First laboratory detection of an absorption line of the first overtone electric quadrupolar band of N2 by CRDS near 2.2 μm

Calculation of electric quadrupole linestrengths for diatomic molecules: Application to the H2, CO, HF, and O2 molecules

We present a unified variational treatment of the electric quadrupole (E2) matrix elements, Einstein coefficients, and linestrengths for general open-shell diatomic molecules in the general purpose diatomic code Duo. Transformation relations between the Cartesian representation (typically used in electronic structure calculations) to the tensorial representation (required for spectroscopic applications) of the electric quadrupole moment components are derived. The implementation has been validated against accurate theoretical calculations and experimental measurements of quadrupole intensities of ¹H₂ available in the literature. We also present accurate electronic structure calculations of the electric quadrupole moment functions for the X¹Σ⁺ electronic states of CO and HF, as well as for the a¹Δ_g-b¹Σ_g ⁺ quadrupole transition moment of O₂ with the MRCI level of theory. Accurate infrared E2 line lists for ¹²C¹⁶O and ¹H¹⁹F are provided. A demonstration of spectroscopic applications is presented by simulating E2 spectra for ¹²C¹⁶O, H¹⁹F, and ¹⁶O₂ (Noxon a¹Δ_g-b¹Σ_g ⁺ band).

Electric quadrupole transitions in carbon dioxide

Recent advances in high sensitivity spectroscopy have made it possible, in combination with accurate theoretical predictions, to observe, for the first time, very weak electric quadrupole transitions in a polar polyatomic molecule of water. Here, we present accurate theoretical predictions of the complete quadrupole rovibrational spectrum of a non-polar molecule CO₂, important in atmospheric and astrophysical applications. Our predictions are validated by recent cavity enhanced absorption spectroscopy measurements and are used to assign few weak features in the recent ExoMars Atmospheric Chemistry Suite mid-infrared spectroscopic observations of the Martian atmosphere. Predicted quadrupole transitions appear in some of the mid-infrared CO₂ and water vapor transparency regions, making them important for detection and characterization of the minor absorbers in water- and CO₂-rich environments, such as those present in the atmospheres of Earth, Venus, and Mars.

Detection of electric-quadrupole transitions in water vapour near 5.4 and 2.5 μm

Nowadays, the spectroscopic databases used for the modeling of Earth and planetary atmospheres provide only electric-dipole transitions for polyatomic molecules (H2O, CO2, N2O, CH4, O3…). Very recently, electric-quadrupole transitions have been detected in the high sensitivity cavity ring down spectrum (CRDS) of water vapour near 1.3 μm [A. Campargue et al., Phys. Rev. Res., 2020, 2, 023091, DOI: 10.1103/PhysRevResearch.2.023091]. This discovery paved the way to systematic searches of quadrupole transitions in water vapor and other polyatomic molecules. In the present work, on the basis of high accuracy ab initio predictions, H216O quadrupole lines are detected for the first time in the 5.4 μm and 2.5 μm regions where they are predicted to have their largest intensities (up to 10-26 cm per molecule). A total of twelve quadrupole lines are identified in two high sensitivity Fourier transform spectra recorded with a 1064 m path length. Ten lines in the 4030-4150 cm-1 region are assigned to the ν3 band while the lines near 1820 and 1926 cm-1 belong to the ν2 band. The derived line intensities which are largely above the dipole intensity cut-off of the standard spectroscopic databases, agree nicely with the theoretical predictions. We thus conclude that the calculated line list of quadrupole transitions, validated by the present measurements, should be incorporated in the spectroscopic databases.

Random vibration-driven continuous-wave CRDS system for calibration-free gas concentration measurement

Random vibrations were employed to pick up each monochromatic component in a continuous-wave cavity ringdown spectroscopy (CRDS) system using a bichromatic laser source. Light frequencies were selected within flat portions of an absorption profile to suppress the jitter in laser frequency during measurements. An interference effect caused by cavity length variations was suppressed by optimizing the initial fit point for each ringdown transient. The difference in exponential decay rates of two frequencies determined the gas mole fraction, and no calibration of empty cavity losses was necessary. Experiments on varying humidity were conducted, and the results agreed with the readings of a commercial hygrometer.