Performance comparisons of conventional and line-Focused surface Raman spectrometers

Four Raman spectrometer configurations were compared for obtaining spectra of surface monolayers, including a new design that employed a line rather t han point focus. Each spectrometer used a 514.5 nm laser and charge-coupled device (CCD) detector, but they differed in collection efficiency and samp ling optics. Previously defined figures of merit for Raman signal and signa l-to-noise ratio (SNR) were determined for each spectrometer, to aid quanti tative comparison, The figure of merit fur SNR, F-SNR, is demonstrated to b e useful for comparisons because it permits prediction of SNR for a given s pectrometer, sample, and measurement conditions. A rigorous definition of F -SNR is based on power density and takes into account the laser damage thre shold of the sample. A simpler but less rigorous definition is based on las er power at the sample rather than power density and may he more useful to users who rarely determine the laser spot size. A new spectrometer design e mploying line focusing and collection is presented, with f/2 optics and a 6 mm slit image at the CCD. A proprietary aberration correction prevents sli t image curvature common to most spectrographs with low f#, and permits ful l height binning of the CCD, The line-focused spectrometer yielded an SNR a nd F-SNR which are comparable to those for a point focus using the same col lection optics and slightly lower than those for the most efficient spectro graph examined, However, the line focus permitted much lower power densitie s to be employed, or yielded much larger signal far the same power density at a point focus. In quantitative terms, the new line-focused design yielde d an SNR which is 67 times that of the best point-focused system, for the s ame sample, measurement time, and laser power density.