Collision-induced absorption between O2-CO2 for the a1Δg (v = 1) ← X3Σ (v = 0) transition of molecular oxygen at 1060 nm.

Agniva Banerjee,Frans Harren,Julien Mandon,David H Parker

doi:10.1039/c8cp06778c

Abstract

Collision-induced absorption between O2 and CO2 molecules associated with the a1Δg (v = 1) ← X3Σ-g (v = 0) band of oxygen around 1060 nm was measured using cavity ring-down spectroscopy. The lineshape for this transition is measured for the first time, and the integrated cross-section is found to be smaller than the only previous report. For pure oxygen, we find an integrated absorption value of (2.10 ± 0.31) × 10-4 cm-2 amg-2 which is in good agreement with the previous reported values. For O2-CO2 collisions we report an integrated value of (6.37 ± 1.09) × 10-5 cm-2 amg-2 which is small but still significant and not accounted for by theory.

Highlights

Absorption of sunlight by atmospheric constituents is a complex phenomenon where the efficiency depends on the quantum nature of the molecules together with the details of collisions taking place among them
For pure O2, we see a quadratic dependence on density and from simple fitting it was possible to extract the Rayleigh Scattering (RS) coefficients, the values obtained from measurements had large error (Z50%) and deviated from the theoretical values provided by Bates.[32]
The relatively large O2–CO2 collision-induced absorption (CIA) cross section we find for the X (v = 0) - a (v = 1) compared to X (v = 0) - a (v = 0) is not consistent with a recent model of CIA by Karman et al.[23] which predicts a very low cross section since CO2 is diamagnetic and cannot follow the favoured total spin exchange mechanism

Summary

Introduction

Absorption of sunlight by atmospheric constituents is a complex phenomenon where the efficiency depends on the quantum nature of the molecules together with the details of collisions taking place among them. When one oxygen molecule collides with other atmospheric species such as O2, N2, or CO2, a transition moment can be induced, lifting up the symmetry restrictions slightly,[14] the efficiency of this phenomenon depends on the quantum character of the collision partner. Mixing by spin–orbit coupling of the X3SÀg ground state with the higher lying d1Pg and/or C3Pg Rydberg state gives this transition magnetic dipole allowed character.[15] While a strong decrease in the magnetic dipole transition for vibrational levels a (v 4 0) due to unfavourable Frank–Condon factors is predicted,[16] the collision induced absorption coefficient was found to be surprisingly large.[17,18].

1.85 Â 10À4

Experimental section

Analysis

Results and discussions

Conclusion