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An Introduction to Raman spectroscopy
Raman spectroscopy is a form of vibrational spectroscopy, much like infrared (IR) spectroscopy. However, whereas IR bands arise from a change in the dipole moment of a molecule, Raman bands arise from a change in the polarizability. In many cases, transitions that are allowed in Raman are forbidden in IR, so these techniques are often complementary. This article briefly explains the Raman effect, some advantages of Raman spectroscopy, and the basics of Raman instrumentation. The Raman Effect When a beam of light is impinged upon a sample, photons are absorbed by the material and scattered. The vast majority of these scattered photons have exactly the same wavelength as the incident photons and are known as Rayleigh scatter, but a tiny portion (approximately 1 in 107) of the scattered radiation is shifted to a different wavelength. These wavelength shifted photons are called Raman scatter. Most of the Raman scattered photons are shifted to longer wavelengths (Stokes shift), but a small portion are shifted to shorter wavelengths (anti-Stokes shift). Figure 1 shows a diagram of Rayleigh scattering, Stokes Raman scattering, and anti-Stokes Raman scattering. In each case, the incident photon excites an electron into a higher “virtual” energy level (or virtual state) and then the electron decays back to a lower level, emitting a scattered photon.  In Rayleigh scattering the electron decays back to the same level from which it started. In both types of Raman scattering the electron decays to a different level than that where it started. Stokes Raman scattering occurs when the final energy level is higher than the initial level, while anti-Stokes Raman scattering occurs when the final energy level is lower than the starting level. Stokes scattering is much more common than anti-Stokes scattering because at any given time an electron in the most common temperature range is most likely to be in its lowest energy state, in accordance with the Boltzmann distribution.  As mentioned above, Raman is a form of vibrational spectroscopy. This means that these energy transitions arise from molecular vibrations. Because these vibrations involve identifiable functional groups, when the energies of these transitions are plotted as a spectrum, they can be used to identify the molecule. The Raman Spectrum A Raman spectrum is a plot of the intensity of Raman scattered radiation as a function of its frequency difference from the incident radiation (usually in units of wavenumbers, cm–1). This difference is called the Raman shift. Note that, because it is a difference value, the Raman shift is independent of the frequency of the incident radiation. Typically, only the Stokes region is used (the anti-Stokes spectrum is identical in pattern, but much less intense). Figure 2 contains Raman spectra of two different methyl chlorosilanes plotted on the same set of axes. Note that each has a characteristic set of peaks that allows it to be distinguished from the other.  Advantages of Raman Spectroscopy Raman spectroscopy is useful for chemical analysis for several reasons: it exhibits high specificity, it is compatible with aqueous systems, no special preparation of the sample is needed, and the timescale of the experiment is short. Specificity: Because Raman detects fundamental vibrations, Raman bands have a good signal-to- noise ratio and are non-overlapping. This allows a Raman spectrum to be used for everything from “fingerprinting” of samples to constructing complex chemical models of reaction processes. Analysis of aqueous systems: The IR spectrum of water is strong and relatively complex, making IR inadequate for analysis of aqueous solutions due to heavy interference by the water bands. However, the Raman spectrum of water is weak and unobtrusive, allowing good spectra to be acquired of species in aqueous solution. No sample preparation: Unlike most other chemical analysis techniques, Raman requires no special preparation of the sample. In fact, no contact with the sample is needed at all because Raman involves only illuminating a sample with a laser and collecting the scattered photons. Note especially that this makes Raman spectroscopy non-destructive. Short timescale: Because a Raman spectrum can be acquired in as little as a few seconds, Raman can be used to monitor chemical reactions in “real time.” Raman Instrumentation A typical Raman spectrometer is made up of three basic parts: the laser, the collection device, and the spectrograph. Laser: A laser is used to excite Raman spectra because it gives a coherent beam of monochromatic light. This gives sufficient intensity to produce a useful amount of Raman scatter and allows for clean spectra, free of extraneous bands. Lasers used for Raman spectroscopy must exhibit good wavelength stability, low background emission and the wavelength may be selected to minimise interference from fluorescence. Collection: The collection device collects the scattered photons. It can be a simple lens attached to a handhel instrument, a microscope for looking at small samples or a fibre-coupled probe which filters out the Rayleigh scatter,any background signal from the fiber optic cables, and sends the Raman scatter to the spectrograph. Many probes also focus and deliver the incident laser beam and the PhAT probe samples from large areas rather than a small focal spot. Spectrograph: When Raman scattered photons enter the spectrograph, they are passed through a transmission grating to separate them by wavelength and passed to a detector, which records the intensity of the Raman signal at each wavelength. This data is plotted as the Raman spectrum. References: 1. Analytical Applications of Raman Spectroscopy; Pelletier, M.J., Ed.; Blackwell: Oxford, 1999. 2. Handbook of Raman Spectroscopy: From the Research Laboratory to the Process Line; Lewis, I.R., Edwards, H.G.M., Eds.; Marcel Dekker: New York, 2001
