Intensity vibrational Raman scattering

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)...

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.

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

Raman optical activity (RO A) Due to molecular chirality there is a difference in the intensity of Raman scattered right and left circularly polarized light. Raman optical activity (ROA) is a vibrational spectroscopic technique that is reliant on this difference and the spectrum of intensity differences recorded over a range of wavenumbers reveals information about chiral centers within a sample molecule. It is a useful probe to study biomolecular structures and their behavior in aqueous solution especially those of proteins, nucleic acids, carbohydrates, and viruses. The information obtained is in realistic conditions... 

This article reviews all the published work concerned with the study of vibrational optical activity in chiral molecules from measurements of a small difference in the intensity of Raman scattering in right and left circularly polarized incident light. The history and basic theory are described briefly, followed by an account of the instrumentation and the precautions that must be observed in order to suppress spurious signals. The various theories that have been proposed in order to relate stereochemical features to the observations are then outlined, this being followed by a survey of all reported Raman optical activity spectra. 

The Raman approach to vibrational optical activity is based on measurement of a small difference in the intensity of Raman-scattered light from chiral molecules in right and left circularly polarized incident light, and several reviews have appeared previously1 -S). However, another review is now timely because important experimental and theoretical developments have since brought Raman optical activity (ROA) to a new level of maturity. 

Thus the condition for observation of Raman spectrum is a nonzero cf which is the selection rule. The intensity of Raman scattering, however depends on a number of factors, which includes the frequency of the scattered light (energy separation and hence populations), the density of vibrational states, the damping constant etc. In the case of glasses, an expression due to Shuker and Gammon (1971) for the Raman intensity of the Stokes lines is given by. 

Figure 3.8. The time dependence of the intensities of Raman scattering for vibrational modes of trans-stilben. The figure insert displays a fitting. [Reprinted with permission from S. L. Schultz, J. Qian, and J. M. Jean, J. Phys. Chem., AlOl (1997), p. 1000. Copyright 1997, American Chemical Society.]...

The radiant intensity / (k) of the vibrational Raman scattering along the direction of the wave vector k is, for isolated molecules [5],... 

RRS has also introduced the concept of a Raman excitation profile (REPy for thefth mode) [46, 4lZ, 48, 49, 50 and M]. An REP. is obtained by measuring the resonance Raman scattering strength of thefth mode as a fiinction of the excitation frequency [, 53]. Flow does the scattering intensity for a given (thefth) Raman active vibration vary with excitation frequency within an electronic absorption band In turn, this has led to transfomi theories that try to predict... 

---