Raman spectroscopy relies upon the change in polarization of a molecular bond that occurs when photons interact with the bond. The incoming photon interacts with the molecule and induces a dipole moment that in turn radiates a photon when the dipole moment it decays. The vast majority of these emissions are elastic. Elastic scattering, also known as Rayleigh scattering, accounts for the vast majority of scattered photons. In a small portion (about 1 in 107-108) of the scattered photons, the photon will have a shift in wavelength due to inelastic collisions called Raman scattering. Most of these energy-shifted photons have longer wavelengths than the incident radiation (known as a Stokes shift) but a small portion will have shorter wavelengths (known as an anti-Stokes shift).
Because Raman scattering constitutes such a small fraction of the scattered light, it produces a relatively weak signal. Raman Spectroscopy usually measures the Stokes shifted photons since they constitute the strongest signal. Although it is possible to measure the much weaker anti-Stokes shifted photons, the signal is almost always too weak for practical environmental monitoring applications. A typical Raman spectroscopy system uses a laser capable of producing intense, monochromatic light to generate a "packet" of scattered light that can be collected, dispersed through a monochromator, and processed to produce a spectrum. Like infrared spectra, Raman spectra are unique to a given compound and hence can be used to "fingerprint" or uniquely identify as well as quantify chemicals on a surface, in a liquid, or in air. Unlike infrared spectroscopy, the Raman technique is not affected by chemical species such as water, water vapor, and carbon dioxide that can "swamp out" infrared systems. Fluorescent molecules that are often present in the environment can interfere with Raman spectroscopy. There are, however, methods that will overcome fluorescent interference while maintaining a strong Raman signal.
Because Raman scattering produces a very weak signal, the source must be very intense and monochromatic, which invariably requires a laser. Several types of lasers have been demonstrated for Raman use outside of a laboratory setting. Choosing the best laser depends on the application (distance to target compounds and media they are found in) and wavelength requirements for the target compounds. Note that the intensity of the Raman scatter is strongly affected by the wavelength of the incident light. The longer the wavelength, the less intense the scattering; therefore, blue and ultraviolet light produces the strongest Raman scattering. Shorter wavelengths translate to higher detection sensitivities.
Because lasers pose optical hazards to both operators and bystanders, OSHA safety standards for laser operation should be observed for safe operation. Laser operation requires significant training and should not be relegated to novice users.
Optics and Filters
Open path systems direct an appropriately chosen laser beam along the axis of a receiving telescope. The laser beam generally passes through a beam expander to reduce the possibility for eye damage. The scattered light is collected by a telescope or other focusing device and directed onto a filter. The filter rejects reflected stray laser light and the Rayleigh scatter leaving the Raman scatter to pass through a monochromator that separates the scattered light into individual wavelengths before directing it to a detection device. In laboratory settings double and triple monochromators are commonly used for Raman Spectroscopy. These systems reject the stray light and separate the scattered light with sufficient efficiencies to allow effective measurement of Raman spectra without using a filter.
Two types of filters are commonly used in Raman spectroscopy. The edge filter functions by blocking all the radiation below or above a specified wavelength and letting the remaining light through. The notch filter blocks the wavelength of incident radiation and a small area to either side therefore allowing the remaining light to pass through. Examples of notch filters include monochromator, absorption filters, dielectric filters, and holographic filters. Because of their wavelength specificity, cost, and ease of use, the latter two find most of their use in industrial process control where a known environment is constantly queried.
A spectrograph separates the incoming light into its wavelength components and directs these onto a detector to quantify the amount of light at each wavelength. Several spectrograph designs are available, and each has strengths and weaknesses. Features to consider when selecting a design are the resolution required, the wavelength range(s) that needs to be captured, stray light treatment, and light throughput capabilities. A typical design has collimating optics that direct the incoming light onto a grating or prism, which separates it into component wavelengths. The wavelengths are reflected to a focusing mirror or other optic that directs them onto a detector. Other more-or-less complicated designs are possible.
Raman spectroscopy should have a detector with a wide dynamic range, all noise sources at levels below the shot noise, a wide wavelength range, and high quantum efficiency. Older devices used a photomultiplier tube. While these can still be used, they have been largely replaced by diode arrays and silicon charge-coupled-device (CCD) arrays. CCDs are the preferred arrays because of their ability to measure many wavelengths at once, and because they have a large wavelength range (400-1000 nm), large dynamic range, high quantum efficiency, low read noise, and low dark noise (Lewis and Edwards 2001). In order to enhance the sensitivity of this technology, an intensified charge-coupled-device (ICCD) can also be used. The intensifier, as the name implies, directly enhances the signal from any incoming light, which is important considering that the Raman signal is so weak. This device can also be "gated," which means that it can be automatically turned on and off to measure light scattered by the incident laser pulse. It also rejects background light and collects more of the Raman-scattered light, therefore enhancing its signal.
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