Mechanisms and Relative Rates of MX2* Chemiluminescence in the Reactions of Ca, Sr, and Ba(1S) Atoms with Dihalogen Molecules

Abstract

Chemiluminescent reactions of ground state (Ca, Sr, and Ba) atoms with (Cl2, Br2, I2, ICl, and IBr) were studied in a beam-gas arrangement. The MX2* pseudo-continua were measured as a function of target gas pressure in the 0.0001−0.25 Pa range. To identify the main reaction channels that contribute to MX2* formation and to obtain their relative contributions, kinetic models were fitted to the data. The following channels were considered: (1) radiative two-body recombination (R2BR), (2) radiative three-body recombination (R3BR), (3) two consecutive harpooning steps involving MX† intermediate (two-step chemiluminescent reaction TSCR), and (4) combinations of the above. On their own, none of the mechanisms (1−3) provide satisfactory data fits. The best agreement is obtained by a model involving both R2BR and TSCR. The branching ratios of these channels were determined for p = 0−0.25 Pa. At 0.16 Pa these R2BR fractions vary from 7% (for Ca, Sr + ICl) to 79% (for Ba + IBr). Absolute CL cross sections and lower limits of photon yields were estimated by cross-calibrations. Photon yields for R2BR varied from 0.0004% to 0.037%, depending on the collision partners.