Raman backscattering spectrum

Figure 1.23 Raman backscattering spectrum of as-grown single-costal ZnO after background subtraction. The sample was irradiated with the 488 nm line of an Ar laser and a power of 190 mW. The solid line represents a least-square fitofsixGaussian lines to the data. The dashed lines indicate the individual local vibrational modes. The peak positions are indicated in the plot.

Raman backscattering measurements were performed in the wave number range between 200 and 4300 cm. Besides the well known phonon modes that are located between 200 and 800 cm additional lines were observed at wave numbers ranging from 2800 to 3150 cm" (see spectrum (a) in Fig. 1). At higher wave numbers Raman lines were not observed. 

Fluorescence from pharmaceutical capsule shells and tablet coatings has hindered measurement of their composition by Raman spectroscopy. By switching from the conventional backscattering mode to a transmission mode, Matousek et al. demonstrated that fluorescence could be eliminated in many instances [8]. Backscattering- and transmission-mode Raman spectra of several samples are shown in Figure 7.5. Each spectrum was acquired in 10s with 80mW 830-mn laser power. Matousek et al. also speculate that signal acquisition times could be relatively easily shortened to well below 0.1 s when the transmission mode is combined with optimized optics [8]. 

The Raman spectra were recorded in the backscattering geometry on a Labram I (Jobin-Yvon, Horiba Group, France) microspectrometer in conjunction with a confocal microscope. To avoid any thermal photochemical effect, we have used a minimum intensity laser power on sample of 370 pW with the 514.5 nm incident line from an Ar-Kr laser from Spectra Physics. Detection was achieved with an air cooled CCD detector and a 1800 grooves/mm, giving a spectral resolution of 4 cm-1. An acquisition time of 120 s was used for each spectrum. The confocal aperture was adjusted to 200 pm and a 50 X objective of 0.75 numerical aperture was used. 

Fig. 1.53. Backscattered Raman spectrum of a polished sintered pellet of YBa2Cu307 3 [151].

We also recorded the Raman spectra in a backscattering configuration with the 488 nm line of an Ar-ion laser at low power to avoid darkening of the dots. The spectrum of 3.2 nm ZnSe QDs is shown in Fig. 3. The LO-phonon peak in ZnSe QDs is shifted to lower frequency (4.9 cm" ) relative to the frequency of bulk ZnSe (indicated as LO). 

For the case of interconnect line features patterned on a single crystal Si substrate, a common procedure for the use of micro-Raman spectroscopy entails determination of the stress in the interconnect from the distortion produced in the substrate and the dielectric which immediately surround the interconnect (DeWolf et al. (1992) Ma et al. (1995) DeWolf et al. (1999)) the Raman spectra can be measured in the backscattering mode using an argon laser. The piezo-spectroscopic shift in the Raman spectrum, which is induced by such local distortions, can be measured to a spatial resolution on the order of 1 pan. so that stress in individual lines can be estimated. 

Figure 8. A Raman spectrum of a single TGRL particle exhibits distinct peaks that can be assigned to lipid vibrations. Inset shows a single trapped lipoprotein particle visualized by the backscatter of the incident laser light used to optically trap the particle in solution.

Of the two backscattering ROA schemes predominantly in use these days, the ICP approach enjoys an advantage of experimental simplicity, while the DCPj enjoys an advantage of higher ROA intensity per Raman intensity, particularly in regions of strongly polarized Raman bands where no such intensity enters the DCPj Raman spectrum. 

In many Raman spectrometers, the use of hbre optics and waveguides can produce a big advantage. Samples that cannot be introduced into the Raman spectrometer due to their size or hazardous nature can be investigated in situ. A lens is fixed on the end of the fibre optic probe to focus the laser beam and to function as the collecting windows in backscattering geometry. The fibre is pointed at the sample and Raman spectrum is acquired. Small sample spots (about 1 pm) can be analysed this way (Smith and Dent 2005). Moreover, the portable Raman spectrometers can be used for rapid and in sim Raman analysis, which wiU be demonstrated in Chap. 4. 

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