UV absorption spectra of H 2 O/HNO 3 films

The ultraviolet (UV) absorption spectra were measured for amorphous and crystalline H 2 O/HNO 3 films representative of polar stratospheric clouds (PSCs). Film compositions were determined using laser induced thermal desorption (LITD). Pure HNO 3 films showed a broad absorption peak with a maximum absorbance at λ ≈ 195 nm. The absorbances of amorphous H 2 O/HNO 3 films were also broad and peaked at λ=196–198 nm. The absorption cross sections increased slightly for more dilute films, although the band shape was nearly independent of the H 2 O content This increase in cross section was consistent with a change in the absorbing species from molecular HNO 3 to ionic NO 3 − . The major absorbing species in crystalline NAT, NAD, and NAM is also NO 3 − and these films all had very similar UV absorption spectra. Their absorption peaks were all at λ ≈ 200 nm and the band shapes were nearly identical.