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.