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 c.m.→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.