Dielectric Function of Oriented Hollow Vegetal Fibers Retrieved from XPS, Diffuse Reflection, and Photoacoustic Spectroscopies

Diffuse reflectance spectroscopy (DRS) and photoacoustic absorption spectroscopy (PAS) are investigated in model systems that mimic topological structures found in vegetal fibers, e.g., made of oriented hollow channels. Among this wide family, the shiny optical aspect of Golden grass stems has motivated detailed investigations of their chemical composition, microstructure, and electronic properties. In this work, diffuse reflectance and absorption of a photon flux within a collection of parallel hollow fibers are addressed using 2D optical models, neglecting interface scattering. Analytical results obtained for hollow square tube (HST) and plane parallel membrane (PPM) arrays are validated by full ray-tracing calculations. Optical paths with multiple reflection/refraction events at air-fiber interfaces selectively enhance DRS and PAS intensities and thus explain the shiny golden-like aspect of Golden grass, in contrast with specular reflectance calculated from the complex refractive index. A simple method is proposed to retrieve the complex dielectric function of fiber walls from the measured PAS and DRS intensities, using analytical equations obtained with hollow fiber topologies, in a low-energy region not easily accessible by photoelectron energy loss spectroscopy.