On the absorption spectra and linkage of inorganic nitrates and sulphates in the vapour state

The Physicochemical Properties of Pure Nitric Acid.

“…For the vapor of the pure acid, continuous absorption has been found in the region 2980-2520 A., with a maximum located at 2620 A. (90). A later study showed the absorption in the vapor to be the same as in the liquid acid, except for a more distinct band (47).…”