Scattering and Molecular Scattering and Raman spectra

Molecular Scattering

When light is passed through the particle having molecular dimention, it gets csatterd. The phenomenon of scattering of light through such the particles having molecular dimension is called scattering. Such theory were established by Rayleigh.

When light enters into the atmosphere (i.e. gaseous medium), the wavelength of visible spectrum appears in the order of constituent of molecules of the atmosphere and hence it get scattered. Or simply the dimension of the molecules of the gas is comparable to the wavelength of light in visible range. So, molecular scattering can be easily experienced in the gaseous medium. The percentage scattering of incident light is inversely proportional to the fourth power of its wavelength. This limitations is called Rayleigh criterian for the molecular scattering.

i.e. according to Rayleigh, the percentage scattering of light \(\propto\frac{1}{(wavelength)^4}\)

examples are; blue colour of sky, red colour used in the danger single, red color of sky during sunrise and sunsine.

Sky appears blue

When light enters into the atmosphere, visible spectrum may be scattered from the atmospheric constituents having molecular dimension. According to Rayleigh, the amount of light scattered is inversely proportional to the fourth power of wavelength. That means the red color scatters least and violet colour scatters much. Therefore, the color of sky should be violet. But violet and indigo colour are less sensitive to our eyes and their mixture forms a blue colour. Therefore the sky appears blue.

Raman Effect

When monochromatic radiation is incident on the solid, it gets scattere. The scattered beam consist of not only the frequency of incident beam, but also the beam having frequency higher and lower than the frequency of incident beam. The process in which the frequency of incident beam undergoes definite change is called Raman effect.

The frequency of scattered beam remains unchanged if the collision between photons and molecules of the substance is elastic. The frequency of the scattered beam changes if the collision between the photons of incident beam and the molecules is elastic.

Let us suppose \(f_i\) and \(f_s\) be the frequency of incident beam and the scattered beam, then the change in Raman shift is $$\Delta f=f_i-f_s$$

When the frequency of the scattered beam is greater than the frequency of incident beam, the radiation is called anti-stokes radiation. In the reverse order, when the frequency of scattered beam is lower than the frequency of incident beam, the radiation is called Stokes radiation.

Stokes lines are frequently much more intense than the anti-stokes lines. Raman shift \(\Delta f\) ,lies within the range of 1000 per cm to 3000 per cm, which falls in far and near infrared regions of the spectrum.

Intensity of Transmitted wave through a Transparent Medium:

if x is the thickness of the medium, \(\mu\) is the absorbtion coefficient, then the intensity of transmitted wave is $$I=I_\circ e^{-\mu x}$$

If \(\mu_\circ\) anda \(\mu_s\) are the absorbtion and scattering coefficients of the system used, then total intensity transmitted through the system can be expressed as

$$I=I_\circ e^{-(\mu_a+\mu_s)x}$$

Raman Spectra and Fluorescence Spectra:

The Raman effect is different from fluorescence, which is the re-emission of light at various frequencies after absorption of the exciting light.

Raman spectra:

1. Spectral lines have frequencies greater or lesser than the incident frequency.
2. frequency of Raman lines are determined by incident frequency and not by the scatter.
3. Raman lines are strongly polarished.
4. Raman lines are weak in intensity due to which concentrated solution are prefered as sample to give enough intensity.

Fluorescence spectra:

1. Line frequency is always less than the incident frequency.
2. frequencies of the fluorescence lines are determined by the nature of the scatterer.
3. Lines are not polarized.
4. The intensity of fluorescence lines is considerable and samples at concentration as low as 1 part in \(10^9\) are used.

Raman Spectra and Infrared Spectra:

Raman shift falls in far and near infrared regions of the spectrum but Raman spectra is quite different from infrared spectra as pointed out below.

Ramna spectra:

1. It arisses due to scattering of light by the vibrating molecules.
2. It is the polarizibility of the molecule which determines whether the Raman spectra will be observed or not.
3. For Raman lines, beiing of weak intensity, concentrated solutions as samples are preferred. Water can be used as solvent.

Infrared Spectra:

1. It arises due to absorbtion of light by vibrating molecules.

2.The molecules must possess permanent dipole moment to exhibit spectra.

3. Generally dilute solutions are preferred and water, being opaque can not be used as solvent.

Applications

1. Raman spectroscope is used to detect using laser beams from safe distance.
2. It can be used in the determination of the degree of dissociation of strong electrolytes in aqueous solution.
3. It can be used in the chemistry , since vibrational information is specific to the chemical bonds and symmetry of molecules.
4. with this, we can have the idea about the nature of the chemical bond existing between the atoms.
5. it can be used to analyze the nano wires to better understand the composition of the structures.
6. it can be also used in the solid state physics spontaneous Raman spectroscopy is used to characterize materials, measure temperature and find the crystallographic orientation of a sample.

References:

Adhikari, P.B, Daya Nidhi Chhatkuli and Iswar Prasad Koirala. A Textbook of Physics. Vol. II. Kathmandu: Sukunda Pustak Bhawan, 2012.

Jenkins, F.A and H.E White. Fundamental of optics. New York (USA): McGraw-Hill Book Co, 1976.

wood, R.W. Physical Optics. New York (USA): Dover Publication , 1934.