Energy level diagrams of Rayleigh scattering, Stokes Raman scattering and anti-Stokes Raman scattering Schematic diagram of a Raman spectrometer
Raman spectroscopy is a form of vibrational spectroscopy, much like infrared (IR) spectroscopy. However, whereas IR bands arise from a change in the dipole moment of a molecule, Raman bands arise from a change in the polarizability. In many cases, transitions that are allowed in Raman are forbidden in IR, so these techniques are often complementary. This article briefly explains the Raman effect, some advantages of Raman spectroscopy, and the basics of Raman instrumentation. The Raman Effect When a beam of light is impinged upon a sample, photons are absorbed by the material and scattered. The vast majority of these scattered photons have exactly the same wavelength as the incident photons and are known as Rayleigh scatter, but a tiny portion (approximately 1 in 107) of the scattered radiation is shifted to a different wavelength. These wavelength shifted photons are called Raman scatter. Most of the Raman scattered photons are shifted to longer wavelengths (Stokes shift), but a small portion are shifted to shorter wavelengths (anti-Stokes shift). Figure 1 shows a diagram of Rayleigh scattering, Stokes Raman scattering, and anti-Stokes Raman scattering. In each case, the incident photon excites an electron into a higher “virtual” energy level (or virtual state) and then the electron decays back to a lower level, emitting a scattered photon.  In Rayleigh scattering the electron decays back to the same level from which it started. In both types of Raman scattering the electron decays to a different level than that where it started. Stokes Raman scattering occurs when the final energy level is higher than the initial level, while anti-Stokes Raman scattering occurs when the final energy level is lower than the starting level. Stokes scattering is much more common than anti-Stokes scattering because at any given time an electron in the most common temperature range is most likely to be in its lowest energy state, in accordance with the Boltzmann distribution.  As mentioned above, Raman is a form of vibrational spectroscopy. This means that these energy transitions arise from molecular vibrations. Because these vibrations involve identifiable functional groups, when the energies of these transitions are plotted as a spectrum, they can be used to identify the molecule. The Raman Spectrum A Raman spectrum is a plot of the intensity of Raman scattered radiation as a function of its frequency difference from the incident radiation (usually in units of wavenumbers, cm–1). This difference is called the Raman shift. Note that, because it is a difference value, the Raman shift is independent of the frequency of the incident radiation. Typically, only the Stokes region is used (the anti-Stokes spectrum is identical in pattern, but much less intense). Figure 2 contains Raman spectra of two different methyl chlorosilanes plotted on the same set of axes. Note that each has a characteristic set of peaks that allows it to be distinguished from the other.  Advantages of Raman Spectroscopy Raman spectroscopy is useful for chemical analysis for several reasons: it exhibits high specificity, it is compatible with aqueous systems, no special preparation of the sample is needed, and the timescale of the experiment is short. Specificity: Because Raman detects fundamental vibrations, Raman bands have a good signal-to-noise ratio and are non-overlapping. This allows a Raman spectrum to be used for everything from “fingerprinting” of samples to constructing complex chemical models of reaction processes. Analysis of aqueous systems: The IR spectrum of water is strong and relatively complex, making IR inadequate for analysis of aqueous solutions due to heavy interference by the water bands. However, the Raman spectrum of water is weak and unobtrusive, allowing good spectra to be acquired of species in aqueous solution. No sample preparation: Unlike most other chemical analysis techniques, Raman requires no special preparation of the sample. In fact, no contact with the sample is needed at all because Raman involves only illuminating a sample with a laser and collecting the scattered photons. Note especially that this makes Raman spectroscopy non-destructive. Short timescale: Because a Raman spectrum can be acquired in as little as a few seconds, Raman can be used to monitor chemical reactions in “real time.” Raman Instrumentation A typical Raman spectrometer is made up of three basic parts: the laser, the collection device, and the spectrograph. Laser: A laser is used to excite Raman spectra because it gives a coherent beam of monochromatic light. This gives sufficient intensity to produce a useful amount of Raman scatter and allows for clean spectra, free of extraneous bands. Lasers used for Raman spectroscopy must exhibit good wavelength stability, low background emission and the wavelength may be selected to minimise interference from fluorescence. Collection: The collection device collects the scattered photons. It can be a simple lens attached to a handhel instrument, a microscope for looking at small samples or a fibre-coupled probe which filters out the Rayleigh scatter,any background signal from the fiber optic cables, and sends the Raman scatter to the spectrograph. Many probes also focus and deliver the incident laser beam and the PhAT probe samples from large areas rather than a small focal spot. Spectrograph: When Raman scattered photons enter the spectrograph, they are passed through a transmission grating to separate them by wavelength and passed to a detector, which records the intensity of the Raman signal at each wavelength. This data is plotted as the Raman spectrum. References: 1. Analytical Applications of Raman Spectroscopy; Pelletier, M.J., Ed.; Blackwell: Oxford, 1999. 2. Handbook of Raman Spectroscopy: From the Research Laboratory to the Process Line; Lewis, I.R., Edwards, H.G.M., Eds.; Marcel Dekker: New York, 2001
An Introduction to Raman spectroscopy
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