Photochemical Studies in Flash Photolysis. III. Photolysis of Acetone in Different Wavelength Regions

Abstract

The flash photolysis of acetone was studied at wavelength regions centered around 260, 280, and 300 mμ, using absorbed intensities of the order of 10^(19) quanta/cc/sec for each wavelength region. The light source was an exploding wire, and the maximum temperature increase per flash was calculated to be not more than 5°C. The products, analyzed by gas chromatography using a sensitive electric discharge detector, consisted of C_2H_6, CO, biacetyl, and, in smaller amounts, CH_4. A search was made for other products as well, and detection limits are given. The C_2H_6/CO ratio decreased with increasing acetone pressure at all wavelength regions and was independent of light intensity in the range investigated. The R_(CO)/p_(acet) ratio, measured over a wide pressure range, was pressure‐independent at 260 and 300 mμ but increased appreciably with pressure at 280 mμ. At low pressures, where the C_2H_6/CO ratio approached a limiting value, the ratio decreased with wavelength according to the order 280>260>300 mμ. The lifetime of the excited acetone molecules at the respective wavelengths was estimated, from a steady‐state treatment, to be 0.6×10^(—9), 1×10^(—9), and 4×10^(—9) sec at 260, 280, and 300 mμ, assuming a collision deactivation efficiency of unity. At 300 mμ there was a marked difference in value of C_2H_6/CO at flash and low intensities: certain second‐order reactions involving excited states appear to occur completely at the relatively high concentrations prevailing under flash conditions. The transition region of intensity effects is described in subsequent papers of this series.