Infrared look at the spectral effects of submicron confinements of CO2 gas

We have recorded, near 4.3 mu m, transmission spectra of pure CO2 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 CO2-surface collision is sufficient to interrupt the rotating-dipole coherence. This gives a proof, here for the CO2-sapphire system, of an assumption proposed many years ago and opens promising perspectives for the optical probing of porous materials.