Excitation diagrams for the molecular emission towards SMM3b. Column densities are given in base 10 and natural logarithms in the left and right hand sides of each plot. On the top right, open circles and rectangles correspond to observed and de-reddened H₂ fluxes. On the bottom left filled circles and rectangles correspond to ortho and para H₂O transitions. CO (top left) and H₂ (top right) plots show a break at E_u ~ 1600 K, defining a warm and a hot gas component at temperatures ~300 K and 900 K, respectively. H₂O (lower left) and OH (lower right) display lower temperatures at ~100 K and significant scatter of data points.