Infrared-based and Raman-based Observation of Molecular Vibrations towards Physicochemical Analysis of Ionic Liquids

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Two non-linear, time-domain Raman methods have been developed towards physicochemical analysis of ionic liquids.(1)Interface-specific observation of molecular vibrations was achieved with visible light by a fourth-order optical process. An aqueous solution of oxazine dye interfaced with air was irradiated with 16-fs pluses at 600 nm. A pump pulse excited Raman-active nuclear motions of the dye. A probe pulse interacted with the excited molecules to generate second harmonic light, the intensity of which was oscillated as a function of the pump-probe delay. Fourier-transformation of the oscillation provided a vibrational spectrum of 0 - 1000 cm^<-1> for the dye located at the air-solution interface.(2)Femtosecond time-resolved photoinduced reflectivity change was measured for the first time on a spincoated dye (cresyl violet) of submonolayer coverage on a fused silica substrate. We found that the sensitivity of the reflection measurement was much higher than that of the absorption measurement for thin monolayers of dye. A prominent beating structure due to vibrational coherence of the dye was observed on the time-resolved reflection signal. A vibrational spectrum of the dye at submonolayer coverage, which corresponded its resonance Raman spectrum, was obtained in the range of 0-800 cm^<-1> by a Fourier transform analysis of the reflection signal.