Intensity fluctuation spectroscopy

Dynamic processes at thermodynamic equilibrium that occur within a time range from sub-microseconds to seconds can be probed without the imposition of a transient disturbance by optical intensity fluctuation spectroscopy. As such, dynamic light scattering (DLS) [155] measures the fluctuation of quasielastic scattering intensity and fluorescence correlation spectroscopy (FCS) [156-158] measures concentration fluctuations of specific fluorescent molecules... 

Quasi-elastic laser light scattering (also called intensity fluctuation spectroscopy, light-beating spectroscopy or photon correlation spectroscopy) is an accurate method to measure the translational diffusion coefficients of macromolecules. The diffusion coefficient is a parameter, that depends on the size and shape of the macromolecules and on the thermodynamic and hydrodynamic interaction between the macromolecules. 

More recently we have found that fluctuations in the scattered light intensity around its mean value give information about dynamics of the ripple motions in the film (intensity fluctuation spectroscopy). The rest of this chapter treats the light-scattering method in more detail. 

Because of relaxation of surface elevations, the scattered light has a broadened spectral distribution compared with the incident light. The broadening is too small to be analyzed by the conventional Fabry-Perot interferometry," however, so the more recent technique of light beating" must be used. We call this technique intensity fluctuation spectroscopy (IFS). 

Intensity fluctuation spectroscopy was used in our laboratory to study the dynamic behavior of surface ripples on thin liquid films. Both squeezing and bending modes were examined. To our knowledge one other group of researchers has obtained dynamic light-scattering data from thin soap films but as far as we know, nothing has been published in the official literature. Also some experiments were reported on lipid bilayers in water. ... 

The literature [61,63-82] r rs to QELS by many differoit names, some of which are spediic methods of implemratation. The.se include dynamic light scattering, laser scattering, laser Doppler velocimeby, intensity fluctuation spectroscopy, photon correlation spectroscopy (PCS), light beating spectroscopy and homo- and heterodyne spectroscopy. Most of the techniques discussed here are based on PCS. 

R. Foord, E.R. Pike, E. Jakeman, R.J. Blagove, E. Wood, A.R. Peacocke, Determination of diffusion coefficients of haemocyanin at low concentration by intensity fluctuation spectroscopy of scattered laser light. Nature 227(5255), 242-245 (1970). doi 10.1038/227242a0... 

Brauer S, Stephenson GB, Sutton M, Bmning R, Dufresne E, Mochrie SGJ, Grabel G, Alsnielsen J, Abernathy DL (1995) X-ray-intensity fluctuation spectroscopy observations of critical-dynamics in Fe3al. Phys Rev Lett 74(11) 2010-2013... 

Livet F, Bley F, Caudron R, Geissler E, Abernathy D, Detlefs C, Grubel G, Sutton M (2001) Kinetic evolution of unmixing in an AlLi alloy using x-ray intensity fluctuation spectroscopy. 

The formal approach of 2D correlation analysis to time-dependent spectral intensity fluctuations has been extended to UV, Raman [1010], and near-IR spectroscopy [1011-1014] 2D fluorescence is upcoming. 

This technique does not require the equilibrium of the chemical system to be perturbed to measure the host-guest binding dynamics. Fluorescence correlation spectroscopy is based on the measurement of changes in the fluorescence intensity of individual molecules, which lead to intensity fluctuations.58 63 For this reason, the measurements are made by detecting the emission from a small sample volume (femtoliters to microliters) containing a small number of fluorophores. 

To measure the droplet size distribution of the primary emulsion (W/O in W/O/W or O/W in O/W/O) that has a micron range (with an average radius of 0.5-1.0 pm), a dynamic light-scattering technique (also referred to as photon correlation spectroscopy PCS) can be apphed. Details of this method are described in Chapter 19. Basically, the intensity fluctuation of scattered light by the droplets as they undergo Brownian diffusion is measured from this, the diffusion coefficient of the droplets can be determined, and in turn the radius can be obtained by using the Stokes-Einstein equation. 

When assessing a nanoemulsion formation, the normal approach is to measure the droplet size distribution using dynamic light scattering techniques, including photon correlation spectroscopy (PCS). In this technique, the intensity fluctuation of light scattered by the droplets is measured as they undergo Brownian motion... 

Cavity Ring-Down Spectroscopy was introduced in 1988 by O Keefe and Deacon as a spectroscopic method for absorption measurements (O Keefe and Deacon, 1988). It is a versatile high sensitivity absorption technique. One of the most essential advantages of CRDS in contrast to usual absorption methods is that the CRDS signal is not affected by intensity fluctuations of the laser since only the decay time of the signal, which does not depend on the laser intensity, is detected. 

Picosecond photon correlation experiments have some similarities to fluorescence correlation spectroscopy (PCS). PCS investigates the fluctuations of the fluorescence intensity of a small number of molecules confined in a small sample volume (see Sect. 5.10, page 176). The intensity fluctuations are correlated on a time scale from microseconds to milliseconds. Therefore, PCS differs from picosecond correlation in the way the photons are correlated. Moreover, PCS effects are driven by diffusion, conformational changes, or other sample-internal effects, while antibunching is driven by the absorption of the photons of the excitation light. 

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