PARTIAL PHOTODISSOCIATION CROSS SECTIONS FOR $O_{2}$ IN THE 1205 {\AA} REGION
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Date
1997
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Ohio State University
Abstract
Measurements of the photoabsorption cross section of $O_{2}$ and the partial photodissociation cross section yielding $O(^{1}D)$ have been made over the wavelength region of the Tanaka second band, $(1,0)E^{3} \Sigma_{u} - X^{3} \Sigma^{-}_{g}. ^{h}$ Photodissociation in this wavelength region predominantly results in the atomic species $O(^{1}D) + O(^{3}P)$, from predissociation via the $B^{3}\Sigma^{-}_{u}$ 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 Schr\""{o}dinger equation (CSE) model, involving a manifold of electronic states and interactions previously used to explain experimental predissociation linewidths of the $np\pi_{a} ^{3}\Sigma_{a}^{-}$ Rydberg $states^{j}$. 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 $(4,0) 3p\pi_{u} D{^{3}}\Sigma^{+}_{u} - X ^{3}\Sigma^{-}_{g}$ $band.^{k}$ This band is known to interfere strongly with the second $band^{l}$. We conclude that, whereas the $np\pi_{u} ^{3}\Sigma^{-}_{u}$ Rydberg states dissociate entirely into $O(^{1} D) + O(^{3} P)$ due to a strong Rydberg-valence interaction, the $3p\pi_{u} ^{3}\Sigma^{+}_{u}$ Rydberg state dissociates via two pathways, involving direct and indirect predissociation. The direct predissociation occurs from Rydberg-valence mixing of the $^{3}\Sigma^{+}_{u}$ states, with the electrostatic interaction an order of magnitude smaller than for the $^{3}\Sigma^{-}_{u}$ states, resulting in only $O(^{3} P)$ products. Indirect predissociation arises from a spin-orbit interaction between the $^{3}p\pi_{u}$ Rydberg $^{3}\Sigma^{+}_{u}$ and $^{3}\Sigma^{-}_{u}$ states, yielding $O(^{1} D)$. This interaction also provides the intensity of the $(4,0)D^{3}\Sigma^{+}_{u} - X^{3}\Sigma^{-}_{g}$ band.
Description
$^{h}$The mixed Rydberg-valence E electronic state is also labelled $B'$ by some authors. $^{i}$L.C. Lee, T. G. Slanger, G. Black, and R.L. Sharpless, J. Chem. Phys. \textbf{67}, 5602 (1977). $^{j}$S. S. Banerjee, Ph.D. Thesis, The Australian National University (1996). $^{k}$The {D} electronic state also labelled $\beta$ by some authors. $^{l}$B.R. Lewis, S.T. Gibson, M. Emami, and J.H. Carver, J. Quant. Spectrosc. Radiat. Transfer \textbf{40}, 469 (1988).
Author Institution: Research school of physical Sciences and Engineering, The Australian National University
Author Institution: Research school of physical Sciences and Engineering, The Australian National University