The collisions between two oxygen molecules give rise to O4 absorption in the Earth atmosphere. O4 absorption is relevant to atmospheric transmission and Earth’s radiation budget. O4 is further used as a reference gas in Differential Optical Absorption Spectroscopy (DOAS) applications to infer properties of clouds and aerosols. The O4 absorption cross section spectrum of bands centered at 343, 360, 380, 446, 477, 532, 577 and 630 nm is investigated in dry air and oxygen as a function of temperature (203–295°K), and at 820 mbar pressure. We characterize the temperature dependent O4 line shape and provide high precision O4 absorption cross section reference spectra that are suitable for atmospheric O4 measurements. The peak absorption cross-section is found to increase at lower temperatures due to a corresponding narrowing of the spectral band width, while the integrated cross-section remains constant (within o3%, the uncertainty of our measurements). The enthalpy of formation is determined to be ΔH250 = -0.12±0.12 kJ mol-1, which is essentially zero, and supports previous assignments of O4 as collision induced absorption (CIA). At 203°K, van der Waals complexes (O2-dimer) contribute less than 0.14% to the O4 absorption in air. We conclude that O2-dimer is not observable in the Earth atmosphere, and as a consequence the atmospheric O4 distribution is for all practical means and purposes independent of temperature, and can be predicted with an accuracy of better than 10-3 from knowledge of the oxygen concentration profile.
The collision‐induced absorption spectra have been measured at room temperature and at 87°K for bands in the 1Δg + 1Δg ← 3Σg− + 3Σg− and 1Δg + 1Σg+ ← 3Σg− + 3Σg− simultaneous electronic systems for oxygen. The binary absorption coefficients were found to increase with decreasing temperature for 1Δg + 1Δg ← 3Σg− + 3Σg−. The band shapes for this system suggest that the Hamiltonian which is responsible for intensity borrowing depends on the angular orientation of the O2 molecules in the collision pair since ΔK = 0,± 2,± 4 selection rules are needed to account for the Δν1 / 2 ∼ 200cm−1 bandwidth. The relative intensity of the (1–0) and (0–0) bands indicates that the interaction Hamiltonian is also strongly modulated by the vibrational coordinates of O2. The frequency shift of this simultaneous transition indicates that the intermolecular distance parameter for 1Δg⋅⋅⋅1Δg is 3% larger than for 3Σg−⋅⋅⋅3Σg−. The unusual band shape for the 1Δg + 1Σg+ ← 3Σg− + 3Σg− band is interpreted in terms of an exiton interaction for the 1Δg⋅⋅⋅1Σg>+ combination. Although bound state (O2)2 molecules undoubtedly exist at low temperatures these data provide no unambiguous spectroscopic evidence of their presence.