Abstract
The ground-state rotational spectrum of the heterodimer (NH3,HI) has been observed by pulsed-nozzle, Fourier-transform microwave spectroscopy. A quartz oreservoir/nozzle attached to the outlet of a solenoid valve allowed pulses of ammonium iodide vapour/argon to be produced without encountering metal surfaces. The observed spectroscopic constants are consistent with a hydrogen-bond description H3NHI rather than an ion-pair H3N I description of the observed species.