The “Refractor”: A Refractive Grazing Incidence Specular Reflection Attachment

Abstract

External Reflection Spectroscopy (ERS) is one of the six established spectroscopic techniques for recording optical spectra. In this technique, light—usually polarized—is specularly reflected from samples having a smooth surface. External reflection spectroscopy is particularly useful for recording spectra of thin films on metal surfaces. External reflection spectra of a thin (~ 500 A) of SiO2 film on an Al substrate are shown in Fig. 1. These spectra can be recorded only with light having parallel polarization and reflected from the substrate at or near Brewster's angle. Brewster's angle is approximately 89° and, hence, the grazing incidence, for highly conductive metals such as Al, Au, Ag, etc. It is impractical to try to attain such a high angle with conventional spectrometers where the angular light beam spread is typically ±5°. Furthermore, the light beam width striking the sample increases with both increase in angle of incidence (θ) and increase in angular beam spread (±φ). Because of this beam spread, external reflection spectroscopy, in spite of its sensitivity, is not recommended for probing small (micrometer size) areas of surfaces. Under the right conditions (viz., use of polarized light at angles near the Brewster's angle, as shown in Fig. 1), high sensitivity is attained. Because the reflectivity of Al is low (~70%) under these conditions, little is gained by employing many reflections. In most cases a single reflection is adequate—which also simplifies the instrumentation and limits the required size of the sample. Spectra of films less than one monolayer thick have been recorded via external reflection spectroscopy.