Simultaneous collision-induced transitions in H$_2$O+CO$_2$ gas mixtures
Résumé
A collision induced absorption (CIA) band has been measured near 6000 cm$^{-1}$ in a spectrum of humidified CO$_2$ recorded by cavity ring down spectroscopy (CRDS) at low pressure (<1 atm). This absorption adds a contribution to the H$_2$O+CO$_2$ continuum mostly originating from far wings of the CO$_2$ and H$_2$O resonance lines broadened by collisions with H$_2$O and with CO$_2$, respectively. The observed CIA corresponds to a simultaneous excitation of 12 CO$_2$ and H$_2$O colliding molecules in the $\nu_3$ antisymmetric and $\nu_1$ symmetric stretching mode, respectively. CRDS spectra recorded near 5940 cm$^{-1}$ with a highly enriched $^{13}$CO$_2$ sample provide a confirmation of the assignment since the measured CIA isotopic spectral shift (of about-68 cm$^{-1}$) coincides with that between the $\nu_3$ bands of $^{12}$CO$_2$ and $^{13}$CO$_2$. The integrated binary coefficient of the two CIA is evaluated and found to be on the order of 2.3×10$^{-3}$ cm$^{-2}$ amagat$^{-2}$. Classical molecular dynamics simulations (CMDS) of the considered CIA are also presented, based on the dominant dipole induction mechanism associated with the vibrational matrix elements of the dipole of CO$_2$ ($\nu_3$) and isotropic polarizability of H$_2$O ($\nu_1$). The results of the calculations are found in good agreement with the observations, thus further validating the attribution of the observed CIA structure to the above mentioned double transitions.
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