The Emission Spectrum of Diatomic Arsenic

The emission spectrum of ${\mathrm{As}}_2$ consists principally of an extensive group of bands in the ultraviolet, which have been analyzed into two overlapping band systems. These two systems have the same lower state. Their upper states lie at about the same electronic energy, have similar vibrational intervals and perturb one another strongly. In one system, called $A\to X$, predissociation sets in at two points in the upper state, namely, at $v^{\prime }=9\mathrm{}$ and $v^{\prime }=14\mathrm{}$. In the regions where predissociation is occurring absorption bands appear (for low $v^{\prime \prime }$) on the emission plates. The heat of dissociation obtained depends upon what assumption is made as to the pair of atomic states standing at the predissociation energy. If this pair is ${}_{}{}^4{}_{}{}^{}S+{}_{}{}^2{}_{}{}^{}D$, $D\left(X\right)\mathrm{}$ is about 4.0 volts in agreement with the value obtained by extrapolation of the vibrational levels, but if this pair is ${}_{}{}^2{}_{}{}^{}D+{}_{}{}^2{}_{}{}^{}D$, as appears to be required by the Kronig rules for perturbations and predissociations, $D\left(X\right)\mathrm{}$ is 2.6 volts.