Theoretical relation between the ionization potential (E_IP) of a DIB carrier and the expected observational slope (α_T) from the PIE model of S97. The relation is plotted for three different R_V = 3.1,4.0, and 5.5. The vertical orange line and shaded region represent our measured value of α_T for DIB λ8621 (from Fig. 4) and its 1σ uncertainty, respectively. The intersection of our measurement with the R_V = 3.1 curve implies E_IP ≈ 12 eV.