Journal of Physical Chemistry A, Vol.102, No.19, 3293-3298, 1998
Ultraviolet-visible absorption cross sections of gaseous HOBr
The UV-visible absorption cross sections for HOBr between 260 and 545 nm have been measured at 295 K in either 80 or 300 Torr of He bath gas. HOBr was generated in situ by laser photolytic production of OH radicals in the presence of Br-2, and its absorption of light was detected using both a gated diode array camera (220 nm windows) and a photomultiplier, Calibration was performed relative to Br-2 loss. The UV-visible spectrum of HOBr consists of three broad absorption bands between about 240 and 550 nm, with lambda(max) at 284 nm (sigma = (25.0 +/- 1.2) x 10(-20) cm(2)), 351 nm (sigma = (12.4 +/- 0.6) x 10(-20) cm(2)), and 457 nm (sigma = (2.3 +/- 0.2) x 10(-20) cm(2)). The existence of the third, long-wavelength absorption band has been confirmed unambiguously in this work. J-value calculations show that absorption into the third band is responsible for up to 50% of the total photolysis rate at high zenith angles close to the surface and at least 25% at all other altitudes and zenith angles down to 40 degrees. At this zenith angle the total J value ranges from 3.2 x 10(-3) s(-1) at 15 km to 2.2 x 10(-3) s(-1) at the surface, corresponding to lifetimes of 5.2 and 7.6 min, respectively.