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Photolyses of ketene at 3130 A and 3340 A

University of British Columbia

Previous research on the photolyses of ketene at 2700 A and 3650 A has shown that considerably different mechanisms are necessary to explain the experimental results. Exactly how the process of deactivation of the excited ketene molecule was affected by change in wavelength was not fully understood. It was felt that investigations of the primary quantum yields at intermediate wavelengths would be fruitful in obtaining a better understanding of the variation of reaction mechanism with wavelength. At 2700 A, using pure ketene, the only primary process is dissociation and therefore the primary quantum yield is unity, while at 3650 A dissociation occurs by way of an excited state which has a finite lifetime during which the excited ketene molecule may undergo collisional deactivation and internal conversion. At 3650 A therefore the primary quantum yield is much less than unity even at low pressure, and decreases with increasing pressure. This research has shown that in the case of ketene at 3130 A the primary quantum yield is approximately unity at low pressures, 20 mm., and decreases to 0.6 at one atmosphere. At 3340 A the primary quantum yield is approximately 0.7 at 26 mm. and 0.2 at 400 mm. and increases with increasing temperature. The dependence of primary quantum yield on pressure at 3130 A and 3340 A was anticipated and can be explained by almost the same mechanism as that proposed for 3650 A radiation. The amount of radiation absorbed during each run was very accurately measured and it was therefore possible to determine quantum yields to within ± 2% at 3130 A, and within ± 10% at 3340 A. For the photolysis at 3650 A the ratios of the rate constants of collisional deactivation and product formation and of internal conversion and product formation at 26° C are 4.6 x 10⁴ litres/mole and 28 respectively. At 3130 A and 37°C this research has shown these ratios to be 16.8 litres/mole and zero respectively, while at 3340 A the ratios are 1.64 x 10² litres/mole and 0.25 at 37°C and 0.99 x 10² and 0.1 at 100°C.

Text

2012-01-18

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http://hdl.handle.net/2429/40150

Chemistry

University of British Columbia

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