Describe the experiment conducted by Sir C.V. Raman to demonstrate Raman effect. What is the use of this effect?
When a light beam is travelled through the substance, the light is deflected by the molecules of the substance. It means that the particles of the substance absorb photons of the light energy and subsequently emit light. In this process, the frequency of the absorbed and emitted light will exactly be the same. Thus the light coming out of the substance will have the exact energy as that of the incident light, therefore there is no energy loss. This is called as Raleigh scattering of light. But, Sir C. V. Raman observed a very little volume (1 in 10 million parts) of scattered light has a slightly different wavelength compared to the incident light wavelength. This change in the wavelength of light beam is called as Raman effect. It forms an important part of spectroscopy. This happens due to some absorption of energy by molecules.
Experiment:
Sir C. V. Raman performed a series of measurements where he focused sunlight on a liquid probe. He used a monochromatic filter (excitation filter) which let only light with a specific wavelength reach the probe. The measured scattered light showed a broader spectrum with additional wavelengths. A second filter (emission filter) behind the probe allowed blocking the incident wavelength. The observed residual scattered light could now be clearly distinguished from the incident light.
The observations which Sir Raman made can be explained by the fact that photons which are not absorbed by the probe will be scattered. In “UV-Vis” absorption spectroscopy, electrons in the ground state are excited to a so‑called excited electronic state. For this, the photon energy (depending on the wavelength) has to match the difference in the energy states. As a result, those absorbed wavelengths cannot be found in the transmitting light. When light is scattered, electrons are also excited from their ground state. However, the photon energy is does not have to be resonant. Molecules can be excited to a virtual energy state, see Figure 2.
Scattered light itself can be distinguished between elastic and inelastic scattering. The major part scatters elastic which means that the energy (i.e. wavelength) of the incident light is equal to the emitted light. This phenomenon is referred to as Rayleigh scattering. Only a minor part scatters inelastically where a small fraction of energy is transferred between molecule and photon. It causes changes in the polarization of the molecule which are induced by molecular vibrations. Hence energy and wavelength of incident and scattered light are not equal anymore. This effect was observed by Sir Raman in his experiments. As a result, this kind of spectroscopy is called Raman spectroscopy.
Inelastic scattering can be further distinguished between two different forms, depending on the energy state of the molecule (see Figure 2). In case one, the molecule is initially in its ground state. After excitation, the molecule falls back to a vibrational energy state above the ground state. As a result, the emitted photon has less energy than before and the scattered light will shift to a higher wavelength. This effect is called Stokes‑Raman‑scattering. The second type of inelastic scattering assumes that the molecule is already in a higher vibrational state. After excitation, the photon falls back to the molecule’s ground state. The emitted photon has a higher energy than before. The wavelength shifts to lower values. This effect is called Anti‑Stokes‑Raman scattering. Anti-Stokes-Raman scattering is mostly weaker than Stokes-Raman scattering as most molecules are initially in their ground state. Hence Stokes‑Raman scattering is mainly measured in Raman spectroscopy.
Uses of Raman effect:
Raman analysis is one of the few technique which can provide key information, easily and quickly, detailing the chemical composition and the structure of the investigated materials.Raman spectrometers detect also spin waves (magnons) in semi-magnetic crystals. As for photons, spin waves with small wave number (near zero, that means at the centrum of the Brillouin zone) are only detected.
When a light beam is travelled through the substance, the light is deflected by the molecules of the substance. It means that the particles of the substance absorb photons of the light energy and subsequently emit light. In this process, the frequency of the absorbed and emitted light will exactly be the same. Thus the light coming out of the substance will have the exact energy as that of the incident light, therefore there is no energy loss. This is called as Raleigh scattering of light. But, Sir C. V. Raman observed a very little volume (1 in 10 million parts) of scattered light has a slightly different wavelength compared to the incident light wavelength. This change in the wavelength of light beam is called as Raman effect. It forms an important part of spectroscopy. This happens due to some absorption of energy by molecules.
Experiment:
Sir C. V. Raman performed a series of measurements where he focused sunlight on a liquid probe. He used a monochromatic filter (excitation filter) which let only light with a specific wavelength reach the probe. The measured scattered light showed a broader spectrum with additional wavelengths. A second filter (emission filter) behind the probe allowed blocking the incident wavelength. The observed residual scattered light could now be clearly distinguished from the incident light.
The observations which Sir Raman made can be explained by the fact that photons which are not absorbed by the probe will be scattered. In “UV-Vis” absorption spectroscopy, electrons in the ground state are excited to a so‑called excited electronic state. For this, the photon energy (depending on the wavelength) has to match the difference in the energy states. As a result, those absorbed wavelengths cannot be found in the transmitting light. When light is scattered, electrons are also excited from their ground state. However, the photon energy is does not have to be resonant. Molecules can be excited to a virtual energy state, see Figure 2.
Scattered light itself can be distinguished between elastic and inelastic scattering. The major part scatters elastic which means that the energy (i.e. wavelength) of the incident light is equal to the emitted light. This phenomenon is referred to as Rayleigh scattering. Only a minor part scatters inelastically where a small fraction of energy is transferred between molecule and photon. It causes changes in the polarization of the molecule which are induced by molecular vibrations. Hence energy and wavelength of incident and scattered light are not equal anymore. This effect was observed by Sir Raman in his experiments. As a result, this kind of spectroscopy is called Raman spectroscopy.
Inelastic scattering can be further distinguished between two different forms, depending on the energy state of the molecule (see Figure 2). In case one, the molecule is initially in its ground state. After excitation, the molecule falls back to a vibrational energy state above the ground state. As a result, the emitted photon has less energy than before and the scattered light will shift to a higher wavelength. This effect is called Stokes‑Raman‑scattering. The second type of inelastic scattering assumes that the molecule is already in a higher vibrational state. After excitation, the photon falls back to the molecule’s ground state. The emitted photon has a higher energy than before. The wavelength shifts to lower values. This effect is called Anti‑Stokes‑Raman scattering. Anti-Stokes-Raman scattering is mostly weaker than Stokes-Raman scattering as most molecules are initially in their ground state. Hence Stokes‑Raman scattering is mainly measured in Raman spectroscopy.
Uses of Raman effect:
Raman analysis is one of the few technique which can provide key information, easily and quickly, detailing the chemical composition and the structure of the investigated materials.Raman spectrometers detect also spin waves (magnons) in semi-magnetic crystals. As for photons, spin waves with small wave number (near zero, that means at the centrum of the Brillouin zone) are only detected.
