Observation of Fluorescence of the HeH Molecule

This Letter reports the first spectroscopic detection of HeH, the simplest excimer molecule that can exist. He${\mathrm{H}}^{*}$ is produced in electronically excited states in reactions between He and ${\mathrm{H}}_2$ excited selectively by synchrotron radiation into the $C{}_{}{}^1{}_{}{}^{}\Pi _{\mathrm{u}}^{}{}_{}{}^{}(v^{\prime }>~1)$ or $B{}_{}{}^1{}_{}{}^{}\Sigma _{\mathrm{u}}^{+}{}_{}{}^{}(v^{\prime }>~11)$ state. He${\mathrm{H}}^{*}$ emits a broad characteristic fluorescence continuum between 200 and 400 nm which is attributed to $B{}_{}{}^2{}_{}{}^{}\Pi \to X{}_{}{}^2{}_{}{}^{}\Sigma _{}^{+}{}_{}{}^{}$.