15min:

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

Measurements of the photoabsorption cross section of O₂ and the partial photodissociation cross section yielding O(¹\!D) have been made over the wavelength region of the Tanaka second band, (1,0) E\,³_u^--X\,³_g^-. Photodissociation in this wavelength region predominantly results in the atomic species O(¹\!D) + O(³\!P), from predissociation via the B\,³_u^- electronic state.

We have found a residual component of the cross section which does not yield O(¹\!D) and therefore arises from a different path to dissociation. The origin of this component is inferred from a coupled-channel Schrödinger equation (CSE) model, involving a manifold of electronic states and interactions previously used to explain experimental predissociation linewidths of the np_u\,³_u⁺ Rydberg states. The CSE calculations give results consistent with the measured branching ratio into the O(¹\!D) and O(³\!P) dissociation channels. The residual cross section is identified as arising from the (4,0) 3p_u\,D\,³_u⁺-X\,³_g^- band. This band is known to interfere strongly with the second band.

We conclude that, whereas the np_u\,³_u^- Rydberg states dissociate entirely into O(¹\!D) + O(³\!P) due to a strong Rydberg-valence interaction, the 3p_u\,³_u⁺ Rydberg state dissociates via two pathways, involving direct and indirect predissociation. The direct predissociation occurs from Rydberg-valence mixing of the ³_u⁺ states, with the electrostatic interaction an order of magnitude smaller than for the ³_u^- states, resulting in only O(³\!P) products. Indirect predissociation arises from a spin-orbit interaction between the 3p_u~Rydberg ³_u⁺ and ³_u^- states, yielding O(¹\!D). This interaction also provides the intensity of the (4,0) D\,³_u⁺-X\,³_g^- band.