Non-intrusive spectroscopic probing of weakly bound van der Waals complexes forming in gaseous carbon dioxide is generally performed at low pressures, for instance in supersonic jets, where the low temperature favors dimers, or in few-atmosphere samples, where the signature of dimers varying as the squared gas density is entangled with the dominating collision-induced absorption. We report experimental and theoretical results on CO2 dimers at very high pressures approaching the liquid phase. We observe that the shape of the CO2 -dimer bands undergoes a distinctive line-mixing transformation, which reveals an unexpected stability of the dimers despite the collisions with the surrounding particles and negates the common belief that CO2 dimers are short-lived complexes. Our results furnish a deeper insight allowing a better modeling of CO2 -rich atmospheres and provide also a new spectroscopic tool for studying the robustness of molecular clusters.
Transmittances recorded in the ν1/2ν2 Fermi dyad region at 300°K and at 80.7 Amagat for Ar. (b) 14.9-amagat CO2 mixed with various quantities of Ar. The absorption induced by collisions of CO2 monomers with Ar atoms has only a small influence on the band shape evolution which is dominated by the dimer signatures.