Keiji Kamogawa

Improved Fluorescence Rejection in Measurements of Raman Spectra of Fluorescent Compounds

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 (ST) and a pure fluorescence spectrum (SF), with a high S/N ratio, and to obtain a Raman spectrum (SR) as their difference. With a cw Ar+ laser, ST and SF 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 (IR/IF) of greater than 0.01. With a mode-locked and Q-switched Nd:YAG laser, ST and SF were obtained from different timings of the detector gate (6 ns), i.e., −6 to 0 ns for ST and 3 to 9 ns for SF. 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 IK/IF 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.

Evidence for direct intermolecular interactions as an origin of the hydration shifts of the C—H stretching vibrations: 1,4-dioxane/water system

Abstract Frequency shifts of the C—H stretching vibrations (νCH) of 1,4-dioxane (DX) and dimethylsulfoxide in aqueous solutions were observed as a function of concentration using a dual-beam difference Raman apparatus. A nonlinear positive increase of the shift was observed upon dilution for the νCH modes of DX, similar to the case of alcohol/water mixtures, while negative and zero shifts were observed for its C—O and C—C stretching modes, respectively. The shift of the νCH mode is not the secondary effect arising from a change in hydrogen bonding at oxygen. The ΔνAA versus χA curve of the νCH mode exhibited a deep minimum around χA = 0.08, indicating the formation of molecular assemblies of DX with the C—H groups inside in the concentration region of 0.08

Observation of resonance Raman spectra of S1, T1, and S0 pyrene in solution: Application of a fluorescence rejection technique

Abstract The new technique of fluorescence rejection during Raman measurements was applied to observe transient resonance Raman (RR) spectra ofpyrene in electronically excited states and a UV RR spectrum in its ground state. The method gave well-defined spontaneous RR spectra with a nanosecond source despite the strong fluorescence.