Abstract
We present the first non-resonant and non-enhanced Raman correlation spectroscopy experiments. They are conducted on a confocal microscope combined with a Raman spectrometer. The thermal fluctuations of the Raman intensities scattered by dispersions of polystyrene particles of sub-micrometric diameters are measured and analysed by deriving the autocorrelation functions (ACFs) of the intensities. We show that for particles of diameter down to 200 nm, RCS measurements are successfully obtained in spite of the absence of any source of amplification of the Raman signal. For particles of diameter ranging from 200 to 750 nm, the ACFs present a time-decay behaviour in accordance with the model of free Brownian particles. For particles of 1000 nm in diameter, the AFCs present a different behaviour with a much smaller characteristic time. This results from the dynamics of a single-Brownian particle trapped in the confocal volume by the optical forces of the focus spot.
Highlights
Dynamic light scattering (DLS) and Fluorescence Correlation Spectroscopy (FCS) belong to the same widespread class of optical fluctuation spectroscopy experiments by which the thermal fluctuations in colloidal solutions or in reacting systems may be investigated
Before performing the Raman correlation spectroscopy (RCS) measurements, the temperature of the solvent and the size of the confocal volume were calibrated by measuring the DLS and FCS autocorrelation functions (ACFs) on commercial solutions containing red fluorescent carboxylate-modified polystyrene (PS) beads (Sigma-Aldrich) of known diameters
RCS measurements were done by deriving the ACF of the intensity of the most intense Raman band namely the one at 3005 cm−1 corresponding to the aromatic CH stretch of the PS [27]
Summary
Dynamic light scattering (DLS) and Fluorescence Correlation Spectroscopy (FCS) belong to the same widespread class of optical fluctuation spectroscopy experiments by which the thermal fluctuations in colloidal solutions or in reacting systems may be investigated The origins of these techniques have their roots in the early 20th century with, on the one hand, the work of Gouy [1] and that of Einstein [2] who understood the thermal origin of the Brownian motion and thereby the relationship between the diffusion coefficient D of a particle and its size, and the temperature and the viscosity of the solvent. A group proposed an alternative experimental implementation for RCS measurements [18] aiming at exploiting the coherence of the Raman process In their case, the experimental set-up is different than the ones used in the previous studies and in FCS. We show that in this situation the RCS measurements correspond to that of a Brownian particles optically trapped in the confocal volume
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