Cloud chamber study of the gas phase photooxidation of sulfur dioxide

Abstract

Nucleation and growth promises to be a powerful quantiative detector and amplifier for many chemical physical phenomena. We have utilized vapor phase nucleation in a study of the photo-oxidation (by disproportionation) of SO2 through adaptation of the upward diffusion cloud chamber. Our results are unique to the extent that the photolysis of SO2 is performed in the presence of high partial pressures of water vapor. At the moment (because of interference from the formation of a sulfuric acid mist) it does not seem possible to study this reaction under these conditions by any other means. Our detection scheme is based on the nucleation of water droplets by H2SO4, a product of photolysis. In these experiments H2SO4 molecules are detected in real time at concentration levels in the neighborhood of 108 cm−3, and reaction rates of the order of 108 cm−3 s−1 are also detected. The photolysis of SO2 to form SO3 and SO (by disproportionation) can result from uv excitation in either the singlet (2900 Å) or triplet (3800 Å) absorption bands of SO2. Quantitative experimental results obtained from excitation in these two respective bands (the singlet excitation leads to intersystem crossing to the triplet manifold) are entirely consistent with one another. Furthermore, the results are consistent with mechanisms previously proposed and verified for the disproporationation taking place in the absence of water vapor, with one exception. The crucial step in this mechanism involves SO2 in the 3B1 state as the principal intermediate which participates in disproportionation through a reaction with a ground state SO2 molecule. The rate constant for this step yielded by previous studies is k1a=4.2×107 l mole−1s−1 while our result is approximately 2×1011 l mole−1 s−1. This is nearly as large as the estimated collision frequency rate constant, namely, 4×1011 l mole−1 s−1. The increased rate constant in the presence of water vapor is rationalized by assuming that SO2 in our experiments, mixed with water vapor at 283% relative humidty, is hydrated so that the activated complex contains an incipient H2SO4 molecule which is stable enough to present only a negligible activation energy. By adopting a suitable strategy, our rate constants are determinable without appeal to the theory of nucleation. However, direct use of the existing theory of nucleation together with the observed rate of nucleation yields a value for the rate constant in satisfactory agreement with that derived by the means which avoids nucleation theory. In this sense our experiments also provide some confirmation of existing nucleation theory. The paper contains a detailed account of the many special technical problems (many of them solved) involved in the adaptation of the cloud chamber to the detection of chemical physical processes. In this sense the accomplishments are twofold: (1) the investigation of the gas phase photoinduced disproportionation of SO2 in the presence of water vapor, and (2) the demonstration of the use of the diffusion cloud chamber as a quantiative tool for the detection and amplification of chemical physical phenomena.