Light scattering spectroscopy introduction

Introduction of two very important physical techniques in polymer science, viz. Light Scattering for molecular weight determination (by Debye) and Infra-red Spectroscopy for structural analysis (by Thompson)... 

The relationship between spectroscopy, transport coefficients, and time-correlation functions is derived using linear response theory. The excellent review by Zwanzig (1965) provides a simple didactic introduction to this subject and contains a broad bibliography of the important papers on this subject. Berne and Forster (1971) describe progress in this and related areas and give a review of various computer experiments used to calculate time-correlation functions. In a recent manuscript Forster (1975) presents an up-to-date treatment of topics relevant to light scattering. 

In 1953, Stockmayer and Fixman observed that the ratio, h, of hydrodynamic or Stokes radii (/ h) of the branched and linear macromolecules could also be used as a measure of branching. The relatively recent introduction of online quasielastic light scattering (QELS, also known as dynamic light scattering and as photon correlation spectroscopy) detectors for SEC permits direct determination of h by... 

Additional methods to measure crystal size distribution based on particle mass employ light scattering/diffraction methods. Simultaneous measurements of particle size (based on particle volume) and their number are also carried out by Coulter counters. There are a variety of other techniques, based on centrifugal sedimentation, electroacoustic spectroscopy, microelectrophoresis, gravitational sedimentation, scarming electron microscopy, etc. An introduction to these techruques is available in a NIST (National Institute of Standards and Technology) publication by Jillavenkatesa et al. (2001). 

Although the effectiveness of the dynamic methods using a small-amplitude oscillatory strain to the rheo-optical studies of polymeric materials was demonstrated successfully from early days for birefringence, " light scattering, " and X-ray diffraction, relatively little had been reported on the use of IR probe, " " " until the introduction of DIRLD spectroscopy. " Because of the specificity of IR to certain submolecular constituents of the system, such a combination provides useful and direct information on the molecular origin of localized time-dependent deformation and relaxation behavior of materials. 

The book starts with a short introduction to the fundamentals of optical spectroscopy, (Chapter 1) describing the basic standard equipment needed to measure optical spectra and the main optical magnitudes (the absorption coefficient, transmittance, reflectance, and luminescence efficiency) that can be measured with this equipment. The next two chapters (Chapters 2 and 3) are devoted to the main characteristics and the basic working principles of the general instrumentation used in optical spectroscopy. These include the light sources (lamp and lasers) used to excite the crystals, as well as the instrumentation used to detect and analyze the reflected, transmitted, scattered, or emitted light. 

An e1ementary but complete introduction to the Raman effect appears in a text by Guillory (2) which also discusses several other molecular spectroscopic structural probes. More rigorous treatments of the subject may be found in works dealing primarily with vibrational spectroscopic methods (3)-(6). A brief introduction to the theory of Raman spectroscopy follows and is based on the inelastic scatter ng of light by a nonabsorbing medium. 

Here, we provide a very brief introduction to the coherent excitation of atoms and molecules. The coherent excitation establishes definite phase relations between the amplitudes of the atomic or molecular wave functions this, in turn, determines the total amplitudes of the emitted, scattered or absorbed radiation. The combination of ultrafast light pulses with coherent spectroscopy allows, for the first time, the direct measurements of wave packets of coherently excited molecular vibrations and their decay (see also Box 18.2 in Chapter 18). 

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