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The photodissociation processes of ketene at 3130 A, 3340 A and 3650 A Taylor, Gladstone Altamont

Abstract

Previous research on the photolysis of ketene has shown that the kinetics of photodissociation processes were not fully understood and that more accurate data were needed to evaluate the mechanism of the rate of dissociation of the electronically and vibrationally excited molecules. There was some evidence that the primary quantum yields at shorter wavelengths extrapolated to a value greater than unity at zero pressure, if this were not within experimental error, it would provide evidence for a process of multistage deactivation of the excited ketene molecules. In the kinetic studies of the dissociation of excited molecules, intersystem crossing to the triplet state had been included to account for phosphorescence. Theoretical consideration from this had led to predictions of the effect of dissociation from the triplet state on the quantum yields of carbon monoxide on the photodissociation of ketene. Experimental verification was now needed to determine the effect, if any, derived from triplet dissociation. Previous attempts have been made to apply the unimolecular theory of dissociation to the photodissociation of excited molecules, but the data available yielded physically impossible results. Hence, only reasonable theoretical values of the parameters involved are given. This research attempted to obtain more accurate data on the quantum yields of ketene at 3130 Å, 3340 Å and 3650 Å at various temperatures. From the results it is now possible to differentiate within the limits of the experiment, between the various theories of the dissociation process. It is established that triplet dissociation is either not a real effect or is small enough to be undetectable under the experimental conditions. It is possible to discount a theory of a cascade collisional deactivation process involving more than three collisions of the excited molecules, but differentiation is not made between one, two and three collisions under experimental conditions. The unimolecular theory of dissociation is applied to the results of the photodissociation process and values determined for the parameters involved. Reasonable agreement with the theoretical predictions is obtained.

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