Observed and best mean hemispherical emissivities. Top: hemispherical emissivities as a function of wave number for observation #1: mean observed (Eq. (9), red), mean of best models (Eq. (10)) and their standard deviations (dotted lines), and best model across all spectra (black) obtained with model M_o. Bottom: spectral residuals R²(w_n) (Eq. (7)) and ${\overline{R}}^2$(w_n) (Eq. (8)) obtained with blackbody model ($R_B^2$) or smooth model, M_o($R_o^2$ or ${\overline{R}}_o^2$). Mean and mean-global residuals are plotted as dashed lines. The intra-molecular mixture is AmC and Th at 0.1%, each within a matrix of Ih crystalline ice. The lattice vibration at 222 cm⁻¹ and instrumental fluctuation about 90 cm⁻¹ are indicated (vertical dotted lines).