PRODUCT CHANNELS IN THE NEAR-UV PHOTODISSOCIATION OF OZONE

The relative quantum yields for the formation of O-2(a (1) Delta(g)) f rom the photolysis of ozone have been measured between 270 and 329 nm at room temperature, and between 300 and 322 nm at 227 K, near the fal l-off region for the formation of spin-allowed singlet products O-2(a (1) Delta(g)) and O(D-1(2)). The molecular fragment was detected by re sonance enhanced multiphoton ionisation at 331.5 nm. The measurements were put on an absolute scale by comparison with previous measurements in the short-wavelength region. The results at room temperature are i n excellent agreement with the recommended quantum yields for O(D-1(2) ) production at wavelengths up to 310 nm, but at longer wavelengths ex hibit a pronounced tail of 10-20% out to at least 329 nm. Measurements at 227 K are identical to those at room temperature between 300 and 3 09 nm, and do not show a shift in the fall-off curve to shorter wavele ngths as has been reported in the literature for O(D-1(2)). For wavele ngths between 309 and 319 nm the yield of O-2(a (1) Delta(g)) is small er than that at room temperature and this, together with the results o f measurements at fixed wavelengths as a function of temperature, conf irms that the photolysis of internally excited ozone provides a major source of O-2(a (1) Delta(g)) at wavelengths just beyond 310 nm. For w avelengths greater than or equal to 320 nm the quantum yield is found to be approximately constant at the two temperatures, and suggests tha t spin-forbidden dissociation of ozone is taking place as the dominant process in this long-wavelength region. The results are compared with recent modelling calculations for the formation of the O(D-1(2)) prod uct in the fall-off region which take into account the spin-allowed di ssociation; of internally excited ozone molecules. Good agreement is f ound at wavelengths up to 320 nm, particularly at room temperature, an d suggests that the formation of singlet products extends noticeably b eyond the fall-off region. The implications of this for stratospheric and atmospheric modelling are briefly discussed.