Raman spectra with lasers

Figure 4.14 Comparison of dichroic IR and Raman spectra of drawn PET. Upper IR spectra measured with E parallel and perpendicular to the draw axis. Lower Raman spectra with laser polarised parallel and perpendicular to the draw axis. Note how the 1615 cm and 1018 cm bands have parallel character, while the 875 cm band has perpendicular character with respect to the draw direction (and hence the polymer chain axis).

The thermal conductivity of suspended graphene has been calculated by measuring the frequency shift of the G-band in the Raman spectrum with varying laser power. These measurements yielded a value for thermal conductivity of 4840 5300 W m 1 K 1 [23], better than that of SWCNTs, with the exception of crystalline ropes of nanotubes, which gave values up to 5800 W m 1 K 1 [24]. Even when deposited on a substrate, the measured thermal conductivity is 600 W m 1 K 1 [25], higher than in commonly used heat dissipation materials such as copper and silver. 

The different schemes above can also be distinguished by using TRRR techniques. At the moment this technique might take more effort than the optical methods. However, it can be done with more accuracy since vibrational Raman bands are better resolved than optical absorption bands. A detailed study of the observed change of the resonance Raman spectrum with time and with probe laser frequency should, in principle, enable one to distinguish between the different schemes given above. This will be possible if the photoproducts in a certain scheme are produced with different rates or have different optical absorption maxima (and thus different resonance Raman enhancement profiles). 

Figure 103. Raman spectrum of laser-grown SWCNTs measured with a 514.5 nm laser focused to -1 nm through a 50x objective. Laser power density was maintained below 3 kW/cm2 to avoid heating.

It has often been reported that prolonged exposure of a specimen to a laser beam will produce a Raman spectrum with enhanced signal to noise (the little-understood physical phenomenon of fluorescence burnout which is illustrated in Figure 1) by this means, good quality Raman spectra can be recorded in the presence of fluorescent material. However, the removal of the fluorescence emission is only temporary and the method is not always appropriate. Some biomaterials, such as skin and wool, exhibit strong... 

Figure 5.17 shows the rotational Raman spectrum of N2 obtained with 476.5 nm radiation from an argon ion laser. From this spectrum a very accurate value for Bq of 1.857 672 0.000 027 cm has been obtained from which a value for the bond length tq of 1.099 985 0.000 010 A results. Such accuracy is typical of high-resolution rotational Raman spectroscopy. 

Fig. 23. Experimental room temperature Raman spectrum from a sample consisting primarily of bundles or ropes of single-wall nanotubes with diameters near that of the (10,10) nanotube. The excitation laser wavelength is 514.5 nm. The inset shows the lineshape analysis of the vibrational modes near 1580 cm . SWNT refers to singlewall carbon nanotubes [195].

The third common level is often invoked in simplified interpretations of the quantum mechanical theory. In this simplified interpretation, the Raman spectrum is seen as a photon absorption-photon emission process. A molecule in a lower level k absorbs a photon of incident radiation and undergoes a transition to the third common level r. The molecules in r return instantaneously to a lower level n emitting light of frequency differing from the laser frequency by —>< . This is the frequency for the Stokes process. The frequency for the anti-Stokes process would be + < . As the population of an upper level n is less than level k the intensity of the Stokes lines would be expected to be greater than the intensity of the anti-Stokes lines. This approach is inconsistent with the quantum mechanical treatment in which the third common level is introduced as a mathematical expedient and is not involved directly in the scattering process (9). 

Fig. 31 Evolution of the Raman spectra of a high-pressure and photo-induced sample of Se while decreasing the pressure at ca. 300 K [109]. The spectrum at 3.9 GPa shows the onset of the transformation S6 p-S. The asterisks indicate the Raman signals typical for p-S whereas the peaks of two stretching vibrations of p-S coincide with those of Se at about 458 cm and 471 cm (not indicated by asterisks). The Raman spectrum of the sample recovered at ambient pressure (0 GPa) is evidently a superposition of the spectra of a-Sg and polymeric sulfur, Sj, arrows indicate plasma lines of the Ar ion laser at 515 nm, which have been used for calibration). For Raman spectra under increasing pressure, see Fig. 23 in [1] and references cited therein...

Fig. 3 a UV-Vis DRS spectra of dehydrated TS-1 catalyst reporting the typical 208 nm (48000cm i) LMCT hand, see Fig. 2h also reported are the four excitation laser lines used in this Raman study near-lR (dotted), visible (full), near-UV (dashed) and far-UV (dot-dashed), b Raman spectra of dehydrated TS-1 obtained with four different lasers emitting at 7 = 1064,422,325, and 244 nm (dotted, full, dashed, and dot-dashed lines, respectively). Raman spectra have been vertically shifted for clarity. Although the intensity of each spectrum depends upon different factors, the evolution of the 7(1125)//(960) ratio by changing the laser source is remarkable. The inset reports the Raman spectrum collected with the 244 nm laser in its full scale, in order to appreciate the intensity of the 1125 cm enhanced mode. Adapted from [48] with permission. Copyright (2003) by The Owner Societies 2003... 

The vibrational spectrum of cis- and fr s-[Pt(II) (NH3)2C12] in the crystalline phase has also been measured with laser-Raman techniques by Dr. Hoeschele of the Biophysics Department, Michigan State University. This technique shows great promise. 

---