Spectroscopy of Triatomic Hydrogen Molecules in a Beam

The formation of beams of triatomic deuterium ${\mathrm{D}}_3$ and the mixed isotopic molecule ${\mathrm{D}}_2$H was optically observed for the first time when the corresponding ions were neutralized by charge exchange. The molecules were detected via the transitions between the Rydberg states $n=3\mathrm{}$ and $n=2\mathrm{}$. An assignment of the ${\mathrm{D}}_2$H spectrum is given. The total lifetimes of rotational sublevels of different electronic states with $n=3\mathrm{}$ of ${\mathrm{D}}_3$ were measured from the radiative decay of molecules along their path behind the charge-exchange region. Agreement with theory is satisfactory.