Perturbations and Predissociation in theS2Band Spectrum

The ${\mathrm{S}}_2$ bands belonging to the ${}_{}{}^3{}_{}{}^{}{\Sigma }_u_{}^{-}{}_{}{}^{}\leftrightarrow {}_{}{}^3{}_{}{}^{}{\Sigma }_g_{}^{-}{}_{}{}^{}$ transition have been photographed with relatively large dispersion in absorption and in emission between $\lambda 3500$ and $\lambda 2400$. Several perturbations have been found in the vibrational levels of the excited ${}_{}{}^3{}_{}{}^{}{\Sigma }_u_{}^{-}{}_{}{}^{}$ state. These became apparent when the quantum assignment of the band heads were determined. The following revised equation was found to fit the unperturbed heads: $\nu =32250.9+429.5(v^{\prime }+\frac{1}{2})-2.75\mathrm{}{(v^{\prime }+\frac{1}{2})}^2-727.4(v^{\prime \prime }+\frac{1}{2})+2.91\mathrm{}{(v^{\prime \prime }+\frac{1}{2})}^2$ Weak bands were observed (mainly in absorption) about 20 ${\mathrm{cm}}^{-1\mathrm{}}$ to the violet of the heads in the $v^{\prime \prime }=0\mathrm{}$ progression. These bands are believed to be due to another system. Perturbations of about +17 ${\mathrm{cm}}^{-1\mathrm{}}$ have been found in the heads of the bands with $v^{\prime }=0, 2, 4$, which are presumably due to perturbations of the rotational levels near the band heads by another electronic state or perturbations in the $U\left(r\right)\mathrm{}$ curve of the ${}_{}{}^3{}_{}{}^{}{\Sigma }_u_{}^{-}{}_{}{}^{}$ state. Perturbations found in the levels with $v^{\prime }>9\mathrm{}$ are attributed to a ${}_{}{}^3{}_{}{}^{}\Pi _u^{}{}_{}{}^{}$ state which causes predissociation in these bands.