We have recorded, near 4.3 $\mu m$, transmission spectra of pure $\mathrm{C}{\mathrm{O}}_2$ gas inserted between the windows of an extremely thin absorption cell. This was done for three pressures using a Fourier transform spectrometer and five optical paths between 0.17 and 1.15 $\mu m$. For these conditions, the line broadening induced by molecule-surface collisions can be studied under “clean” confinement conditions, i.e., between two parallel well-polished crystal surfaces separated by a known distance. This is in opposition with previous investigations using porous materials which involve pores of unknown dimensions with corrugated inner surfaces of ill-defined shapes. The analysis of the spectra shows that the line broadening due to the collisions of the molecules with the cell windows is independent of the optical transition and inversely proportional to the confinement length. Furthermore, the measured values are quantitatively reproduced if one assumes that a single $\mathrm{C}{\mathrm{O}}_2$-surface collision is sufficient to interrupt the rotating-dipole coherence. This gives a proof, here for the $\mathrm{C}{\mathrm{O}}_2$-sapphire system, of an assumption proposed many years ago and opens promising perspectives for the optical probing of porous materials.

• Received 16 September 2015
• Revised 7 December 2015

DOI:https://doi.org/10.1103/PhysRevA.93.012516