 as a BWD band will follow the same approach based on a comparison with theory, as applied in [34], [46], [35], [38].…”

“…A dimer-based model including both quasibound and bound WD contributions was developed and found to fit reasonably well to the measured self-continuum. About half of the measured continuum was found to have a WD origin, the quasibound contribution being larger than the BWD contribution for the considered bands [38].…”

“…According to [38], the intensity of the BWD band predicted at 8530 cm -1 [37], is 2.35×10 -21 cm/molecule while the value of the equilibrium constant at room temperature is about 0.025 atm -1 . It leads to a calculated value of 5.87×10 -23 cm×molecule -1 atm -1 for the integrated cross-section of the band at 8530 cm -1 .…”

“…Distinct (broad) spectral peaks were found to coincide with BWD calculated spectrum [23], [36], [37] leaving little doubt on the BWD contribution to the in-band continuum. Very recently, similar comparison between WD predicted spectrum and FTS self-continuum measurements were performed in the region of the bands at 8800 and 10600 cm -1 [38]. The measurements were performed at high temperature (398 K and 431 K) in order to allow for using high water vapor pressure values (1-4 atm).…”

“…The characterization of the atmospheric absorbers in this particular window is of importance as the 1.27 µm oxygen band (including rovibronic lines superimposed over a broad collision induced absorption (CIA) band -see e. g. [39], [40]) is considered for air-mass determination in a number of satellite missions [41]. In addition, near 8500 cm -1 , the studied region overlaps the above mentioned FTS investigation at high temperature [38] which will allow for a discussion of the temperature dependence. Finally, a BWD absorption band is predicted with significant intensity in the studied spectral interval and its detection would be the first experimental evidence of the BWD contribution to the continuum in the near infrared transparency windows.…”