The photolysis of ozone by ultraviolet radiation. IV. Effect of photolysis wavelength on primary step

Abstract

Quantum yields for the photolysis of ozone have been measured at six wavelengths in the region 248 to 334 nm. The quantum yield for the photolysis of pure ozone increases linearly with ozone concentration over the pressure range employed for $\lambda $ $\leq $ 310 nm, but is virtually independent of [O$_{3}$] at $\lambda $ = 334 nm. At all wavelengths the quantum yield, extrapolated to zero ozone pressure, is near 4. Addition of H$_{2}$ to the ozone markedly increases the quantum yield at all wavelengths except $\lambda $ = 334 nm. Dilution of the ozone by O$_{2}$ reduces the quantum yield; at high [O$_{2}$] the quantum yield tends to zero. It is shown that the results are consistent with the production of singlet molecular oxygen (in the $^{1}$\Delta $_{\text{g}}$ or $^{1}$\Sigma $_{\text{g}}^{+}$ states) in the primary step at all wavelengths in the ultraviolet region. For $\lambda $ $\leq $ 302 nm the atomic product is in the excited, $^{1}$D, state, O$_{3}$ + h$\nu $$_{(\lambda \leq 302\text{nm})}$ $\rightarrow $ O$_{2}$($^{1}$$\Sigma _{\text{g}}^{+}$ or $^{1}$\Delta $_{\text{g}}$) + O($^{1}$D), (1) but with increasing wavelength the efficiency of O($^{1}$D) production drops rapidly, and at $\lambda $ = 334 nm the atomic product is almost exclusively O($^{3}$P). O$_{3}$ + h$\nu $$_{(\lambda =334\text{nm})}$ $\rightarrow $ O$_{2}$($^{1}$$\Sigma _{\text{g}}^{+}$ or $^{1}$\Delta $_{\text{g}}$) + O($^{3}$P). (6) Thus it appears that ozone photolysis in the weak absorption tail of the Huggins band does not proceed with conservation of spin. Some possible implications of the laboratory observations in atmospheric photochemistry are considered.