Rayleigh frequency

Because of the high monochromaticity of the incident laser beam, transitions differing by even a fraction of a wavenumber from the Rayleigh frequency can be detected, which makes pure rotational transitions accessible to laser-Raman spectroscopy. 

Scattering in the Rayleigh Frequency Region A. Optically Isotropic Molecules 1. Theory... 

IV. SCATTERING IN THE RAYLEIGH FREQUENCY REGION A. Optically Isotropic Molecules... 

Rayleigh Frequency Distribution - A mathematical representation of the frequency or ratio that specific wind speeds occur within a specified time interval. 

Rayleigh frequency we obtain in some of our probe spectra, a narrow, small absorption peak. This feature is situated in the middle of the dispersion shaped spectra shown in Fig. 3(a) and has not been reported so far. 

An additional type of scattering is obtained due to the fact that molecules themselves are vibrating at frequencies corresponding to various normal modes of motion. These characteristic vibrational frequencies can mix with the exciting light to form sum and difference frequencies in the scattered radiation. Although weak, these frequencies may be detected as shifts from the Rayleigh frequency and are called normal Raman spectra. The measure of these shifts reflects the characteristic vibrations of the molecule and may be utilized as a complement to infrared spectroscopy. 

The excitation current was fixed for the realized probe at 1mA. The computed field resulting for this current value is lower than 100 Am, in order to be located in the linear zone of the hysterisis diagram (Rayleigh). Whatever the type of the chosen probe or the excitation frequency, the same zone is controlled. The surface of this zone is 100 mm (10x10). 

Based upon a piezoelectric 1-3-composite material, air-bome ultrasonic probes for frequencies up to 2 MHz were developped. These probes are characterized by a bandwidth larger than 50 % as well as a signal-to-noise ratio higher than 100 dB. Applications are the thickness measurement of thin powder layers, the inspection of sandwich structures, the detection of surface near cracks in metals or ceramics by generation/reception of Rayleigh waves and the inspection of plates by Lamb waves. 

The first temi results in Rayleigh scattering which is at the same frequency as the exciting radiation. The second temi describes Raman scattering. There will be scattered light at (Vq - and (Vq -i- v ), that is at sum and difference frequencies of the excitation field and the vibrational frequency. Since a. x is about a factor of 10 smaller than a, it is necessary to have a very efficient method for dispersing the scattered light. 

Due to the rather stringent requirements placed on the monochromator, a double or triple monocln-omator is typically employed. Because the vibrational frequencies are only several hundred to several thousand cm and the linewidths are only tens of cm it is necessary to use a monochromator with reasonably high resolution. In addition to linewidth issues, it is necessary to suppress the very intense Rayleigh scattering. If a high resolution spectrum is not needed, however, then it is possible to use narrow-band interference filters to block the excitation line, and a low resolution monocln-omator to collect the spectrum. In fact, this is the approach taken with Fourier transfonn Raman spectrometers. 

Figure 1-1 The Blackbody Radiation Spectrum. The short curve on the left is a Rayleigh function of frequency.

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. 

In the low frequency region, the calculations predict nanotube-specifiic Eig and E g modes around 116 cm and 377 cm respectively, for (10,10) armchair naiiotubes, but their intensities are expected to be lower than that for the A g mode. However, these Eig and E2g modes are important, since they also show a diameter dependence of their mode frequencies. In the very low frequency region below 30 cm a strong low frequency Raman-active E2g mode is expected. However, it is difficult to observe Raman lines in the very low frequency region, where the background Rayleigh scattered is very strong. 

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