Crossed beams chemistry: Reactions of Ba, Sr, Ca, and Mg with Cl2 and Br2

Reactions of Ba, Sr, Ca, and Mg with Cl₂ and Br₂ have been studied in a crossed beam apparatus employing an electron bombardment ionizer‐mass filter detector. Product center‐of‐mass (c.m.) recoil energy and angle distributions have been fit to the measured monohalide product (MX) laboratory (LAB) angular distributions by averaging the $\mathrm{c.m.}\to \mathrm{LAB}$ transformations over the measured (nonthermal) beam speed distributions. These experiments give no indications of a dihalide product (MX₂) and indicate that the formation of MX₂ (by a two‐body radiative association) cannot account for more than a small fraction (<∼5%) of the reactive collisions. All eight reactions favor forward product scattering (i.e., MX scattered in the direction defined by the incident M) with relatively low recoil energies (∼10%–20% of the reaction exoergicity). For reaction with either halogen molecule, the fraction of the product scattered into the forward c.m. hemisphere increases in the sequence: Mg, Ca, Sr, Ba. Similarities and differences between the results of the present study and features of the reactions of alkali atoms are discussed.