Notes. ^(a) The rest frequencies are taken from Amano & Hirao (2005) for o-H₂D⁺, and from Amano et al. (2005) for N₂H⁺ and N₂D⁺. ^b)Temperatures are reported on the main-beam temperature scale. ^(c)[1] this work; [2] Sabatini et al. (2020). ^(d) Critical densities computed for a temperature <20 K, considering downward collision rates in a multilevel system. The collisional rates are taken from Hugo et al. (2009) for o-H₂D⁺, Balanca et al. (2020) for N₂D⁺ and Lin et al. (2020) for N₂H⁺. ^(e)ɡ_u is the statistical weight. Spectroscopic data from The Cologne Database for Molecular Spectroscopy (Müller et al. 2005). Note: the upper level energy of the o-H₂D⁺ is defined here with respect to o-H₂D⁺ 1₁₁ level (see Vastel et al. 2006).