Cluster–cluster reaction rate constants are obtained from measurements of coalescence in nozzle-beam expansions. Coalescence is described by the Smoluchowski equations, modified to accommodate changes in volume and temperature. The asymptotic form of the cluster size distribution can reveal the scaling of the second-order rate constants Kjk with the sizes of colliding particles. Scalings derived in the past are demonstrably incorrect and physically unjustifiable. Here, we use the physically reasonable scaling, Kjk = K11jμkμ, to study H2 coalescence in a nozzle beam. Experimental size distributions fit the predicted asymptotic form, Nk = Akae−bk, very well, with deviations identifying magic numbers. We obtain the base agglomeration cross section K11 and the parameter μ, and thus all the rate constants, for H2. By comparing with products of geometric cross section and relative velocity, we find sticking coefficients. Experimental results for a nozzle-beam expansion of He are analyzed similarly, to yield K11 and μ, and hence sticking coefficients, for He. These are compared to those for H2.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry