Excitation diagram of H₂. N_u is the column density of the transition upper level, g_u is the degeneracy of the upper level and T_u is the upper level energy in Kelvin. The squares show the data. The H₂ ortho-to-para ratio is taken equal to 3. The error bars are small in size compared to the symbol size. The arrows indicate upper limits. The stars show the PDR model predictions. For each object, H₂ line intensities were computed from linear interpolation of our grid of models with the suitable values of the FUV incident radiation field and gas density (see Table 1). We use χ = 4.5 and n = 10³ cm^-3 for L1721; χ = 19 and n = 5 × 10³ cm^-3 for the California; χ = 51 and n = 10⁴ cm^-3 for N7023E; χ = 60 and n = 10⁴ cm^-3 for the Horsehead; χ = 250 and n = 10⁴ cm^-3 for rho Oph; χ = 550 and n = 5 × 10⁴ cm^-3 for N2023N. Error bars due to uncertainties in the UV field are given. For comparison of the predicted and observed line intensities, in the model the PDRs have been tilted with respect to the line of sight with cos(θ) = 0.15.