  The recent detections of electric-quadrupole (E2) transitions in water vapor
and magnetic-dipole (M1) transitions in carbon dioxide 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 micron
transparency window of carbon dioxide. The studied spectral interval
corresponds to the region where M1 transitions of the n2+n3 band of carbon
dioxide were recently identified in the spectrum of the Martian atmosphere.
Here, both M1 and E2 transitions of the n2+n3 band were detected by OFCEAS.
Using recent ab initio calculations of the E2 spectrum of 12C16O2, intensity
measurements of five M1 lines and three E2 lines allow us to disentangle the M1
and E2 contributions. Indeed, E2 intensity values (on the order of a few 10-29
cm per molecule) are found in reasonable agreement with ab initio calculations
while the intensity of the M1 lines (including an E2 contribution) agree very
well with recent very long path measurements by Fourier Transform spectroscopy.
We thus conclude that both E2 and M1 transitions should be systematically
incorporated in the CO2 line list provided by spectroscopic databases.
