The Absorption Spectrum of Arsenic

A new band-system has been found when ultraviolet light is absorbed by arsenic vapor, heated to 1100°C. Some eighty band-heads have been detected between the wave-lengths of 2200 and 2750A. An analysis of the vibrational levels has been made, which shows that the spectrum can be ascribed to the diatomic molecule. The value of ${\omega }_e$ is 432 ${\mathrm{cm}}^{-1\mathrm{}}$ for the lower and 271 ${\mathrm{cm}}^{-1\mathrm{}}$ for the upper state. The vibrational levels of the excited molecule are greatly perturbed, as is shown by irregularities in the plot of $\Delta G\left(v^{\prime }\right)$ against $v^{\prime }$. The continuous absorption of arsenic in the ultraviolet is considerable in extent. It is therefore probable that its potential energy curve is cut by another, due to a third state of the molecule, which causes predissociation. Energies of dissociation have been calculated and, by analogy with phosphorous, suggestions have been made as to the states of the atoms produced by dissociation. A doubling of the band-heads in a restricted region of the system has been observed. A critical survey of previous work on the fluorescence spectrum of arsenic is included and an attempt has been made to allot, as far as is possible, vibrational quantum numbers to the bands there observed.