ATTACKING ATOM EFFECTS IN THE M + CH3I -] MI + CH3 COLLISION DYNAMICS (M = NA, K, RB, CS)

This paper reports on new results on the dynamics of the M + CH3I --> MI + CH3 (M = K, Cs) reaction family obtained from crossed molecular beam studies. With the K + CH3I reaction as standard, absolute values of the M (K, Rb, Cs) + CH3I excitation functions have been obtained over a significant collision energy range. A comparison of these reactive processes is presented revealing the attacking atom effects in their collision energy dependence. Although the common electron jump mechanism seems to govern the main dynamics, important differences appear associated to the size and ionization potential of the attacking atom. Some of them are as follows: (a) The backward deviation of the reactively scattered MI increases slightly as the size of the atom increases. Particularly for the Cs reaction there is a shift from backward to near sideways as the collision energy increases. (b) The collision energy dependence of the products' average translational energy shows a linear dependence without important kinematic effects. (c) In relation with the reaction cross section and its collision energy dependence clear trends are noted: (i) At low collision energy the lower the ionization potential of the attacking atom, the smaller the reaction threshold and the higher the reaction cross section. (ii) At intermediate collision energy and providing that maxima and/or minima in the reaction cross section are present, the lower the ionization potential the smaller, the collision energy at which the reaction cross section, sigma-R, shows a maximum and/or minimum value. These trends are discussed in relation to possible kinematical and dynamical effects involved in the (collision energy dependence of the) electron transfer required in such collision processes.