Knowledge Bank

Measurements of the photoabsorption cross section of $O2$ and the partial photodissociation cross section yielding $O(1D)$ have been made over the wavelength region of the Tanaka second band, $(1,0)E3\Sigma u-X3\Sigma g-.h$ Photodissociation in this wavelength region predominantly results in the atomic species $O(1D)+O(3P)$, from predissociation via the $B3\Sigma u-$ electronic state. We have found a residual component of the cross section which does not yield $O(1D)$ and therefore arises from a different path to dissociation. The origin of this component is inferred from a coupled-channel Schr""{o}dinger equation (CSE) model, involving a manifold of electronic states and interactions previously used to explain experimental predissociation linewidths of the $np\pi a3\Sigma a-$ Rydberg $statesj$. The CSE calculations give results consistent with the measured branching ratio into the $O(1D)$ and $O(3P)$ dissociation channels. The residual cross section is identified as arising from the $(4,0)3p\pi uD\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Sigma u+-X3\Sigma g-$ $band.k$ This band is known to interfere strongly with the second $bandl$. We conclude that, whereas the $np\pi u3\Sigma u-$ Rydberg states dissociate entirely into $O(1D)+O(3P)$ due to a strong Rydberg-valence interaction, the $3p\pi u3\Sigma u+$ Rydberg state dissociates via two pathways, involving direct and indirect predissociation. The direct predissociation occurs from Rydberg-valence mixing of the $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Sigma u+$ states, with the electrostatic interaction an order of magnitude smaller than for the $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Sigma u-$ states, resulting in only $O(3P)$ products. Indirect predissociation arises from a spin-orbit interaction between the $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}p\pi u$ Rydberg $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Sigma u+$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Sigma u-$ states, yielding $O(1D)$. This interaction also provides the intensity of the $(4,0)D3\Sigma u+-X3\Sigma g-$ band.