A THEORETICAL STUDY OF THE $X^{1}\Sigma^{+}-E^{1}\Sigma^{+}$ and $X^{1}\Sigma^{+}-A^{1}\Pi$ BAND SYSTEMS OF SiO

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Ohio State University

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The SiO molecule has long been of interest owing to its astrophysical occurrences. Additional interest in SiO at NASA arises because of the consideration of ``volume reflecting heat shields” made of $SiO_{2}$ for Jupiter entry. During Jupiter entry a relatively thick and cool boundary layer, with high concentrations of SiO, forms on the surface of the heat shield. The degree to which this boundary layer can absorb (or block) high temperature H and Si emission from the bow shock layer is an important consideration in determining the mass requirements of the best shield. We have, therefore, carried out extensive SCF-CI calculations on the $X^{1}\Sigma^{+}, E^{1}\Sigma^{+}$, and $A^{1}\Pi$ states of SiO using a large Slater basis. Excellent agreement is obtained with the experimental RKR potential curves for these states. The theoretical ground state dissociation energy is 8.2 eV, in excellent agreement with the most reliable experimental result of 8.26 $\pm$ 0.13 eV. Further, our computed $X^{1}\Sigma^{+}$ dipole moment curve is in excellent agreement with recent microwave experiments. Our results for the sum of the squares of the electronic transition moments for the$X^{1}\Sigma^{+}-E^{1}\Sigma^{+}$ and $X^{1}\Sigma^{+}-A^{1}\Pi$ band systems are critically compared with previous experimental and theoretical determinations. To assess the extent of ``radiation blockage” by SiO during Jupiter entry, we have combined our theoretical transition moments for the band systems specified above with experimental transition energies and RKR Franck-Condon factors to determine line-by-line absorption cross section between 160 and 230 nm. Using these cross sections and a three slab model for the thermochemical profiles, we have solved the radiative transport equations to unambiguously show that the incident shock layer radiation in this wavelength region is attenuated by nearly two orders of magnitude by $X^{1}\Sigma^{+}-A^{1}\Pi$ and $X^{1}\Sigma^{+}-E^{1}\Sigma^{+}$ band systems of SiO.


S. R. Langhoff: NRC Associate.
Author Institution: NASA, Ames Research Center