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Lecture 14

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chichi's version from 2018-04-29 13:07

Section 1

Question Answer
Types of Raman ScatteringNormal Raman Spectroscopy, ,Confocal raman spectroscopy, micro-raman spectroscopy, FT-Raman spectroscopy, Resonanace raman spectroscopy, surface enhanced raman spectroscopy, SEM-raman spectroscopy, SPM-Raman spectroscopy
Raman Spectroscopylight scattering technique, so all that is required for the collection of a spectrum is to place the sample into the excitation beam and collect the scattered light
Raman spectroscopy advantages #1unlink other vibrational techniques, its operational wavelength range is usually independent of the vibrational modes being studied
Raman spectroscopy advantages #2since Raman spectroscopy measures the shift in frequency from that of the excitation laser, it can be performed using any operating range from UV to NIR
Raman spectroscopy advantages #3It thus permits access to vibrational mode information normally associated with wavelengths ranging from 2-200 microns
Raman spectrscopy advantage #4This makes Raman ideal for the study of inorganic materials that have vibrational frequencies in the far-infrared that are otherwise difficult to reach
Raman spectroscopy advantage #5Dispersive Raman microscopy using visible excitation wavelengths delivers 1 micron spatial resolution, and is widely used in the analysis of micron-level sample contaminants
Raman spectroscopy advantage #6Raman spectrometers are based on one of two technologies: dispersive Raman and Fourier transform Raman. Each technique has its unique advantages and each is ideally suited to specific types of analysis
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Section 2

Question Answer
The theory of Raman scattering show 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
An important advantage of Raman spectra over infraredlies in the fact that water does not cause interference; indeed, Raman spectra can be obtained from aqueous solutions
An important advantage of Raman spectra over infrared #2In 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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Section 3

Question Answer
How does Raman work? #1Raman spectra are acquired by irradiating a sample with a powerful laser source of visible or near-infrared monochromatic radiation
How does Raman work? #2During irradiation, the spectrum of the scattered radiation is measured at some angle (often 90 deg) with a suitable spectrometer
How does Raman work? #3At the very most, 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.
IR is sensitive to functional groups and to highly polar bonds,
Raman is more sensitive to backbone structures and symmetric bonds
IR and Raman are both used forto provide twice the information about the vibrational structure than can be obtained by using either alone.
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Section 4

Question Answer
3 Types of emitted radiationstokes scattering, anti-stokes scattering, rayleigh scattering
Why is the stokes scattering generally used?Anti-stokes lines are appreciably less intense that the corresponding stokes lines
Mechanism of Raman and Rayleigh Scattering #1The 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 hv.
Mechanism of Raman and Rayleigh Scattering #2The second and narrower arrow shows the type of change that would occur if the molecule is in the first vibrational level of the electronic ground state
Mechanism of Raman and Rayleigh Scattering #3The relative populations of the two upper energy states are such that Stokes emission is much favored over anti-stokes. Rayleigh scattering has a considerably higher probability of occurring than Raman because the most probable event is the energy transfer to molecules in the ground state and reemission by the return of these molecules to the ground state
Mechanism of Raman and Rayleigh ScatteringThe ratio of anti-Stokes to Stokes intensities will increase with temperature because a larger fraction of the molecules will be in the first vibrationally excited state under these circumstances
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Section 5

Question Answer
Raman vs IR #1In contrast, scattering involves a momentary distortion of the electrons distributed around a bond in a molecule, followed by reemission of the radiation as the bond returns to its normal state
Raman vs IR #2In its distorted form, the molecule is temporarily polarized; that is , it develops momentarily an induced dipole that disappears upon relaxation and reemission. The Raman activity of a given vibrational mode may differ makedly from its infrared activity
Intensity of Normal Raman Peaks #1The intensity or power of a normal Raman peak depends in a complex way upon the polarizability of the molecule, the intensity of the source, and the concentration of the active group
Intensity of Normal Raman Peaks #2The power of Raman emission increases with the fourth power of the frequency of the source; however, advantage can seldom be taken of this relationship because of the likelihood that ultraviolet irradiation will cause photodecomposition. Raman intensities are usually directly proportional to the concentration of the active species
Raman Depolarization Ratios #1Polarization is a property of a beam of radiation and describes the plane in which the radiation vibrates. Raman spectra are excited by plane-polarized radiation
Raman Depolarization Ratios #2The scattered radiation is found to be polarized to various degrees depending upon the type of vibration responsible for the scattering
The depolarization ratio p is defined asp=I(sub upside down T)/I
The depolarization ratio is dependent uponthe symmetry of the vibrations responsible for scattering
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