Chemiluminescent spectra of alkali-halogen reactions

Using a beam-gas arrangement, we have obtained the chemiluminescence spectra of alkali-halogen reactions at pressures less than I mtorr. The spectra in general are characterized by a broad molecular feature on which are superimposed sharp atomic alkali transitions. Under higher resolution (better than 5 A) the molecular feature consists of a single progression of regularly spaced peaks extending over several thousand angstroms. Spectra are presented for the chemiluminescent reactions Mz+Xz(XY) ~MX*+MX(MY), where Mz=Kz, Rbz, CSz and XZ<XY)= Clz, Brz, Iz, IBr, ICI, CIF. The MX· emitter is identified by comparison of the vibrationally resolved spectra with previous absorption and emission studies of the alkali halides. The spectra consist of transitions from the bound or quasibound vibrational levels of a shallow homopolar excited state to high-lying vibrational levels of the ionic ground state. Differences in the onsets and cutoffs between our spectra and those of flame studies are noted and rationalized in terms of the different effective rotational potentials describing the MX· emitters produced under the single-collision conditions employed here and under the conditions prevailing in flame studies. For the mixed halogen reactions, the MY ground state is the more stable salt molecule that can be formed from the halogen atoms of the XY pair. A four-center mechanism is deduced from the dependence of the chemiluminescent intensity on reagent concentrations. The observed atomic emission apparently results from a different mechanism. While the alkali reactions with Fz do not yield appreciable molecular emission, atomic transitions from high Rydberg levels are prominent for these reaction systems. Both atomic and molecular chemiluminescence are shown to be consistent with electron-jump mechanisms.