Rotational ground state transition J = 1−0 of CF⁺ with resolved hyperfine structure (black histogram) measured by storing about 8000 mass-selected CF⁺ ions for 600 ms in a cryogenic 22-pole ion trap (4.2 K) in the presence of a high number density of helium (~ 4 × 10¹⁴ cm^-3) and continuous irradiation with tunable narrowband mm-wavelength radiation (~ 10μW power in the trap region). The relative depletion signal for both lines is of the order of a few percent, with an absolute number of around 400 helium complexes formed over the storage time. The spectrum is averaged over 320 iterations, and the line centre positions were extracted from fitting a Voigt line shape function to the data (grey curves).