Raman Effect The Raman Effect is the phnomenon of scattering of light, one of the most convincing proofs of the quantum theory Was discovered in 1928 Raman was awarded the Nobel Prize in Physics in 1930

Theory of Raman effect If light of a definite frequency is passed through any substance in gaseous, liquid or solid state, the light scattered at right angles contains radiations not only of the original frequency (Rayleigh Scattering) but also of some other frequencies which are lower or higher than the frequency of the incident light.

Radiation scattered by molecules contains photons with the same frequency as the incident monochromatic radiation (laser) (orange in the diagram), but also contains photons with shifted frequency (red in the diagram), the Raman radiation. The spectrum of this wavelength-shifted light is called the Raman spectrum. Conceptualization of Raman effect Conceptualization of Raman effect

The violet light of the solar spectrum is isolated with a violet filter and passed through the liquid sample. Most of the light emerging from the liquid sample is the same color as the incident violet beam: the so-called Rayleigh scattered light. However, Raman was able to show that some of the scattered light was a different color, which they could isolate by using a green filter placed between the observer and the sample. Demonstration of Raman Effect

When a monochromatic light is scattered by matter, two types of interaction take place and result into two distinctive types of scattered light Rayleigh scattering Raman scattering The scattering of light may be thought of as the redirection of light that takes place when an electromagnetic (EM) wave (i.e. an incident light ray) encounters an obstacle (solid,liquid or gas)

Rayleigh scattering is elastic scattering where there is no energy exchange between the incident light and the molecule. This scattering is responsible for the blue color of the sky; it increases with the fourth power of the frequency and is more effective at short wavelengths. The second type of interaction involves energy exchange between the incident photon and material's molecules.Hence,the scattered photon will have a new frequency, or energy, which is simply equal to the sum or the difference between the frequencies of the incident photon and the natural frequency of the thermally excited and kinetically active species in the material. This type of scattering is inelastic in nature and is referred to as Raman scattering.

W hen a sample is irradiated with a strong monochromatic light source( usually a laser). Most of the radiation will scatter “off” the sample at the same wavelength as that of the incoming laser radiation, a process known as Rayleigh scattering. However, a small amount will scatter from that sample at a wavelength shifted from the original laser wavelength. Hence we get modified lines or Raman lines The molecules of the sample take up energy from or give up energy to the photons, which are thereby scattered with diminished or increased energy, hence with lower or higher frequency. This results in the emission of lights having higher or lower frequency than incident radiation. The energies corresponding to the Raman frequency shifts are found to be the energies associated with transitions between different rotational or vibrational states of the scattering molecule.(contd) What are Raman lines? How Raman lines are formed ?

If the vibrational energy of the molecule is increased after the collision, the energy of the scattered photons is decreased for the same amount and, therefore, can be detected at longer wavelengths. The respective spectral lines are called Stoke’s lines. If the vibrational energy of the molecule is decreased after the collision, the energy of the scattered photons is increased for the same amount and, therefore, can be detected at shorter wavelengths.The respective spectral lines are called anti-Stoke’s lines. Stokes and Anti-stoke lines

Level transistions in Raman Scattering

The scattering liquid is taken in a horn like glass tube called Raman tube. One end of the tube is closed with an optically plane glass plate constituting a window W, for scattered light to emerge. The other end of the tube is drawn out to the shape of a horn (H) and its outside is blackened to provide a contrasting background, suitable for observation. The liquid is illuminated by mercury arc lamp S. R is a metal reflector to increase the intensity of illumination further. To prevent overheating, the tube is surrounded by a jacket J through which cold water circulates.F is a filter and it filters and permits only highly monochromatic light. The convex lens L is arranged in front of window focuses and directs the scattered radiation upon the slit of spectroscope. The light is scattered in a transverse direction and is observed through a spectroscope Experimental arrangement for Raman Spectrum

Raman SpectrumFluorescence Spectrum Spectral lines have frequencies greater than and less than incident frequency Frequency of the lines always less than the incident frequency Raman lines are strongly polarized Lines are not polarized Frequency shift of the Raman lines are characteristics of the scattering substance Frequency shift are determined by nature of the substance Comparison of Raman spectrum & Fluorescence Spectrum

Molecular structure analysis Crystallography Nanotechnology Life sciences Semiconductor fabrication Polymer/physical /organic chemistry Pharmaceuticals and Cosmetics Geology and Mineralogy Applications of Raman effect –an overview