Light, scattering frequency

Raman effect When light of frequency Vo is scattered by molecules of a substance, which have a vibrational frequency of j, the scattered light when analysed spectroscopically has lines of frequency v, where... 

The scattering techniques, dynamic light scattering or photon correlation spectroscopy involve measurement of the fluctuations in light intensity due to density fluctuations in the sample, in this case from the capillary wave motion. The light scattered from thermal capillary waves contains two observables. The Doppler-shifted peak propagates at a rate such that its frequency follows Eq. IV-28 and... 

RS Raman spectroscopy [210, 211] Scattered monochromatic visible light shows frequency shifts corresponding to vibrational states of surface material Can observe IR-forbidden absorptions low sensitivity... 

Laser Doppler Velocimeters. Laser Doppler flow meters have been developed to measure Hquid or gas velocities in both open and closed conduits. Velocity is measured by detecting the frequency shift in the light scattered by natural or added contaminant particles in the flow. Operation is conceptually analogous to the Doppler ultrasonic meters. Laser Doppler meters can be appHed to very low flows and have the advantage of sensing at a distance, without mechanical contact or interaction. The technique has greatest appHcation in open-flow studies such as the deterrnination of engine exhaust velocities and ship wake characteristics. 

Spectroscopic examination of light scattered from a monochromatic probe beam reveals the expected Rayleigh, Mie, and/or Tyndall elastic scattering at unchanged frequency, and other weak frequencies arising from the Raman effect. Both types of scattering have appHcations to analysis. 

Light-scattering processes involve the interaction of light with gases or particles in such a manner that the direction or frequency of the light is altered. Absorption processes occur when the electromagnetic radiation interacts with gases or particles and is transferred internally to the gas or particle. 

If the detected frequency of the flashing light scattered by a microscopic object when crossing the fringes is multiplied by the fringe distance, the veltKity component of the scattering object normal to the beam bisector and parallel to the laser beam plane is determined. 

The quantum mechanical view of Raman scatering sees a radiation field hvo inducing a transition from a lower level A to a level n. If vnlc is the transition frequency, then the inelastically scattered light has frequency v0 — v t. That is, the molecule removes energy hv k from an incident photon. This process corresponds to Stokes scattering. Alternatively, a molecule under-... 

The third common level is often invoked in simplified interpretations of the quantum mechanical theory. In this simplified interpretation, the Raman spectrum is seen as a photon absorption-photon emission process. A molecule in a lower level k absorbs a photon of incident radiation and undergoes a transition to the third common level r. The molecules in r return instantaneously to a lower level n emitting light of frequency differing from the laser frequency by —>< . This is the frequency for the Stokes process. The frequency for the anti-Stokes process would be + < . As the population of an upper level n is less than level k the intensity of the Stokes lines would be expected to be greater than the intensity of the anti-Stokes lines. This approach is inconsistent with the quantum mechanical treatment in which the third common level is introduced as a mathematical expedient and is not involved directly in the scattering process (9). 

Pettinger B., Wenning U., Wetzel H., Surface-plasmon enhanced Raman-scattering frequency and angular resonance of Raman scattered-light from pyridine on Au, Ag and Cu electrodes, Surf. Sci. 1980 101 409-416. 

Phase-Doppler Anemometry. Theoretical analyses on dualbeam light scattering with off-axis detection 670 showed that the spatial frequency of the scattered interference fringe pattern is... 

For very accurate line profile measurements, a heterodyne technique has been developed 240) which can be briefly explained as follows the light, scattered into a cone within the angle 0 b9 (50< 1 °). is focused onto the cathode of a photomultiplier. The photocurrent is proportional to the square of the incoming light amplitude but cannot follow the rapid light frequency. Any beat frequencies, however, resulting from interference between the... 

If a S> 1, collective effects play an important role and the light scattering is no longer caused by individual electrons but by electron density fluctuations 280), Jn this case the spectrum shows a central line at Xq and two narrow lines located symmetrically about Xq, at a distance governed by the electron plasma frequency. The linewidth is smaller than in the case X < 1 and is determined rather by the thermal motion of the ions, not that of the electrons. The line shape depends on the ratio of electron to ion temperatures. Therefore, a measurement of the shape and width of this central line allows, under certain assumptions, a direct determination of the ion temperature. 

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