CLASSICAL LIGHT SCATTERING1st 古典光散射.ppt

1、Sept. 4, 2006,CLASSICAL LIGHT SCATTERING1st 古典光散射,3nd Presentation for LLS Book 2: p.2-19,For more infos please login on: .tw/rheology/entry_basic.htm,Overview A. Scattering Intensity (by gases or by macromolecules in solution) B. Concentration Dependence C. Size Dependence (Str

2、ucture factors, Rg, Zimm Plots),Atmospheric scattering,Preliminary Remarks,Light scattering has provided an important method for characterizing macromolecules for at least three decades Through the use of laser light, spectrum analyzers, autocorrelators, and the technology for clarifying samples, ex

3、periments can now be used to study molecular motion We will have finished introducing the classical light scattering (古典光散射) theories (especially their assumptions and limitations) in the next presentation (9/18) We will provide the basic concepts of dynamic light scattering (動態光散射) on 9/18 and 10/2

4、 before some applications of LLS (e.g., laser doppler velocimetry) and advanced theories (e.g., SAXS) are given,A. Scattering Intensity 由古典理論求得散射光強,Scattering by an Isolated Dipole and by Gases 小粒子、小分子的散射,Ez interacts with electrons in an atom or molecule to induce and electric dipole moment pz whic

5、h provides the source of the scattered light,For one scatter per unit volume:,Scattering geometry,單位體積內，一個散射粒子所貢獻之光強,Contd. Rayleigh Ratio (experimentally reported value),For N scatters per unit volume:,光電倍增管於散射角 所接收之總光強 = 單位體積之散射光強 x 總散射體積,單位體積內，N 個散射粒子所貢獻之光強,(scattering volume),Scattering by Macro

6、molecules in Solution 巨分子的散射,The scattering intensity per unit volume:,Interference effects of scattered light in the particle become significant! The scattering centers undergo Brownian movement, which produces transient optical inhomogenities in the solution,單位體積所貢獻之光強,濃度起伏的平方值取平均,B. Concentration

7、 Dependence 由微小之濃度起伏求得小粒子散射光強,The magnitude of the average concentration fluctuation will clearly depend on the energy to produce the fluctuation .,Finally, we arrive at,The scattering intensity per unit volume:,期望值,單位體積所貢獻之光強,Applications Obtaining M and B2 LS intensities are sometimes reported in

8、terms of the turbidity The attenuation results from scattering in any direction except at s = 0,C. Size Dependence 由巨分子散射光之干涉結果可推得結構因子,Structure Factors S(q),FIG. The path difference for scattering from a segment in a macromolecule,Step 1 Write down the electric field of the light scattered by the j

9、th segment:,aj contains factors such as the polarizability, the distance R to the detector etc,See book 2 (p.14-16) for the derivation of the structure factor for a uniform sphere of radius R or a uniform rod of length L,The expression for the structure factor:,Step 2 Sum up the scattered waves from

10、 the P segments within the particle Step 3 Calculate the scattering intensity for one particle with a fixed orientation Step 4 Calculate the scattering intensity for an ensemble of randomly oriented particles Step 5 Calculate the scattering intensity (without interference) at s=0,(Divide Step 4 by Step 5),TABLE Structure Factors and Radius of Gyration for Various Particles,Radius of Gyration LS measurements (at sufficiently low s) permit Rg to be determined from S(q) without any assumptions about the shape of the molecule Zimm Plot Simultaneously extrapola