Depolarized Rayleigh Spectra of DNA in Solution
The spectral analysis of scattered light has been extensively used to study dynamical processes in a variety of systems. All the processes which result in a modulation of either the isotropic or the anisotropic part of the optical polarizability of molecules cause shifts of the frequency of the incident light and may be monitored by the polarized or depolarized light scattering spectrum. Different dynamic processes in the scattering medium can be characterized by their characteristic relaxation times (and corresponding frequency shifts) which cover several orders of magnitude in the time (frequency) domain, thus requiring different experimental approaches in order to obtain the dynamical information 1. Photon correlation spectroscopy, which is based on post detection processing of the electronic signal, giving the intensity correlation function by means of a correlator or a spectrum analyser, may be used to monitor processes with characteristic times in the range of 1 to 10−6 s. This time range may be extended down to about 10−8 s by analysing the statistics of times of arrivals of scattered photons 2. Processes faster than 10−8 s cannot be monitored by post detection processing of photocounts in the time domain. The analysis of processes with shorter relaxation times (higher frequencies) may be performed in the frequency domain using optical methods. The frequency shifts of scattered light in the range of 10 MHz to 1000 GHz (corresponding to characteristic times of 0.2 ps to 20 ns) can be measured by means of Fabry-Perot interferometers, while frequency shifts higher than 10 GHz are easily accessible to measurements by means of grating monochromators. A Fabry-Perot interferometer analyses the optical field itself, it acts as a predetection frequency filter and produces a spectrum, i.e. a plot of intensity versus frequency.
KeywordsLaser Light Scattering Characteristic Relaxation Time Short Relaxation Time Polypropylene Glycol Orientational Motion
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