Dependence of Reactivity on Internal and Translational Energy in K+SF6, CCl4, and SnCl4

Abstract

The dependences of the nonreactive scattering of potassium atoms at thermal energies from SF6, CCl4, and SnCl4 on internal energy of the molecules and initial relative kinetic energy are studied as a function of scattering angle. A collimated, velocity-selected beam of potassium atoms is crossed at right angle with a molecular beam whose source temperature is varied over the range of 300°–580°K. The angular dependence of the nonreactive cross sections is analyzed with an optical model of chemical reactions to obtain information about potential parameters in an assumed form of two-body interaction, the threshold conditions for reaction, the probability of reaction as a function of the distance of closest approach and internal energy, as well as an estimate of the total reaction cross section. The threshold conditions, energy and distance, in the reaction K + SF6 are independent of the initial relative kinetic energy at both low and high SF6 temperatures; the threshold distance increases by about 6% from T(SF6) = 300°K to 580°K. The variation of the nonreactive scattering with the SF6 beam source temperature, or SF6 internal excitation, can best be explained by a model in which only the stretching vibrational modes of SF6 are assumed to have an effect on the reactivity with K. The calculated differential cross sections based on this model fit very well to the measured results over the experimental ranges of SF6 temperature and of initial relative kinetic energy (2.05–5.18 kcal/mole). The effect of internal energy in CCl4 on its reactivity with K is similar to that of SF6 but smaller. The effect in SnCl4 is very small and uncertain; the interpretation is complicated by the lack of knowledge about the interaction potential.