Knowledge Bank

The SiO molecule has been observed in circumstellar envelopes and in several sources, in particular in star-forming regions but not in cold clouds. Very recent studies suggest that the formation of this species must certainly be linked to high temperatures. This molecule has been localised in regions where the ultraviolet photons are abundant and it is necessary to know the photo dissociation cross sections and reaction rates. We report have {ab-initio} calculations of the energies of the first singles excited states lying between the electronic ground state and the first dissociation asymptote. The atomic orbital basis act comprised 129 contracted functions including f functions on the two atoms. First, we have calculated the MCSCF (Mult configurational self consistent field) potential energy functions along the stretching coordinate (coov) for the single valence states $(X1\Sigma +,C1\Sigma -,D1\Delta ,A \mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}\Pi )$ and the $E1\Sigma +$ which has some Rydberg character. All these states dissociate into $(Si3P+O3P)$. As first pointed out by several $authors1$, the latter $E1\Sigma +$ presents a barrier to dissociation resulting from interactions with higher $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}\Sigma +$ dissociating into $(Si1D+O1D)$. In the molecular region, the equilibrium distances $Ru$. the harmonic vibrational frequencies $\omega e$ and the excitation energies $Te$, have been calculated in a large scale Ab-initio Multireference Configuration interaction (MRC) for the ground state and the first valence excited states $(C1\Sigma -,D1\Delta ,A1\Pi )$. In the MRCI calculations, all valence excitations from the reference wave function were included. These spectroscopic constants compare well with the experimental $data2$. For the first dipolc allowed transition $(X \mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}\Sigma +A \mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}\Pi )$, we have performed the calculation of the potential energy, electric dipole and transition moment functions of the two electronic states for values of the stretching coordinate ranging from the molecular to the dissociation region. To lest the accuracy of these ab initio results, the calculated potential energies and electric dipole moments have been used in the evaluation of the vibration-rotation energy levels and the absorption spectrum at 300K which are in good agreement with the experimental $spectrum2$. The next step will be the determination of higher excited states which are important for dissociation studies.