Measurement of the ionization potential of triatomic hydrogen

The d Rydberg series of H₃ converging to the lowest rotational level of ortho-H₃⁺ has been detected for n values ranging from 27 to 100 using field ionization. The Rydberg molecules are prepared by photoexcitation from the long-lived species, H₃ B ̃ \~{}{}(2p²{A}₂'', N=K=0), in its lowest vibrational level. Extrapolation of the Rydberg series yields an ionization potential of 29562.6±0.5{cm}^{{{-}}1}$. The amplitudes of the excitation spectra reveal the interaction between the ${H}₃⁺$ cores with ${N}⁺$=1 and 3 via l uncoupling of the d electron, as well as window resonances from interlopers belonging to vibrationally excited ${H}₃⁺$ cores.