Raman-like pumping from long range excited alkali atom-alkali dimer interactions

Abstract

In a study of the reactions of halogen atoms with small supersonically cooled molecules, we observe the manifestation of Raman scattering in the absence of an external light source. The observed features are unique in that they are induced by and correlate with emission from the Na D-line components (D 1, D 2) formed in the chemical reaction, Na 2 + Br → Na ∗ + NaBr . In the chemiluminescent studies, the D-line emission is scattered by cooled sodium dimers (Na 2) formed, through supersonic expansion, in the lowest vibrational levels of their ground electronic state. Multiple Stokes and anti-Stokes features, assigned as resonance Raman progressions, dominate all normal chemiluminescent emission from the Na 2 B 1 Π u → X 1 Σ g + and A 1 Σ u + → X 1 Σ g + band systems. Comparison of higher resolution scans ( Δ ν ∼ 1.15 cm −1 ) with the results of computer simulations confirms the observation of resonance Raman scattering and suggests an unusually large broadening linewidth for the dimer, Γ ∼ 4 cm −1. This linewidth cannot be associated with Doppler or power broadening. Nor can it be attributed to interaction with the atomic bromine in the reaction zone. Pulsed and cw laser probes indicate that these bands are intimately linked to the presence of an excited state sodium atom (3P) constituency, suggesting the possibility of a long range Na 2Na ∗ interaction which mediates the Raman scattering process.