Chemiluminescent Reaction Channel Opened by Photon Absorption During Collision

The reaction exoergicity of K + HgBr→KBr + HgBr ($\Delta D_0=-0.86\mathrm{}\pm 0.09$ eV) plus the energy of a photon of $\lambda <606\mathrm{}\pm 26$ nm provides sufficient energy to open the channel to HgBr* ($\tilde{B}{}_{}{}^2{}_{}{}^{}\Sigma _{}^{+}{}_{}{}^{}$). Although neither reagents nor products absorb near 595 nm, luminescence at 500 nm is observed from the crossing region of beams of K, Hg${\mathrm{Br}}_2$, and photons ($\lambda \sim 595$ nm). This emission is attributed to HgBr* formed by light absorption during reactive collision. The cross section is ∼ ${10}^{-17\mathrm{}}$ ${\mathrm{cm}}^2$ at ∼ 2 MW/${\mathrm{cm}}^2$.