Jordan University Of Science and Technology

Jordan University Of Science and Technology Faculty of science Faculty of science Department of physics Seminar title Raman scattering Presented by : Sakher Abed Al Razaq Hayajneh Superviser name : Dr. Fedda Alzoube 1st semester 2007/ 2008

. TABLE OF CONTENTS : . Introduction . Types scattering of photon . Raman scattering explanation . Application for Raman scattering . Key feature of SERS . Mechanism of SERS . Application for SERS . Conclusion . References

INTRODUCTION : 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 or nonhomogeneity, In our case, the scattering material (solid, liquid or gas). As the EM wave interacts with the matter, the electron orbits within the constituent molecules are perturbed periodically with the same frequency (no) as the electric field of the incident wave.

The oscillation or perturbation of the electron cloud results in a periodic separation of charge within the molecules, which is called an induced dipole moment . The oscillating induced dipole moment is manifest as a source of EM radiation, thereby resulting in scattered light. The majority of light scattered is emitted at the identical frequency (no) of the incident light, a process referred to as elastic scattering.

However, as explained below, additional light is scattered at different frequencies, a process referred to as inelastic scattering. Raman scattering is one such example of inelastic scattering. In summary, the above comments describe the process of light scattering as a complex interaction between the incident EM wave and the material’s molecular/atomic structure, which is useful to study the microscopic structure .

Three types photon of scattering 1 - Thomson scattering 2 - Compton scattering 3 - Raman scattering The last one will be discussed in details.

1.Thomson scattering: Describe electromagnatic radiations in a simple classical process scatter by the electron of ionized gas.where thomson cross-section …eq (1) Where ro = classical radius of electron

In Thomson scattering the radiation is not absorbed but reappears as radiations travelling in different directions .there is no change in frequency in quantum term the photon collides elastically with the electrons . In other words. The photon energy is mush less than the rest mass of the electron .

2.Compton scattering: Compton scattering is observed in x-rays passing through a solid or gas. The essential interaction is between higher photon energy and individual electrons. wether or not that electron is bond to atomic nucleus.

Under conservation of momentum and energy gives compton scattering formula …eq(2) Where λ΄ is the wavelength of the scattered photon & λ is the wavelength of incident photon • .

3 – Raman scattering :- The spentaneous raman effect was discovered by C .V Raman 1928 ,he was a distinguished indian physist who was awarded the noble prize in 1930 for his work on the scattering of light and for the discovery of raman effect . Raman scattering results from the interaction of light with the vibrational modes of molecules constituting the scattering medium raman scattering can equivalently be described as the scattering of light from optical phonons .

*Raman scattering can occur by change vibrational , rotational or electronic energy of the molecules . The difference energy between the incident photon and the raman scattered photon is equal to the energy of a vibration Of the scattering molecules and its described is inelastic scattering .

If there no change in frequency between incident and scattered photon This scattering called “Reyliegh scattering’’ • Figure 1

Raman scattering has small fraction of light such as 1 from 10^7 photon will be inelastic scatter , the event of scattering occurs in 10^-14 seconds or less .

In general the scattered light contains frequencies different from those of excitation source , those components shifted to lower frequency are called “ Stokes lines ” . • Figure 2

and those shifted to higher frequency are called “Anti stoke lines” . • Figure 3

The stockes line are typically orders of magnitude more intense than anti stockes lines because at thermal equilibrium and normal temperatures the population of electrons in level n state is smaller than the population in ground state result from Boltzman energy distribution ( N proportional to )

Figure 4

Stokes scattering result when molecules are in their ground state when it is interact with the beam of light some of energy from the colliding photon is channeled into the vibrational mode of the molecules this causes the light to absorbed and then re-emitted at lower frequency.

Figure 5

Anti stockes scattering occurs when molecules is in avibrationally excited state when it interact with the radiations the interaction can cause to drop to ground state and lose some of it is vibrational energy to the re-emitted a higher frequency light.

