Stokes spectrum

For most purposes only the Stokes-shifted Raman spectrum, which results from molecules in the ground electronic and vibrational states being excited, is measured and reported. Anti-Stokes spectra arise from molecules in vibrational excited states returning to the ground state. The relative intensities of the Stokes and anti-Stokes bands are proportional to the relative populations of the ground and excited vibrational states. These proportions are temperature-dependent and follow a Boltzmann distribution. At room temperature, the anti-Stokes Stokes intensity ratio decreases by a factor of 10 with each 480 cm-1 from the exciting frequency. Because of the weakness of the anti-Stokes spectrum (except at low frequency shift), the most important use of this spectmm is for optical temperature measurement (qv) using the Boltzmann distribution function. 

Block the laser beam or spectrometer entrance slit and adjust the spectrometer to an anti-Stokes shift of 1000 cm Caution Exposure of the sensitive phototube to the intense Rayleigh scattering line can seriously damage the detector. Scan the anti-Stokes spectrum from 1000 to 150 cm in the parallel polarization configuration and, using appropriate sensitivity expansion, j measure the ratio of anti-Stokes to Stokes peak heights for each band. 

Figure 7. Two-body depolarized scattering Stokes spectrum of the Vi Raman band of gaseous CF4 in absolute units at 294.5 K. Full circles ( ) indicate experimental data together with error bars.

CARS CoherenI Anii-Slokes Raman Speclroscopy Solid, liguid(60 pm-3 cm) Pump beam (0),)+ probe beam (co) Anti-Stokes spectrum High resoiulion Raman spectra 14... 

The 1st Stokes spectrum from CS2 droplets was also noted to be asymmetrically broadened. For the Stokes wave, phase modulation can be induced by... 

Figure 4 shows the Stokes spectrum for the 1st- and 2nd-order SRS waves from CS2 droplets. The broadening is only on the longer wavelength side (-400 cm ) of the (658 cm ) and (1322 cm ) SRS peaks. The struc-... 

Raman line intensities are proportional to the number density N of molecules in the initial state /c>, which is in turn proportional to the pertinent Boltzmann factor for that state at thermal equilibrium. Consequently, the relative intensities of a Stokes transition /c> - m> and the corresponding anti-Stokes transition m> -> /c> are 1 and exp — hoj kjkT), respectively. (The factor coicol varies little between the Stokes and anti-Stokes lines, because the Raman frequency shifts are ordinarily small compared to cui.) Hence the anti-Stokes Raman transitions (which require molecules in vibrationally excited initial states) are considerably less intense than their Stokes counterparts, particularly when the Raman shift (o k is large. In much of the current vibrational Raman literature, only the Stokes spectrum is reported (cf Fig. 10.1). 

In fact, Raman spectrum is a plot of the intensity of Raman scattered radiation as a function of its frequency difference from the incident radiation, usually in units of wavenumbers, cm. This difference is called the Raman shift. It is to be noted that, because it is a difference value, the Raman shift is independent of the frequency of the incident radiation. Typically, only the Stokes region is used because the anti-Stokes spectrum is identical in pattern, but it is much less intense. Raman spectroscopy has been found very useful for chemical analysis because of the following reasons it exhibits high specificity, it is compatible with aqueous systems, no special preparation of the sample is needed and the timescale of the experiment is short. 

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