UV absorption spectra of H sub(2)O/HNO sub(3) films

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