Understanding Raman Effect: Spectrum Analysis and Quantum Theory

1. Presentation on Raman Effect:Presented By:Dr. J.P. GuptaDepartment of PhysicsGovt. (Auto.)P. G. College Satna(MP)

2. Raman EffectWhen a monochromatic radiation or radiation of very narrow frequency band is scattered bhy a solid then scattered lightn not only cosists of the radiations of incident frequency but also the radiations of frequencies above and below that pf incident beam frequency . This form of scattering in which the frequency pf incident beam undergoes a definite change was obsderved and studied by Raman is 1928 and is called Raman effect.If Vi is the frequency of the incident radiation and Vs is that of the light scattered by a given molecular species , then the Raman shift is defined by

3. V = Vi-VsThis difference is the characteristic of the substance providing the scattering and does not depend on the frequency of the light employed when. (I) V is positive Vs <Vi Raman spectrum is said to consist of stockes lines. (II) V is negative Vs >Vi Raman spectrum is said to consist of anti-stock line.

4. Stakes lines are frequently much more intense than the anti stoke lines.Raman shift V generally lines within the range of 100cm-1 to 3000cm-1 which falls in far and near infra red region of the spectrum . this leads to conclude that the change in energy of the scattered light in Raman effect correspond to the energy change accompanying rotation and irrational transition in a molecule.

5. Classical theory of Raman EffectThe spectrum of scattered light consist of line of the same frequency as the incident beam (Rayleigh lines)additional weak lines of changed frequency lines on low frequency side of exiling line are called stokes lines while those on the high frequency side are called anti stock lines.Under high resolution equidistant line are observed this is pure rational Raman spectrum . the separation between these lines is nearly twice the spectrum between rotation lines in the far infra red spectrum of the molecule.Under high resolution stoke and anti stoke lines are found to be composed of rational line structure. They constipate the vibration rational Raman spectrum of molecule .

6. QUANTUM Theory of Raman effect :- Inscattering due to the absorption of the incident radiation by scqttering molecules. They are raised to a higher energy state. Now if they return to their original state, then frequney of radiation cmitted is same as that of incident light :

7. but it they return to a higher or lower vibrational or rotational level. The frequency of scattered radiagion is less or greater than that of the incident radiation the amount of this difference is equivalent to the difference in vibrational or rotational energy states. Let us consider a molecure in its intialclower state E“ and is exposed to incident radiation of frequency vi cm. The absorption of this incident radiation could rais this molecure to a level in which its energy is CE” th CVI ). Now suppose it return to a level of energy E” lying above the level E” by losing energy hcVs and emitting (scattered) radiation having absorbed frequency vs. It pillows then

8. E”+hcvi-hcvs= E’E”-E”= hc(vi-Vs)E’-E”= hce vi

9. Which shows as said earlier that Raman shift or is equal to the difference in energy of the two levele represented by E’ and E” . It is obvious that sign of depends upon (E’-E”). If E’ > E” or in other words. If the molecule is inilially in lower state when it obsorbs energy of incident light stokes lines of Raman spectrum (for which should be positive) are produced but if the molecule is intialy in upper state E’ and then retans returns to the lower state E” after emission of scattered Raman radiation then or is negative and hence Raman spectrum will consist of anti stokes lines.

10. The transition are ellustrared in fig. It should be noted that frequency shifts of Raman lines their intensity and polarization are characterstics of the scattering substance classically. Stokes and on antistokes line should have equal intensity. However experiment can Quantum mechanically it is because such transitions are more problem andhence more intense .Furtular the dotted leven shown in the energy level diagram is not a stationary level.It is a virtual level . Since energy is not preserved in a virtual level the photonimmediately returns to lower stationary level after emilting a photon fluorecence effect basically differs form Raman effect .