Stokes bands

For most purposes only the Stokes-shifted Raman spectmm, 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 foUow a Boltzmann distribution. At room temperature, the anti-Stokes Stokes intensity ratio decreases by a factor of 10 with each 480 cm from the exciting frequency. Because of the weakness of the anti-Stokes spectmm (except at low frequency shift), the most important use of this spectmm is for optical temperature measurement (qv) using the Boltzmann distribution function. 

To date, no Raman spectrum of syndiotactic polypropylene has been published although vibrational analyses have been issued by Schacht-schneider and Snyder and also by Peraldo and Cambini during 1965. Recently we have had the opportunity to examine polypropylene in three forms atactic, isotactic and syndiotactic. The results for the last specimen are included in Fig. 6. It will be seen that there is an enormous emission in the 1350—1400 cm-1 region. The nature of this is not known — it may be fluorescence but it cannot be checked, as the anti-Stokes band at v0+ 1350 cm-1 would be vanishingly weak due to the low Boltzmann population 1350 cm-1 above the ground state. A coordinate analysis is available for syndiotactic polypropylene and currently we are working on an assignment of the observed results. 

When a transparent medium was irradiated with an intense source of monochromatic light, and llie scattered radiation was examined spectroscopically, not only is light of the exciting frequency, v, observed (Rayleigh scattering), blit also some weaker bands of shifted frequency are detected. Moreover, while most of the shifted bands are of lower frequency, v - Aii, there are some at higher frequency, v + Aiq, By analogy to fluorescence spectrometry (see below), the former are called Stokes bands and the latter a iti-Stakes bands. The Stokes and anti-Stokes... 

Fig. 8.12 The Raman effect. Monochromatic light of frequency vQ is scattered by a sample, either without losing energy (Rayleigh band) or inelastically, in which a vibration is excited (Stokes band), or a vibra-tionally excited mode in the sample is de-excited (anti-Stokes band). The spectrum is that of the light scattered by the sample. The energy level diagrams illustrate that the scattering process occurs via highly unstable states of high energy.

Yet, the reverse process may also take place. If the collision with a photon brings a vibrationally excited molecule to the unstable state of energy hvQ + hvvij it may decay to the ground state, transferring a net amount of energy h yv,b to the photon, which leaves the sample with a higher frequency equal to vQ + vvl, . This peak, which is termed the anti-Stokes band , has much lower intensity than the Stokes band, because the fraction of vibrationally excited molecules is usually small. 

You will note some structure in the Stokes band near 460 which is due to chlorine isotopic frequency shifts. (See Exp. 37 for a discussion of isotope effects in diatomic molecules.) Rescan this region at higher resolution (1 cm or less) with an expansion of the chart display and measure the frequencies of each of the components. Record the ambient temperature near the Raman cell. 

Figure 6 Energy transfer diagram illustrating Rayleigh and Raman scattering (top), and Raman spectra for CCfr excited at room (298 K) and hqnid-N2 (77 K) temperatures by Ar+ ion laser radiation of Xq = 488.0 nm or vo = 20 492 cm (bottom). The number above the peaks is the Raman shift, Av = vq — Wc cm. Since the fraction of molecules occupying excited states depends on the Boltzmann factor (kT = 207 cm at 298 K), the intensities of anti-Stokes bands fall off rapidly with decreasing temperature (kT = 54 cm at 77 K) and increasing vibrational frequency Vk...

Figure 2.34 A typical Raman spectrum consisting of an intense Rayleigh line (vo) and a series of lower frequency Raman bands corresponding to sample vibrational modes. The higher frequency anti-Stokes bands fall away rapidly and are rarely observed deliberately, except for special purposes...

Figure 1 Laser-excited vibrational transi tions and the relationship of Stokes/Anti Stokes band intensity ratios to temperature.

An efficient mechanism for rejection of the scattered laser-line radiation has to be incorporated. The major new development in this area is the introduction of holographic filters. These devices have the sharp cut-off characteristics of the multilayer dielectrics, but do not exhibit a harmonic structure in the transmission curve. The transmission is high (80-90%) and featureless, as opposed to the dielectrics which have numerous features in the transmission curve because of interference among the multiple layers. Two filters are sufficient to give Rayleigh-line rejection, and spectral information down to 150 cm" can be obtained. Another filter type is the Chevron unit [41] which has been shown by Nicolet, and possibly Bruker, to give Raman data down to 60 cm. Additionally, data can be obtained on both the Stokes and anti-Stokes bands [42]. 

Fig. 15. The low frequency region (Av = —200cm —l-200cm" ) of the Raman spectra of crystals I of d(GGTATACC) in the A form, 2 d(CGCGAATTCGCG) in the B form and 3 d(CACGCGTG) in the Z form [88W1]. The central feature is the Rayleigh line and both Stokes (lower frequency) and anti-Stokes bands are shown [88W1].

An alternative laser-tracking approach is available to multidimensional spectrometers which are able to measure both Stokes and anti-Stokes bands simultaneously. In this approach, the band position of the Stokes and the anti-Stokes bands are calculated. Because these bands occur symmetrically about the laser position, any difference between them is related to a shift in the laser position. 

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