The spectrum of SO, which lies in the region 2400-4000A, has been photographed in the 2nd order of a 21-foot grating. The wave-numbers of the lines of the seven bands, (0,4), (0,5), (0,6), (0,7), (0,8), (0,9), and (1,4), are tabulated.

Rotational structure.—The analysis of the rotational structure leads to the assignment of the band system to a ${}_{}{}^3{}_{}{}^{}\Sigma$, ${}_{}{}^3{}_{}{}^{}\Sigma$ electronic transition, and yields the following values of the constants:

Spin fine-structure. The triplets are resolved for $K>\mathrm{}30\mathrm{}$ and for $K<\mathrm{}13\mathrm{}$. However, due to overlapping of the different branches, the resolution for $K<\mathrm{}13\mathrm{}$ could be observed well in the (0,8) band only. It was found that, in order to represent adequately the observed separations of the triplets, it was necessary to modify somewhat the equations in Kramers' theory of spin tripling in ${}_{}{}^3{}_{}{}^{}\Sigma$ states. The equations which best respresent the data are: ${\Delta }_{2-1}=A-\frac{B}{\mathrm{}(2K+3)\mathrm{}}-({\gamma }^{\prime }-{\gamma }^{\prime \prime })(K+1\mathrm{})\mathrm{}$ ${\Delta }_{2-3}=A+\frac{B}{\mathrm{}(2K-1)\mathrm{}}+({\gamma }^{\prime }-{\gamma }^{\prime \prime })K$ where, for the (0,8) band, $A=-2.11\mathrm{}$, $B=-10.83\mathrm{}$, and $({\gamma }^{\prime }-{\gamma }^{\prime \prime })=+0.0150\mathrm{}$. Of these three constants, $A$ is the only one which is different for different bands; it ranges from -1.97 for the (0,4) band to -2.19 for the (0.9) band. We cannot determine which one, if either, of the two terms, $A$ and $B$, represents the value of $3({\epsilon }^{\prime }-{\epsilon }^{\prime \prime })$ in Kramers' theory. The values of ${\Delta }_2{F}_1\mathrm{}\left(K\right)\mathrm{}$ and ${\Delta }_2{F}_3\mathrm{}\left(K\right)\mathrm{}$ indicate that ${\gamma }^{\prime }\sim +0.017\mathrm{}$ and that ${\gamma }^{\prime \prime }\sim -0.003\mathrm{}$.

Predissociation and perturbations. Bands with $v^{\prime }=0, 1, 2, \mathrm{and} 3$ are observed to break off at $K^{\prime }=66, 53, 39, \mathrm{and} 6$, respectively. No bands with $v^{\prime }>3\mathrm{}$ are observed. This effect is explained as predissociation, and is assumed to be caused by a ${}_{}{}^3{}_{}{}^{}\mathrm{I}$I state. Not only do the rotational levels for different $v^{\prime }$ end at different values of $K^{\prime }$, but also the term value at which the break occurs decreases with increasing $v^{\prime }$. If the potential energy curve for the ${}_{}{}^3{}_{}{}^{}\Pi$ state is assumed to have a shallow minimum, a satisfactory explanation of the effect of rotation on predissociation is obtained by an application of Oldenberg's theory of dissociation by rotation. A further indication that the ${}_{}{}^3{}_{}{}^{}\Pi$ potential energy curve has a shallow minimum is given by the observed perturbations of the rotational levels of the upper ${}_{}{}^3{}_{}{}^{}\Sigma$ state. According to our interpretation, the term value at which the rotational levels for $v^{\prime }=3\mathrm{}$ terminate corresponds to dissociation of the normal state of the SO molecule. The energy of dissociation of this state, measured from the $v^{\prime \prime }=0\mathrm{}$ vibrational level, is calculated to be 5.053±0.001 volts.

Vibrational structure. The origins of several bands of the $v^{\prime }=0\mathrm{}$ progression are calculated, and the vibrational constants of the lower state determined from them. Since the bands with $v^{\prime }>1\mathrm{}$ are too much perturbed to permit an analysis of their rotational structure, it was necessary to use measurements of band heads to determine the vibrational constants of the upper state. The constants obtained are: ${{\omega }_e}^{\prime \prime }=1123.73\mathrm{}\pm 0.24$ ${\mathrm{cm}}^{-1\mathrm{}}$, ${x_e}^{\prime \prime }{{\omega }_e}^{\prime \prime }=6.116\mathrm{}\pm 0.017$ ${\mathrm{cm}}^{-1\mathrm{}}$, ${{\omega }_e}^{\prime }=628.7\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$, and ${x_e}^{\prime }{{\omega }_e}^{\prime }=5.65\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$.

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