THEMIS a-C(:H) nanoparticle initial temperatures (upper plot), and in the lower plot photon absorption timescales (blue), and lifetimes (black) for the ISRF conditions (G₀, F_UV E_hν, and A_V) given in Table 4. The thin vertical dashed lines indicate the THEMIS minimum grain size in the diffuse ISM (a ~ 0.4-0.5 nm), and the derived limit for harsh PDR environments (a_- ≲ 0.7-0.8 nm, IC63 and Orion Bar, Schirmer et al. 2022) and (a_- ~ 0.4 nm, Orion Bar, Elyajouri et al. 2024). The horizontal dashed lines mark the million year lifetime (lower plot) and T = 10³ K (upper plot).