Top: measured rotational ground state transition J_K = 1₀−0₀ of ortho-NH₃D⁺ (black histogram). About 10 000 mass-selected NH₃D⁺ ions were stored for 600 ms in the cryogenic ion trap (4 K) in the presence of a high number density of helium (~4−7 × 10¹⁴ cm^-3) and continuous irradiation with tunable narrowband mm-wavelength radiation (~30μW power in the trap region). The absolute number of helium complexes formed over the storage time was ~400−700. The spectrum is averaged over 100 iterations, and the line centre position was extracted from fitting a Gaussian line-shape function to the data (black line). Bottom: the same line recorded with seven times higher radiation power (grey histogram). The estimated hfs components due to the deuterium nuclear quadrupole moment and their relative intensities are shown as black bars (C. Puzzarini, priv. comm.). The Gaussian fit function of the unsaturated line is shown for comparison (black line).