High-resolution threshold-ionization spectroscopy of NH3

High-resolution photoionization, zero-kinetic-energy photoelectron and Rydberg-state-resolved threshold-ionization spectra of ammonia and its deuterated isotopomers have been recorded in the region of the lowest vibrational levels (v2+=0,1) of the X̃+ ground ionic state of NH3+ following single-photon excitation from the ground neutral state using a narrow bandwidth vacuum ultraviolet laser system (bandwidth 0.008 cm−1). The resolution enables the observation of photoionization transitions originating from distinct tunneling components of the ground neutral state and the measurement of the spin-rotational splittings of the ionic energy levels. A new value of the first adiabatic ionization potential of NH3 [I.P.=82 158.751(16) cm−1] has been derived which is more accurate than previous values by almost two orders of magnitude. The photoionization dynamics of NH3 to the lowest vibrational levels of the X̃+(2A2″) ground state of NH3+ is dominated by the emission of even ℓ photoelectron partial waves, and a strong propensity rule is observed that favors transitions with ΔK=K+−K″=0 and, to a lesser extent, ΔK=3.