Spectroscopic investigation of van der Waals molecules. I. The infrared and visible spectra of (O2)2

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Abstract
The infrared and visible spectra of gaseous oxygen have been examined at temperatures around 90°K using a long path absorption cell. At all temperatures the infrared and visible spectra show a broad band which can be assigned as collision‐induced absorption. However, at low temperatures small but discrete features appear with integrated intensities dependent on the square of the gas density. These features are assigned to bound state van der Waals molecules of the type (O2)2. The visible absorption of (O2)2 studied corresponds to the 1 Δg (ν=0)+1 Δg(ν=1) ←3Σg−(ν=0) simultaneous transition. The part of the spectrum attributed to bound dimers shows a progression of eight fine structure bands superimposed on the broad simultaneous transition absorption. The fine structure has been assigned to combinations of electronic and vibrational transitions involving the stretching mode of the van der Waals bond of (O2)2. In the ground state each oxygen molecule is in the 3 Σg−(ν=0) state, while in the excited state one oxygen molecule is in the 1 Δg(ν=0) state and the other is in the 1 Δg(ν=1) state. The spacings and convergence of the dimer vibrational levels provide a determination of the dissociation energy of the ground and excited dimer states, giving De″=87 and De′=50 cm−1 . The infrared spectrum of (O2)2 occurs near the infrared inactive fundamental vibration of O2 and shows three regions of discrete absorption superimposed on the broad collision‐induced band. The discrete absorption bands have been assigned to fundamental and combination bands of (O2)2. The combination band features involve hindered rotor transitions associated with the internal rotations of the O2 molecules within the dimer. From an analysis of the infrared vibration‐rotation band contour of one of the dimer fundamentals, an average distance of 4.8 Å between the centers of mass of the two O2 molecules was determined for the (O2)2 van der Waals molecule. Applying the usual band analysis formulas to determine the geometry is an uncertain procedure since the data indicate that (O2)2 is weakly bonded and has a floppy structure. It was subsequently not possible to choose among possible linear or nonlinear dimer equilibrium configurations with the present experimental or theoretical information. All the spectroscopic evidence obtained here is consistent with the description of the weak bonding in (O2)2 as due to van der Waals‐type interactions. There is no need to suggest a pairing of the electrons in oxygen into some sort of weak chemical bond that might stabilize (O2)2.