WAVELENGTH DEPENDENCE OF THE PRIMARY PHOTODISSOCIATION PROCESSES IN ACETONE PHOTOLYSIS

The quantum yields of CH4 and CO were determined in the photolysis of acetone-HCl and acetone-HCl-CO2 mixtures at 285, 300, and 313 nm and 25 +/- 2-degrees-C. In these mixtures HCl is used in order to ensure conversion of CH3 into methane, thus allowing evaluation of primary photodissociation processes from the pressure dependence of PHI(CH4) and PHI(CO). The results are interpreted in terms of the occurrence of two primary photodecomposition modes: CH3COCH3* --> CH3 + CH3CO# (hot) followed by CH3CO# --> CH3 + CO (I) and CH3COCH3* --> CH3 + CH3CO (II). The low pressure quantum yields of these processes (phi(I) and phi(II), respectively) are estimated as being equal to 0.10 and 0.90 at 285 nm, 0.06 and 0.94 at 300 nm, and 0.03 and 0.28 at 313 nm. The observation that PHI(CH4) and PHI(CO) do not exhibit the same pressure dependence suggests that decomposition of acetone proceeds from two distinct excited states. Process II appears to originate from a vibrationally rich first excited triplet state of acetone (T1*), while CO formation can be envisaged as arising from high vibrational levels of first excited singlet (S1).