New ultraviolet absorption cross-sections of BrO at atmospheric temperatures measured by time-windowing Fourier transform spectroscopy

The UV absorption cross-section spectra of the atmospherically important radical BrO have been determined using the recently developed technique of time-windowing Fourier transform spectroscopy (TW-FTS). The absorption spectra of the A2Π3/2–X2Π3/2 band system were recorded in the flash photolysis of a gaseous mixture of Br2 and O3. The bromine-photosensitized decomposition of O3 was observed at five different temperatures between 203 and 298 K. The absolute UV absorption cross-section was determined from the time-dependent observation of reactant and product absorptions and by a kinetic analysis of the BrO behavior. The integrated UV absorption cross-section of BrO was, within the accuracy of the measurements, constant over the temperature range studied, as expected from spectroscopic considerations. For the (7, 0) vibrational band at 29 540 cm−1 (338.5 nm), the peak absorption cross-sections were determined to be 2.19±0.23 at 298 K, 2.23±0.23 at 273 K, 2.52±0.26 at 243 K, 2.75±0.29 at 223 K, and 3.03±0.31 at 203 K (all in units ×10−17 cm2 per molecule, at a spectral resolution of 3.8 cm−1, with error intervals of 2σ). Further, vibrational constants and the dissociation limit for the electronic state A2Π3/2 were derived. The A←X dissociation energy was determined to be D0=35 240±160 cm−1 or 421.6±1.9 kJ/mol.