Estimates of the CRIR of hydrogen molecules, ζ₂, derived for a sub-sample of sources for which H₂D⁺, DCO⁺, H¹³ CO⁺, and C¹⁷ O observationsare available, and assuming ℓ = R_eff in Eq. (5), representing our reference case (dots). The sources are arranged from left to right following their evolutionaryphase (bottom left legend). The bars associated with each point represent the variability of our results with ℓ (i.e. ℓ = 0.5R_eff and ℓ =2R_eff for the upper and lower limit, respectively). The blue line is our mean value for ζ₂ for the reference case, while the blue shaded area is its variability with ℓ. The orange line and shaded area represents the estimates of van der Tak & van Dishoeck (2000), obtained by converting the CRIR for hydrogen atom, ζ_H, via ζ₂ = 2.3 × ζ_H. The green line is the recent upper limit for L1544 reported by Ivlev et al. (2019).