Product channels in the near-UV photodissociation of ozone

Abstract

The relative quantum yields for the formation of O₂(a ¹Δ_g) from 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 fall-off region for the formation of spin-allowed singlet products O₂(a ¹Δ_g) and O(¹D₂). The molecular fragment was detected by resonance 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 in excellent agreement with the recommended quantum yields for O(¹D₂) production at wavelengths up to 310 nm, but at longer wavelengths exhibit 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 309 nm, and do not show a shift in the fall-off curve to shorter wavelengths as has been reported in the literature for O(¹D₂). For wavelengths between 309 and 319 nm the yield of O₂(a ¹Δ_g) is smaller than that at room temperature and this, together with the results of measurements at fixed wavelengths as a function of temperature, confirms that the photolysis of internally excited ozone provides a major source of O₂(a ¹Δ_g) at wavelengths just beyond 310 nm. For wavelengths 320 nm the quantum yield is found to be approximately constant at the two temperatures, and suggests that 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(¹D₂) product in the fall-off region which take into account the spinallowed dissociation of internally excited ozone molecules. Good agreement is found at wavelengths up to 320 nm, particularly at room temperature, and suggests that the formation of singlet products extends noticeably beyond the fall-off region. The implications of this for stratospheric and atmospheric modelling are briefly discussed.