Pressure dependence for the CO quantum yield in the photolysis of acetone at 248 nm: A combined experimental and theoretical study

The quantum yield of CO in the laser pulse photolysis of acetone at 248 nm and at 298 K in the pressure range 20 - 900 mbar (N-2) has been measured directly using quantitative infrared diode laser absorption of CO. It is found that the quantum yield of CO shows a signicant dependence on total pressure with Phi(CO) decreasing with pressure from around 0.45 at 20 mbar to approximately 0.25 at 900 mbar. From a combination of ab initio quantum chemical calculations on the molecular properties of the acetyl (CH3CO) radical and its unimolecular fragmentation as well as the application of statistical ( RRKM) and dynamical calculations we show that CO production results from prompt secondary fragmentation (via (2a)) of the internally excited primary CH3CO* photolysis product with an excess energy of approximately 62.8 kJ mol(-1). Hence, our findings are consistent with a consecutive photochemically induced decomposition model, viz. step ( 1): CH3COCH3 + hv -> CH3CO* + CH3, step (2a): CH3CO* -> CH3 + CO or step (2b) CH3CO* -(+M)-> CH3CO. Formation of CO via a direct and/or concerted channel CH3COCH3 + hv -> 2CH(3) + CO (1') is considered to be unimportant.