Geometrical renormalization approach to calculating optical properties of fractal carbonaceous soot

We develop a theoretical approach ta calculating optical properties of carb onaceous soot in the long-wavelength limit. Our method is based on geometri cal renormalization of clusters; it avoids both the inaccuracy of the dipol e approximation in its pure form and the numerical complexity of rigorous d irest methods of solving the EM boundary problem. The results are verified by comparison With the experimental measurements for specific extinction of diesel soot in the spectral region from 0.488 mum to 0.857 cm that were pe rformed by Bruce et al. [Appl. Opt. 30, 1537(1991)]. The theory leads to an alytical expressions that are applicable to different soots, With various g eometrical properties and optical constants. We show that the functional fo rm of the long-wavelength asymptote of the specific extinction can depend c ritically on a parameter characterizing the sample geometry, and we identif y the critical value of this parameter. (C) 2001 Optical Society of America .