New Infrared Absorption Bands of Alkali Vapors

We have identified two new infrared absorption bands in potassium vapor. The band between 1.1 and 1.6 μm is attributed to the ${\mathrm{K}}_2$ (${{}_{}{}^3{}_{}{}^{}\Sigma _{\mathrm{u}}^{}{}_{}{}^{}}^{+}\to {{}_{}{}^3{}_{}{}^{}\Sigma _{\mathrm{g}}^{}{}_{}{}^{}}^{+}$) transition, the absorption analog of the intense emission continuum of ${\mathrm{H}}_2$. The absorption at wavelengths longer than 1.6 μm is probably due to trimers.