The absorption coefficient for both liquid and vapor H2O2 is essentially the same.
The shape of the curve relating the extinction coefficient to the wavelength is slightly parabolic.
The absorption of ultraviolet radiation by H2O2 results in dissociation of the molecule into two hydroxyl radicals (HO.), although other reactions are possible and may occur to some extent.
Beer’s law is not strictly obeyed by H2O2 solutions, as higher concentrations of H2O2 absorb to a greater extent than Beer’s law would predict (i.e., the molecular extinction coefficient decreases as H2O2 concentration increases > 50% wt.%).
The presence of alkali shifts the absorption curve toward the visible (i.e., increases the absorption coefficient). This is due to the dissociation of H2O2 into the perhydroxyl ion (HO2-) that absorbs more intensely than H2O2.