Accurate Laboratory Measurement of the O₂ Collision‐Induced Absorption Band Near 1.27 μm

Abstract

The atmospheric band of O2 near 1.27 μm plays an important role in determining the air mass from ground or spaceborne atmospheric spectra. This band consists of narrow absorption lines of the anormalΔ1g−XnormalΣnormalg−3()0−00.25emtransitions superimposed to a much broader collision‐induced absorption (CIA) structure. Very accurate CIA binary coefficients, BO2−O2, BO2−N2, and BO2−air, are determined by highly sensitive cavity ring down spectroscopy from low density spectra (0.36 to 0.85 amagat) of pure oxygen and an O2/N2 mixture (20.96%/79.04%) at room temperature over the 7,513–8,466‐cm−1 region. Two cavity ring down spectrometers involving 12 distributed feedback lasers and two external cavity diode lasers were used below and above 7,920 cm−1, respectively. The measurements performed at different densities show a percent level consistency. This accuracy results from (i) a high baseline stability of the spectra, (ii) an accurate determination of the baseline using argon spectra recorded for the same densities, and (iii) a reliable removal of the local contribution of the absorption lines made easier thanks to subatmospheric pressure recordings. The retrieved binary coefficients are compared to the experimental data included in HITRAN2016 database (Maté et al., 1999, https://doi.org/10.1029/1999JD900824), to the dataset used for Total Carbon Column Observing Network spectra as well as to recent ab initio calculations. Although more accurate, the binary coefficients reported here are found in agreement with the Maté et al. measurements using high pressure Fourier transform spectroscopy. The derived value of the integrated CIA band intensity SO2−air= 1.31(3) × 10−4 cm−2 amagat−2 exceeds the corresponding Fourier transform spectroscopy value by about 3%.

Validation of spectroscopic data in the 1.27 µm spectral region by comparisons with ground-based atmospheric measurements

“…The latter corresponds to the recent data measured by Mondelain et al [15] for values at room temperature. Their (small) temperature dependences were taken from Maté et al [12] using:…”

“…Room temperature CIA binary coefficients were recently determined with improved accuracy, from low density spectra [i.e. from (0.97 to 2.3)10 19 molecule/cm -3 equivalent to 0.36 to 0.85 amagat] of pure oxygen and of an O2/N2 mixture by highly sensitive cavity ring-down spectroscopy (CRDS) [15]. Despite the lower sample pressures used in this CRDS study, the retrieved CIA coefficients were found in very good agreement with the FTS values by Maté et al [12].…”

High sensitivity spectroscopy of the O2 band at 1.27 µm: (II) air-broadened line profile parameters

“…In a previous work, highly sensitive cavity ring down spectroscopy (CRDS) was used to determine the room temperature CIA binary coefficients from low density spectra (0.36 to 0.85 amagat) of pure oxygen and an O 2 /N 2 mixture at room temperature, over the wide 7513-8466 cm -1 region [10]. As concerned the study of the absorption lines, a first contribution was dedicated to low pressure spectra of pure O 2 in the 7920-8085 cm -1 interval for pressures up to 150 Torr [11].…”

“…Since lines are quite isolated (see Fig. 1), a spectral range of about 1 to 2 cm -1 was considered at a time, in order to simplify the fitting procedure and to minimize the influence of the contribution of the CIA [10]. The contribution of lines close to the considered spectral region ( cm -1 from the center of the considered line) was nevertheless taken into account ± 1 using HITRAN2016 and fixed in the fitting procedure.…”

“…Figure10. The speed-dependent components of the collisional broadening and shifting for O 2 in air versus the angular momentum quantum number N" of the lower level of the transition.…”