Investigation of the collision‐induced absorption by O 2 near 6.4 μm in pure O 2 and O 2 /N 2 mixtures

The collision‐induced fundamental vibrational band of molecular oxygen has been measured between 1300 and 2000 cm −1 using a Fourier‐transform infrared spectrometer and an optical path length of 84 m. Spectra were recorded for pure O 2 and O 2 /N 2 mixtures at densities up to 10 times the density of an ideal gas at standard temperature (273.15 K) and pressure (101.325 kPa), and for temperatures between 228 and 296 K. The band is dominated by the Δ J = 0, Q branch and the Δ J = 2, S and Δ J = −2, O branch shoulders, with the S branch exhibiting ripples previously attributed to bound dimer transitions, pure quadrupole transitions of O 2 perturbed by line mixing, and intercollisional interferences. The ripples are seen at the same wavenumbers in O 2 ‐Ar mixtures, with intensities dependent on both the O 2 and Ar densities, suggesting that the ripples are not due to bound dimer transitions. The integrated band intensity S is related to the collision‐induced absorption coefficients by S = S O 2 ‐O 2 ρ O 2 2 + S O 2 ‐N 2 ρ O 2 ρ N 2 , where S O 2 ‐O 2 and S O 2 ‐N 2 are the integrated binary collision‐induced absorption coefficients for O 2 ‐O 2 and O 2 ‐N 2 collisions, respectively, and ρ O 2 and ρ N 2 are the O 2 and N 2 gas densities. We find values for S O 2 ‐O 2 = 6.972(66) × 10 −4 cm −2 and S O 2 ‐N 2 = 7.12(22) × 10 −4 cm −2 , respectively, at 296 K, when the gas density is equal to that found at STP (i.e., S O 2 ‐O 2 = 6.972(66) × 10 −4 cm −2 amagat −2 and S O 2 ‐N 2 = 7.12(22) × 10 −4 cm −2 amagat −2 ).