Power absorbed per unit area per grain due to photon and collisional heating at z = 3.5 kpc and R = 22 kpc in NGC 891, where, according to our model, G₀ = 0.1. The collisional heating has been computed for IGM gas with a temperature T_gas = 10⁶   and   10⁷ K and a proton density n_H = 10^-3 cm^-3. In the three black lines there is a discontinuity around 1 nm because of the transition between PAHs and a-C:H grains.