Vibrational Raman scattering

Figure Bl.2.2. Schematic representation of the polarizability of a diatomic molecule as a fimction of vibrational coordinate. Because the polarizability changes during vibration, Raman scatter will occur in addition to Rayleigh scattering.

Figure 6.3 Stokes and anti-Stokes vibrational Raman scattering...

Temkin S. I., Burshtein A. I. Pressure-induced transformation of the Q-branch of the rotational-vibrational Raman-scattering spectrum, JETP Lett. 24, 86-9 (1976) [Pis ma ZhETFU, 99-103 (1976)]. 

Raman spectroscopy comprises a family of spectral measurements based on inelastic optical scattering of photons at molecules or crystals. It involves vibrational measurements as well as rotational or electronic studies and nonlinear effects. Following, Raman will be used in the established but slightly inaccurate way as a synonym for the most important and most common technique of the family, linear vibrational Raman scattering. 

Vibrational Raman Scattering of radiation with changed frequency due lo polarizability change during a vibration (A = visible usually) Qualitative for large molecules 10-Ms 10 Pa (100 ion) (v dependent) Useful for characterization. Some structural information from number of bands, position, depolarization ratios, and possibly Isotope effects. All... 

Fig. 7.1. Two equivalent ROA experiments involving Stokes vibrational Raman scattering at angular frequency oj — ojv in incident light of angular frequency oj far from resonance, a The ICP ROA experiment measures JR - JL, where JR and JL are the scattered intensities (shown here as unpolarized) in right- and left-circularly polarized incident light, respectively, b The SCP ROA experiment measures hi-h., where Jr and JL are the intensities of the right- and left-circularly polarized components, respectively, of the scattered light using incident light of fixed polarization (shown here as unpolarized)...

The tensors which enter theoretical expressions are transition tensors 7, for a transition between an initial state i and a final state /. The Placzek polarizability theory for vibrational Raman scattering [56], which we use here, is valid in the far from resonance limit, i and / are then vibrational states. If we assume that they differ for normal mode p, then the transition tensors can be written as... 

These include temperature, major constituent densities, gas velocity, and correlations of these properties. We discuss here the advantages and limitations of various potential light-scattering probes, and illustrate these with recent results for vibrational Raman scattering flame diagnostics. 

The inelastic processes - spontaneous Raman scattering (usually simply called Raman scattering), nonlinear Raman processes, and fluorescence - permit determination of species densities as well as temperature, and also allow one, in principle, to determine the temperature for particular species whether or not in thermal equilibrium. In Table II, we categorize these inelastic processes by the type of the information that they yield, and indicate the types of combustion sources that can be probed as well as an estimate of the status of the method. The work that we concentrate upon here is that indicated in these first two categories, viz., temperature and major species densities determined from vibrational Raman scattering data. The other methods - fluorescence and nonlinear processes such as coherent anti-Stokes Raman spectroscopy - are discussed in detail elsewhere (5). 

Table III Comparison of Vibrational Raman Scattering Fluctuation Measurement Capabilities for Different Laser Source Characteristics.

These are the quantities to which we are giving our attention. Vibrational Raman scattering is being used for the temperature and density data, and, when taken simultaneously with velocity data from coupled LV instrumentation (.8), provides also the fluctuation mass flux through use of fast chemistry assumptions and the ideal gas law for atmospheric pressure flames. 

Figure 3. Schematic of turbulent combustor geometry and optical data acquisition system for vibrational Raman-scattering temperature measurements using SAS intensity ratios. Also shown are sketches of the expected Raman contours viewed by each of the photomultiplier detectors, the temperature calibration curve, and several expected pdf s of temperature at different flame radial positions. The actual SAS temperature calibration curve was calculated theoretically to within a constant factor. This constant, which accounted for the optical and electronic system sensitivities, was determined experimentally by means of SAS measurements made on a premixed laminar flame of known temperature. Measurements of Ne concentration were made also with this apparatus, based on the integrated Stokes vibrational Q-branch intensities. These signals were related to gas densities by calibration against ambient air signals.

Temperature from Rotational and Vibrational Raman Scattering Effects of Vibrational-Rotational Interactions and Other Corrections... 

Analysis of experimental rotational Raman scattering from N 0and H has been used to determine temperatures in premixed laboratory flames (1,2). Temperatures based upon rotational Raman scattering from N and 02 had lower uncertainties (1-4%) than those based upon vibrational Raman scattering (3-9%) because rotational Raman scattering is generally more intense and gives rise to many more transitions. However, careful application of Raman intensity theory is required. 

Figure 1. Rotational—vibrational line strength correction factors for pure rotational Raman scattering (fM)0 and for O-, S-, and Q-branch vibrational Raman scattering (foh fots, and folQ). The value J is the rotational quantum number of the initial level (O), Stokes (A), anti-Stokes.

Figure 3. Calculated band profiles of Stokes vibrational Raman scattering from Nt at 2000 K assuming a triangular slit function with FWHM = 5.0 cm 1. The bottom curve includes the isotropic part of the Q-branch only. The top curve is a more exact calculation including O- and S-branch scattering, the anisotropic part of the Q-branch and line-strength corrections owing to centrifugal distortion. The base lines have been shifted vertically for clarity.

Temperature-Velocity Correlation Measurements for Turbulent Diffusion Flames from Vibrational Raman-Scattering Data... 

We present here preliminary results for the (temperature x vj ljjcity) probability density function shown in this paper as , where the quantities within the average brackets are instantaneous values. These data have been obtained from a coordinated experimental program utilizing pulsed laser vibrational Raman scattering and cw real fringe laser velocimetry (LV). 

Figure 2. Intensity ratio of anti-Stokes to Stokes vibrational Raman scattering for a trapezoidal slit function. Center position of Stokes bandpass at 6072 A.

Vibrational Raman Scattering ofradiaiion with Qualitative for large changed frequency due molecules lo polarizafaiOty change duriitg a vibration (A = visible usually)... 

This review is largely concerned with vibrational Raman scattering in which the states G> and F> differ only in the possession of one or more quanta of vibrational energy. In systems with electronically degenerate ground states it is impossible to avoid entirely the effects of simultaneous changes in the electronic and vibrational quantum numbers This latter point is discussed briefly later on with respect to the Jahn-Teller effect as well as the electronic Raman effect. 

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