Light can be scattered when it interacts with a molecule, causing it to emit a new photon at the same frequency, known as elastic scattering. Inelastic scattering, such as Raman scattering, occurs when the emitted photon has a lower or higher frequency. Raman scattering can be used to determine the composition of a material sample through Raman spectroscopy.
When light travels through a solid, liquid, or gas, some of the light is scattered, traveling in different directions than the incoming light. Most of the scattered light will retain its original frequency, known as elastic scattering, for example Rayleigh scattering. A small proportion of the scattered light will have a lower frequency than the incoming light and an even smaller proportion will have a higher frequency – this is known as inelastic scattering. Raman scattering is a form of inelastic scattering and is named after Chandrasekkara Venkata Raman, who received a Nobel Prize for his work on the subject in 1930.
While scattering can be thought of as light simply reflecting off small particles, the reality is more complex. When electromagnetic radiation, of which light is one type, interacts with a molecule, it can distort the shape of the molecule’s electron cloud; the extent to which this occurs is known as the polarizability of the molecule and depends on the structure of the molecule and the nature of the bonds between its atoms. Upon interaction with a luminous photon, the shape of the electron cloud can oscillate at a frequency related to that of the incoming photon. This oscillation in turn causes the molecule to emit a new photon at the same frequency, resulting in elastic or Rayleigh scattering. The extent to which Rayleigh and Raman scattering occur depends on the polarizability of the molecule.
Molecules can also vibrate, with the bond lengths between atoms periodically increasing or decreasing by 10%. If a molecule is in its lowest vibrational state, sometimes an incoming photon will push it into a higher vibrational state, losing energy in the process and causing the emitted photon to have less energy and therefore a lower frequency. Less commonly, the molecule may already be above its lowest vibrational state, in which case the incoming photon may cause it to drop back to a lower state, gaining energy which is emitted as a photon with a higher frequency.
This lower and higher frequency photon emission is the form of inelastic scattering known as Raman scattering. If the scattered light spectrum is analyzed, it will show one line at the input frequency due to Rayleigh scattering, with smaller lines at lower frequencies and even smaller lines at higher frequencies. These lowest and highest frequency lines, known as the Stokes and anti-Stokes lines respectively, occur at the same intervals from the Rayleigh line, and the overall pattern is characteristic of Raman scattering.
Because the frequency ranges in which the Stokes and anti-Stokes lines appear depend on the types of molecules light interacts with, Raman scattering can be used to determine the composition of a material sample, such as the minerals present in a piece of rock. This technique is known as Raman spectroscopy and normally employs a monochromatic laser as the light source. Particular molecules will each produce a unique pattern of Stokes and anti-Stokes lines, allowing for their identification.
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