Chemiluminescence in Molecular Beams. Electronic Excitation of Alkali Atoms in Exchange Reactions of Vibrationally Excited Alkali Halides

Abstract

Energy transfer by a chemical-exchange process, KBr†+Na→K*+NaBr (where † denotes vibrational excitation and * electronic excitation), has been observed in a ``triple-beam'' experiment. The KBr† was formed at the intersection of crossed beams of K+Br2 and entered a second scattering chamber containing a Na cross beam, where light emitted by K* was observed with a photomultiplier behind an interference filter. The cross section for the exchange process is estimated to be at least 10 Å2. The angular variation of the light intensity agreed precisely with the angular distribution of KBr measured (by a surface-ionization detector) in the same experiment. The light disappeared when propyl iodide was substituted for Br2 (as expected, since the K+PrI reaction exothermicity is well below the threshold for electronic excitation of K). An attempt was also made to observe the nonreactive exchange process, NaBr†+K→NaBr+K* (by interchanging the K and Na beams). However, this was obscured by background light arising from collisions of K with ambient Br2; at present it can only be concluded that the cross section appears to be <10% of that for the reactive exchange process. The light yield from chemical exchange was about as large when the experiments were repeated with Br2 replaced by ICl. Thus the primary K+ICl reaction must give an appreciable yield of KCl (since the exothermicity is above the threshold for K excitation if KCl is formed but below if KI is formed). In this case, the light intensity falls off more rapidly at wide angles than the angular distribution of the alkali halide. This suggests that the primary reaction also forms KI which scatters mainly at wide angles, whereas the KCl scatters mainly forward with respect to the incident K atoms.