A novel fiber-optic interface for unenhanced external reflection Raman spectroscopy of supported monolayers

A novel sample interface has been developed to acquire the unenhanced exter nal reflection Raman spectra of Langmuir-Blodgett thin films deposited onto solid substrates. This interface does not rely on a surface or resonance e nhancement to amplify the monolayer signal, but rather uses fiber optics an d gradient-index lenses for collection of the scattered light, thus providi ng an efficient collection mechanism for the Raman scattered radiation in a n external reflection arrangement. This design is relatively inexpensive an d straightforward to implement, resulting in a sensitive and fast tool for obtaining high-quality Raman spectra of ultrathin films (greater than or eq ual to 2.5 nm thickness). The results of this study show that a fiber-optic bundle provides an efficient mechanism for transfer of the scattered Raman radiation from the sample to the monochromator. The optical design also in corporates gradient-index lenses for image transfer onto the fiber bundle, thereby significantly reducing chromatic and spherical aberrations in the t ransfer of the image of the sample onto the fiber. Using this optical desig n, the spectrum of a single cadmium arachidate (Cd:Ar) monolayer transferre d onto CaF2 results in a signal-to-noise ratio of similar to 400:1 in the v (C-H) stretching region. Additionally, a high reflectivity dielectric mirro r can be used as an alternative substrate to acquire Raman spectra below 20 00 cm(-1). By use of this dielectric substrate, the unenhanced external ref lection Raman spectrum of a Cd:Ar monolayer in the v(C-C) spectral region w as obtained for the first time.