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Improved Fluorescence Rejection in Measurements of Raman Spectra of Fluorescent Compounds

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Abstract:

Practical ways to achieve fluorescence rejection were developed by using the technique of Raman difference spectroscopy and a gated diode-array detector. In order to improve the signal-to-noise ratio, we had to circumvent the problem presented by the fact that the sensitivities of each element of the diode-array detector are not always uniform and linear—a problem which results in an apparent noise in the observed spectra. Therefore, in this study we tried to separately observe a Raman-plus-fluorescence spectrum (S T) and a pure fluorescence spectrum (S F), with a high S/N ratio, and to obtain a Raman spectrum (S R) as their difference. With a cw Ar+ laser, S T and S T were excited at different wavelengths but recorded in the same wavelength region. This procedure was easy and quite successful for compounds, such as methyl orange and methylene blue, that have a Raman-to-fluorescence intensity ratio (I R/I F) of greater than 0.01. With a mode-locked and Q-switched Nd:YAG laser, S T and S F were obtained from different timings of the detector gate (6 ns), i.e., −6 to 0 ns for S T and 3 to 9 ns for S F. The temporal discrimination between Raman and fluorescence photons combined with the Raman difference technique significantly improved the 532-nm excited Raman spectrum of the tetraphenylporphyrin solution. This method is applicable to compounds with an I R/I F of smaller than 0.01. The time-correlated single photon counting method with a time-to-amplitude converter was improved, to enable it to work with a high repetition rate, and was applied to a Rhodamine 6G solution, which was also examined by the gated Raman difference technique. Advantages and problems associated with each technique and appropriate methods for a given sample will be discussed.

Keywords: Instrumentation, Raman scattering; Molecular structure; Raman spectroscopy

Document Type: Research Article

DOI: http://dx.doi.org/10.1366/0003702884428121

Affiliations: Institute for Molecular Science, Okazaki National Research Institutes, Myodaiji, Okazaki, 444 Japan

Publication date: February 1, 1988

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