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Chapter 18 - Chapter 18 Raman Spectroscopy When radiation...

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Chapter 18 Raman Spectroscopy When radiation passes through a transparent medium, the species present scatter a fraction of the beam in all directions. In 1928, the Indian physicist C. V. Raman discovered that the visible wavelength of a small fraction of the radiation scattered by certain molecules differs from that of the incident beam and furthermore that the shifts in wavelength depend upon the chemical structure of the molecules responsible for the scattering.
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Raman Spectroscopy The theory of Raman scattering shows that the phenomenon results from the same type of quantized vibrational changes that are associated with infrared absorption. Thus, the difference in wavelength between the incident and scattered visible radiation corresponds to wavelengths in the mid-infrared region. The Raman scattering spectrum and infrared absorption spectrum for a given species often resemble one another quite closely.
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Raman Spectroscopy An important advantage of Raman spectra over infrared lies in the fact that water does not cause interference; indeed, Raman spectra can be obtained from aqueous solutions. In addition, glass or quartz cells can be employed, thus avoiding the inconvenience of working with sodium chloride or other atmospherically unstable window materials.
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THEORY OF RAMAN SPECTROSCOPY Raman spectra are acquired by irradiating a sample with a powerful laser source of visible or near-infrared monochromatic radiation. During irradiation, the spectrum of the scattered radiation is measured at some angle (often 90 deg) with a suitable spectrometer. At the very most, the intensities of Raman lines are 0.001 % of the intensity of the source; as a consequence, their detection and measurement are somewhat more difficult than are infrared spectra.
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Excitation of Raman Spectra A Raman spectrum can be obtained by irradiating a sample of carbon tetrachloride (Fig 18-2) with an intense beam of an argon ion laser having a wavelength of 488.0 nm (20492 cm -1 ). The emitted radiation is of three types: 1. Stokes scattering 2. Anti-stokes scattering 3. Rayleigh scattering
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Excitation of Raman Spectra The abscissa of Raman spectrum is the wavenumber shift ∆ν , which is defined as the difference in wavenumbers (cm -1 ) between the observed radiation and that of the source. For CCl 4 three peaks are found on both sides of the Rayleigh peak and that the pattern of shifts on each side is identical (Fig. 18-2). Anti-Stokes lines are appreciably less intense that the corresponding Stokes lines. For this reason, only the Stokes part of a spectrum is generally used. The magnitude of Raman shifts are independent of the wavelength of excitation.
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Mechanism of Raman and Rayleigh Scattering The heavy arrow on the far left depicts the energy change in the molecule when it interacts with a photon. The increase in energy is equal to the energy of the photon h ν .
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