PRESSURE-DEPENDENT YIELDS AND PRODUCT BRANCHING RATIOS IN THE BROAD-BAND PHOTOLYSIS OF CHLORINE NITRATE

The photolysis of chlorine nitrate was studied using broadband flash p hotolysis coupled with long-path ultraviolet-visible absorption spectr oscopy. Branching ratios for the Cl + NO3 and ClO + NO2 product channe ls were determined from time-dependent measurements of ClO and NO3 con centrations. Yields of the ClO and NO3 products displayed a dependence on the bath gas density and the spectral distribution of the photolys is pulse. Product yields decreased with increasing bath gas density re gardless of the spectral distribution of the photolysis pulse; however , the decrease in product yield was much more pronounced when photolys is was limited to longer wavelengths. For photolysis in a quartz cell (lambda > 200 nm) the yield decreased by a factor of 2 over the pressu re 10-100 Torr. In a Pyrex cell (lambda > 300 nm), the yield decreased by a factor of 50 over the same pressure range. When photolysis was l imited to lambda > 350 nm, the yield decreased by a factor of 250. Bra nching ratios for the photolysis channels [ClONO2 + hv --> ClO + NO2 ( 1a) and ClONO2 + hv --> Cl + NO3 (1b)] were determined from the relati ve ClO and NO3 product yields at various pressures. Although the absol ute product yield displayed a pressure dependence, the branching betwe en the two channel was independent of pressure. The relative branching ratios (assuming negligible contributions from other channels) are 0. 61 +/- 0.20 for channel 1a and 0.39 +/- 0.20 for channel 1b for photol ysis with lambda > 200 nm and 0.44 +/- 0.08 for channel 1a and 0.56 +/ - 0.08 for channel 1b for photolysis with lambda > 300 nm. The implica tions of these results for the chemistry of the lower stratosphere are discussed.