Net scattering intensity

When a monochromatic, coherent light is incident into a dilute macromolecule solution, if solvent molecules and macromolecules have different refractive index, the incident light is scattered by each illuminated macromolecule to all directions [9, 10]. The scattered light waves from different macromolecules mutually interfere, or combine, at a distant, fast photomultiplier tube detector and produce a net scattering intensity I(t) or photon counts n(t) which is not uniform on the detection plane. If all macromolecules are stationary, the scattered light intensity at each direction would be a constant, i.e. independent of time. 

Also known as quasi-elastic light scattering, this technique monitors the tempord fluctuations in / (q) (Berne and Pecora, 1976 Chu, 1990). These fluctuations result from random thermal motions, which change the instantaneous spatial arrangement of molecules and thus the net scattered intensity. As these random motions result in microscopic concentration fluctuations, a mutual diffusion coefficient can be determined from the time constant of the decay of the time autocorrelation function of Liq, t). Rapid advances in laser and autocorrelator technology during the last two decades have made this experiment a routine characterization and research tool. 

In these experiments the time-averaged scattered intensity /, is measured as a function of the scattering vector q. The net detected intensity can be computed as the superposition of the signals from each scattering center (e.g., each monomer unit). According to the spatial arrangement of the scatterers, the individual scattered waves may interfere constructively or destructively at the detector. Thus, 7f(q) is proportional to the so-called static structure factor 5(q), which sums the waves with different phases from different locations. 5(q), in fact, reflects the spatial Fourier transform of the distribution of scatterers (i.e., the pair correlation function), and... 

These are plotted in Fig. 10.6, which shows the net intensity envelope in the xy plane as a solid line and represents the horizontally and vertically polarized contributions to the resultant by the broken lines. Since 0 is symmetrical with respect to the x axis, the three-dimensional scattering pattern is generated by rotating the solid contour around the x axis. 

Ren et al. reported a method to prepare a gold tip with a tip apex radius of 30 nm reproducibly [27]. They observed the TERS of a Malachite Green isothiocyanate (MGITC) monolayer on an Au(lll) surface and obtained an enhancement factor of about 1.6 X 10, by using the relation, q= /TERs/lRRs=g /l focus where q is the net increase in the signal. Iters snd rrs the signal intensities for TERS and RRS (resonance Raman scattering), respectively is the TERS enhancement (gis the field enhancement), a denotes the radius of the enhanced field, and Rfocus the radius of the laser focus. 

Around defects, the scattering power differs from that in the perfect crystal because X-rays which do not satisly the Bragg condition in the perfect crystal may be diffracted in the deformed region arotmd the defect. Just as in the Lang projection topograph, these regions behave as small crystals which diffract kinematically and the net result is an increase in the intensity over that from the perfect crystal. 

Since we have checked that the shaiie of the scattering curves does net change with time, integrated intensity within a central window of Q is proportional to the total amount of scattering rods. 

A vertical laser beam has been used by Ashkin (1970) and Ashkin and Dziedzic (1971) to levitate weakly absorbing spherical particles by radiation pressure. Lateral stability results from the dominance of refracted over reflected components of the scattered light (see Table 7.1). Unequal reflection on opposite sides of the particle, which is caused by beam nonuniformity, produces a net force that drives the particle toward lower light levels this instability is countered by refraction, which produces a reaction that drives the particle toward higher light levels. The particle is thus laterally stabilized in the most intense part of the beam. Laser levitation has the disadvantage that it... 

Momentum also plays a role in ordinary spontaneous Raman spectroscopy. When the pump radiation at 532 nm is passed through a sample, the aE term of Eq. (2) produces scattering and, for the first Stokes case shown in Fig. la, the frequency is i si = where is the Raman-active vibration excited in the sample. It should be noted that there is an exchange between the radiation field and molecule not only of energy but also of momentum, represented by the vector ky. The direction and magnitude of k, are determined by the photon-scattering direction, which is random for this spontaneous event. The result is scattering in all directions so that there is no coherent addition of photon amplitudes, as expressed in the summation /(i si) = C8q The net intensity from this inco-... 

The net result of this interaction of light and a scattering particle is that some of the energy which was associated with the incident ray will been radiated in directions away from the initial line of propagation. Thus the intensity of light transmitted through the particle along the incident beam direction is diminished by the amount radiated in all other directions by the dipoles in the particle. 

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