Rydberg spectra of triatomic hydrogen and of the ammonium radical

Abstract

In hollow-cathode discharges through H₂ and D₂ a number of new emission bands have been observed which have been identified as belonging to triatomic hydrogen (H₃ and D₃) in Rydberg states. The information about the structures of these radicals in the upper and lower states of the observed bands is summarized. The geometry is very similar to that of the H₃⁺ ion. A related Rydberg spectrum is expected and observed for the NH₄ radical. Spectra first observed by Schuster in 1872 and by Schüler, Michel and Grün in 1955 in discharges through ammonia are shown by isotope studies to be due to the free ammonium radical. The spectra of ND₄ are more intense and sharper than those of NH₄. A full analysis has not yet been accomplished but there is little doubt that NH₄ is tetrahedral in its Rydberg states.