Knowledge Bank

The collision-induced photoabsorption spectrum of $O2$, pressurised by He, is presented in the 1180-1340 {\AA}-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 $\Delta A=2$ collision-induced $3p\pi u3\Delta u\leftarrow X3\Sigma g-$ Rydberg transition, partly on the basis of comparisons with the photoabsorption spectrum of metastable $O2(1\Delta g).h$ 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 $3p\pi u3\Delta u$ Rydberg state and the $A\prime 3\Delta u$ valence state, in contrast to the situation for the corresponding states of $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Sigma u-$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Pi u$ 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 $3p\pi u\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Delta u\leftarrow X3\Sigma g-$ Rydberg system exceeds that of the collision-induced $A\prime 3\Delta u\leftarrow X3\Sigma g$ valence system by at least an order of magnitude. Possible sources for this intensity are discussed.