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

摘要

The infrared and visible spectra of gaseous oxygen have been examined at temperatures around 90deg;K using a long path absorption cell. At all temperatures the infrared and visible spectra show a broad band which can be assigned as collisionhyphen;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)2studied corresponds to the1Dgr;glpar;ngr;equals;0rpar;plus;1Dgr;glpar;ngr;equals;1rpar; larr;3Sgr;gminus;lpar;ngr;equals;0rpar;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 the3Sgr;gminus;lpar;ngr;equals;0rpar;state, while in the excited state one oxygen molecule is in the1Dgr;glpar;ngr;equals;0rpar;state and the other is in the1Dgr;glpar;ngr;equals;1rpar;state. The spacings and convergence of the dimer vibrational levels provide a determination of the dissociation energy of the ground and excited dimer states, givingDePrime;equals;87thinsp;andthinsp;Deprime;equals;50thinsp;cmminus;1. The infrared spectrum of (O2)2occurs near the infrared inactive fundamental vibration of O2and shows three regions of discrete absorption superimposed on the broad collisionhyphen;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 O2molecules within the dimer. From an analysis of the infrared vibrationhyphen;rotation band contour of one of the dimer fundamentals, an average distance of 4.8 Aring; between the centers of mass of the two O2molecules was determined for the (O2)2van der Waals molecule. Applying the usual band analysis formulas to determine the geometry is an uncertain procedure since the data indicate that (O2)2is 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)2as due to van der Waalshyphen;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.