The absorption spectra of nitrates and nitrities in relation to their photo-dissociation

Summary

The two well-known absorption bands of inorganic nitrates whose long wave-length limits are at 350mµ and 230mµ respectively, are attributed to photo-dissociation of the nitrate into the nitrate and an atom of oxygen in the normal state (³P) and in the excited state (¹D₂) respectively. From the above wave-length limits the heat of dissociation of the oxygen molecule into two normal atoms comes out as 114 kilo cals./gm. mol., and the energy of excitation of the oxygen atom from its normal level (³P) to its (¹)D₂ level comes out as 43 kilo cals./gm. mol. These are in agreement with the values deduced from spectroscopic data.

The observed strong dichroism of both these absorption bands, which are much more intense when the electric vector of the incident light wave lies in the plane of the NO₃ group than when it is along the normal to the NO₃ plane, is interpreted as due to a corresponding difference in the quantum efficiency of the photo-dissociations for the two directions of polsarisation of the exciting light.

The two absorption bands of the nitrites receive a similar explanation as due to the dissociation from the nitrite of one oxygen atom, in the normal state and in the excited state respectively.