Correspondence Regarding the Article 'On the Nature of the Evanescent Wave' (Appl. Spectrosc. 2013. 67[2]:126-130). How 'Total' is Total Reflection from Powdered Samples?

摘要

With great interest I read the article by Milan Milosevic, "On the Nature of the Evanescent Wave." This article fills an important gap in the description of an evanescent wave to a broad audience, centered mainly in the field of applicants of total reflection spectroscopy. As a member of this community, I feel obliged to add a comment to the article. My comment concerns the statement that if the evanescent wave, "were a propagating wave, it would be subject to scattering in those cases where medium 2 is a powder, and, therefore, the supercritical internal reflection of a non-absorbing powdered sample would not be total." With the sentence "It is" after the above-cited statement, the author generated the impression that there is no scattering of evanescent waves. There is absolutely no doubt about the fact that an evanescent wave is a non-propagating wave. On the other hand, it is a well established fact, both experimentally and theoretically, that an evanescent wave can indeed be scattered by non-absorbing objects. An overview of light scattering of evanescent waves was given by Sigel. In particular, the dynamic light-scattering works on colloids all show results in which the diffusion coefficient of the colloids are in some sense related to the bulk diffusion coefficient, which, again, can leave absolutely no doubt that the scattering is caused by particles diffusing in the rarer medium. (I should only briefly mention the routine use of gracing incidence X-ray diffraction (GIXD), to characterize the top layers of crystalline materials by diffraction from an evanescent X-ray wave, e.g., the works of Renner et al.). These experimental and theoretical results do not, however, necessarily contradict the main conclusions in the Milosevic work. Only the statement that a wave with the nature of an evanescent wave cannot be scattered is not correct.