Electric quadrupole and magnetic dipole transitions of CO2 near 3.3 µm.
The recent detections of electric-quadrupole (E2) transitions in water vapor [1,2] and magnetic-dipole (M1) transitions in carbon dioxide [3,4] have opened a new field in molecular spectroscopy. While in their present status, the spectroscopic databases provide only electric-dipole (E1) transitions for polyatomic molecules (H2O, CO2, N2O, CH4, O3…), the possible impact of weak E2 and M1 bands to the modeling of the Earth and planetary atmospheres has to be addressed. This is especially important in the case of carbon dioxide for which E2 and M1 bands may be located in spectral windows of weak E1 absorption. In the present work, a high sensitivity absorption spectrum of CO2 was recorded by Optical-Feedback-Cavity Enhanced Absorption Spectroscopy (OFCEAS) in the 3.3 µm transparency window of carbon dioxide . The studied spectral interval corresponds to the region where M1 transitions of the ν2+ν3 band of carbon dioxide were recently identified in the spectrum of the Martian atmosphere . Here, in addition to M1 transitions, E2 transitions of the ν2+ν3 band could be detected by OFCEAS. This detection was performed on the basis of high accuracy ab initio predictions of the intensities of the E2 transitions . This is the first laboratory detection of electric quadrupole transitions in carbon dioxide. The E2 intensity values (on the order of a few 10–29 cm/molecule) are found in reasonable agreement with ab initio calculations. We thus conclude that both E2 and M1 transitions should be systematically incorporated in the CO2 line list provided by spectroscopic databases.
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