16The developed DRS measurement apparatus is schematically illustrated in Figure 1. The two laser pulses whose wavelengths are 797 nm and 641 nm were focused simultaneously onto a sample with an objective lens. Back-scattered light from the sample was collected using the same objective lens and detected with a photon counting module after extracting the CARS signal using a filter and a spectrometer. The time correlation function was obtained using a data processing scheme developed by us1. Then, I demonstrated the molecular selectivity of DRS by performing DRS measurements on a multi-component dispersion. The black curves in Figure 2(a) and (b) compare the normalized time correlation function obtained from Rayleigh scattering (DLS) and CARS signal (DRS) of a 1:1 mixture of polystyrene and silica nanoparticle dispersions in the back-scattering geometry. The polystyrene and silica particle radii were 60 and 109 nm, and the concentration was 0.5 wt% for each. The time correlation function obtained from the DRS measurement (Fig. 2(b)) clearly deviates from its DLS Research Digest counterpart (Fig. 2(a)). The DRS time correlation function for the mixed dispersion in Fig. 2(b) coincides almost perfectly with that for the polystyrene-only dispersion (red curve in Fig. 2). This the successful molecular-selective detection of the polystyrene diffusion dynamics in the mixture dispersion2.result unambiguously demonstrates Fig. 1. Schematic of the apparatus for the DRS measurement; OPA: optical parametric amplifier; BS: beam splitter; DM: dichroic mirror; SP: short-pass filter; TDC: time-to-digital converter.Fig. 2. Normalized time correlation functions obtained from Rayleigh scattering (DLS) (a) and CARS (DRS) (b) from the nanoparticle dispersion mixture (black lines). Red and blue dashed curves represent the calculated time correlation functions with polystyrene-only and silica-only.References 1) T. Hiroi, S. Samitsu, H. Kano, K. Ishioka, Anal. Sci. 38, 607 (2022). 2) T. Hiroi, S. Samitsu, K. Ishioka, H. Kano, J. Phys. Chem. C 127, 10245 (2023). 1. Outline of ResearchDynamic light scattering (DLS) is a technique that has been widely used for the characterization of diffusion dynamics for polymer solutions and soft materials. In DLS measurement, the fluctuation of Rayleigh scattering, which originates from the Brownian motion of the scatterers, is recorded as a form of the time correlation function. One of the major disadvantages of DLS is the absence of molecular selectivity, which limits the application of DLS to multi-component materials. The basic idea to solve this problem is to replace Rayleigh scattering in DLS with Raman scattering. However, molecular-selective diffusion dynamics cannot be extracted by observing standard (spontaneous) Raman scattering because it is an incoherent process and can therefore contain no information on the positions of the diffusing molecules. To circumvent this problem, I propose monitoring the intensity of coherent anti-Stokes Raman scattering (CARS), which is a stimulated Raman process emitted from a molecular ensemble with coherent molecular vibration. Hereafter I refer to this novel molecular-selective DLS scheme as dynamic Raman scattering (DRS). Because it is a coherent optical process, the CARS signal becomes most intense when the wavevectors of the incoming and outgoing lights are conserved (phase-matching condition). I further show theoretically, however, that only the weaker CARS signal detected in a phase-mismatching condition can carry the information on the positions of the diffusing molecules. I then developed a DRS experimental setup based on the theoretical proposal and demonstrated the molecular selectivity of DRS by detecting the phase-mismatched CARS from a dispersion mixture of polystyrene and silica nanoparticles as a proof-of-principle experiment.2. Research ActivitiesMolecular-selective Dynamics Measurement of Soft MaterialsTakashi HIROI
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