Absolute photoemission cross sections of ultraviolet emissions produced by electron-impact dissociation of boron trichloride

We report absolute photoemission cross sections and appearance potentials for the most intense ultraviolet emissions produced by electron-impact dissociation of boron trichloride, ${\mathrm{BCl}}_3$. The emission features studied include two atomic boron emissions, the 2${\mathit{p}}^2$ ${}_{}{}^2{}_{}{}^{}$D→2p ${}_{}{}^2{}_{}{}^{}$P° line at 208.9 nm and the 3s ${}_{}{}^2{}_{}{}^{}$S→2p ${}_{}{}^2{}_{}{}^{}$P ° line at 249.8 nm, which is the strongest ultraviolet emission feature, as well as the BCl ${\mathit{A}}^1$Π→X ${}_{}{}^1{}_{}{}^{}$ ${\mathrm{\Sigma }}^{+}$ system centered at 272.4 nm, whose previously reported cross section had to be revised upward by about 20% to 3×${10}^{\mathrm{-}18}$ ${\mathrm{cm}}^2$ at 40 eV. The 249.8-nm boron line was found to have the largest emission cross section with a value of 5.4×${10}^{\mathrm{-}18}$ ${\mathrm{cm}}^2$ at 75 eV. All cross sections displayed a prominent double onset structure with the second onset roughly 13 eV above the first one, which is very close to the ionization energy of atomic chlorine. This indicates the presence of a strong channel producing excited-state boron or boron chloride fragments simultaneously with ground-state chlorine ions. An attempt has been made to separate this ‘‘ionic’’ channel from the ‘‘neutral’’ channel in which neutral ground-state atoms are the only by-products accompanying the formation of the excited fragments.