Decay time and distribution area as a function of lignin content. a) Dependence of rapid (τ1), slow (τ2) and average (τ) decay times (open symbols) on the lignin fraction for bagasse fibers with lignin contents between 9 and 28% for an eucalyptus fiber with lignin concentration of 2%. Linear regression curves were fitted to τ2 and τ dependences on the lignin content, whereas the saturation behavior for τ1 cannot be properly adjusted by a function that could express some theoretically based model. The corresponding decay time values (filled square, circle and triangle) for untreated bagasse fibers are also shown separately. A decay time of 4.9 ns was estimated for a non-interacting lignin molecule by extrapolating the τ2 linear dependence for zero lignin fraction. For lignin in a dense film, the decay was estimated to be around 0.3 ns. b) Area below the τ1 and τ2 distributions as a function of the lignin fraction.