Effect of atomic reagent approach geometry on reactivity: Reactions of aligned Ca(1P1) with HCl, Cl2, and CCl4

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Abstract
The reactivity of Ca(1P1) with HCl, Cl2, and CCl4 has been studied as a function of Ca(1P1) alignment with respect to the initial average relative velocity vector of the reagents in a beam‐gas scattering geometry. While the total chemiluminescence cross section for the Ca(1P1)+HCl reaction is insensitive to Ca(1P1) alignment, the branching into the CaCl(A 2Π) and CaCl(B2Σ+) states depends markedly on the approach direction of the atomic p orbital. Parallel approach of the Ca p orbital favors CaCl(B2Σ+) formation while perpendicular approach favors CaCl(A2Π). A weak dependence of these effects on CaCl(B2Σ+) vibrational state is observed. The analogous reaction with Cl2 shows a strong preference for perpendicular p‐orbital approach in both chemiluminescent product channels, which is most prominant for the CaCl(A2Π) state. In contrast, the reaction with CCl4 displays no significant dependence on approach geometry. For Ca(1P1)+Cl2, a chemi‐ionization channel is observed, showing a preference for perpendicular alignment intermediate between that for the CaCl(A2Π) and CaCl(B2Σ+) channels. Chemiluminescence spectra, absolute chemiluminescence cross sections, branching ratios, and emission polarizations are also presented. Results are interpreted in terms of an electron‐jump model in which the symmetry of the reagents is preserved during a reactive encounter.