Figure 6

Figure 7 virtual state virtual state b . Diagram illustrate raman shift w w a b a stokes shift Antistockes shift

Raman scattering explanation : Light is treated as a electromagnatic wave and the molecules is modeled as small spheres connected by spring. The incident light can be described by following equations : E (x, t) = E0 Cos (wℓ t - k’x), …eq (3) The induced dipole is µ = α . E , …eq (4) α is the polarizability tensor , substitute in the electric field of light µ = αE0 Cos (wℓ t) , …eq (5)

The polarizability tensor depend on the conformation of the molecule, it changes as the molecule vibrate . But α = α (Q ) , …eq (6) Q is the normal vibrational coordinates . We can expand α as tayler series , α = αo + [ ∂ (α)/ ∂ (Q )](Q - 0) +……eq (7) where Q = Q0 Cos (wm t), …eq (8) Wm is the frequency for the molecule vibrate

By substitute α we calculate the induced dipole , µ = αoE0 Cos (wℓ t) + [∂ (α)/ ∂ (Q )]. Q0E0 Cos (wℓ t) .Cos (wm t) , …eq (9) µ = αoE0 Cos (wℓ t) + [∂ (α)/ ∂ (Q )] . Q0E0 [Cos ((wℓ - wm )t) + Cos ((wℓ + wm )t)] , …eq (10)

1st term { αoE0 Cos (wℓ t) } mean the incident light frequency is the same for the scattered light frequency which was mentioned ( Reyliegh scattering ) . 2nd term L.H.S [∂(α)/ ∂ (Q )]. Q0E0 [Cos ((wℓ - wm )t) , mean lower shift of frequency which was mentioned (Raman stokes shift )

2nd term R.H.S [∂ (α)/ ∂ (Q )]. Q0E0 [Cos ((wℓ + wm )t) , mean higher shift of frequency which was mentioned (Raman anti stokes shift ) . Note that ,the Raman scattering intensity proportional to [∂ (α)/ ∂ (Q) ]^2 . - The selection rule for raman scattering is ∂ (α)/ ∂ (Q) ≠ 0

Applications for raman scattering : Surface enhanced raman scattering spectroscopy (SERS). This review covers from the basic principles of raman spectroscopy to the advanced technique of surface enhanced raman scattering (SERS) spectroscopy.

SERS was accidentally discovered while people tried to do raman on the electrode in 1974 .the original idea to generate a high surface area on the roughed metal. In 1977 found the rough sliver electrode produce raman spectrum that is a million fold more intense than was expected . This enormously strong signal debuted surface enhanced raman scattering (SERS) .

Key features of SERS: - SERS occurs when molecules are brought to the surface of metal in a variety of morphologies . - large enhancement are observed from silver ,gold and copper ,the particles size for enhancement raman to happen ranges from 20 nm - 300nm. - Molecules adsorbed in the first layer on the surface show largest enhancements (large range effect 10 nanometer ) .

Figure 8

Mechanism of SERS : - several years ago few were fully convinced that SERS enhancement could be as high as 15 orders of magnitude and SERS active materials would include a variety of transition metals and probably semi-conductors .

The enhancement most probably comes from the increasing of the electric field. When the shape of particle was sharp The electric field near the sharp tip would be greatly enhanced , then if we collect two or more particles gather together the electric field could be collective resonance of free electron of the surface of the metal particles which provides great enhancement .

Application for SERS : SERS used in detection of DNA & RNA - Procedure : the gold particles were designed in special way to make use of SERS the dye CY3 was attached to the gold particles and the target was attached to the CY3 then the gold particle thrown onto the DNA chip After that add some silver particles to the solution which gather around the gold particles which give SERS effect .

Now we are in a good shape to obtain Raman signals from the target and locate it with high resolution and will get “ fingerprint ” of the DNA & RNA .

Conclusion As previous overview the important factor for raman scattering that it is more sensitive to different vibrational modes this is the reason why its called “ fingerprint of molecules” after SERS was discovered . SERS is among the most sensitive techniques available to surface science . its capability to delivering specific chemical identification and to couple this with a wide range of instruments, has led to its continuing use in both new and traditional areas of surface science . .

such industrial fields related to surface oxidation ,adhesion, corrosion and catalytic processes and in advanced materials ,biology and sensor research . Finally , SERS became a good tool in all advanced techniques . Physicists study to increase surface enhancement using nano wires technology .

REFERENCES : - MIT course 6.975 handout ,Katrin Kneipp(200) - Introductory Raman Spectroscopy,R.LFerraro,academic prees USA(1994). - Introduction to modern optics, Grant R.Fowles,2nd edition, New Work1975 - The new interfacial ubiquity of surface enhanced Raman spectroscopy, NM ,J.Weaver (2000) - Surface enhanced Raman spectroscopy ,G,C.Weaver,K,Norrod,J.Chem.75,5,621(1998) - Electromagnetic Mechanism of surface enhanced Spectroscopy ,G,C,Schatz,759-774 (2002) - M.Moskovits ,D.P.Dilella,in surface enhanced Raman Scattering, R.K.Chang new work, 243-273(1982)

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Jordan University Of Science and Technology