10min:

L. J. COIN, B. R. LEWIS AND S. T. GIBSON, Research School of Physical Sciences and Engineering, The Australian National University, Canberra, ACT 0200, Australia.

The collision-induced photoabsorption spectrum of O₂, pressurised by He, is presented in the 1180--1340~Å-region. In addition to the collision-broadened wings of the dipole-allowed mixed Rydberg-valence transitions, we have found a series of diffuse peaks whose intensities depend linearly on the foreign-gas pressure. The observed peaks have been assigned as the (1,0)--(5,0) bands of the \Lambda=2 collision-induced 3p_u\,³\!_u <- X\,³_g^- Rydberg transition, partly on the basis of comparisons with the photoabsorption spectrum of metastable O₂(¹\!_g). The relative regularity of the spacings between the observed peaks supports the conclusions of Buenker and Peyerimhoff, in an ab initio study, that there is little interaction between the 3p_u\,³\!_u Rydberg state and the A'\,³\!_u valence state, in contrast to the situation for the corresponding states of ³_u^- and ³_u symmetries. Although the complexity of this spectral region and the diffuseness of the bands precludes any detailed line-profile analyses, it is clear that the integrated intensity of the 3p_u\,³\!_u <- X\,³_g^- Rydberg system exceeds that of the collision-induced A'\,³\!_u <- X\,³_g^- valence system by at least an order of magnitude. Possible sources for this intensity are discussed.