Resonance Raman Spectroscopy

RESONANCE RAMAN SPECTROSCOPY

Overview Ø Brief introduction on Resonance Raman Scattering. Ø Advantages of RRS. Ø Why intensity of the scattered radiation is enhanced? Ø Quantum Mechanical Theory of normal Resonance Raman Scattering. Ø Theory of Resonance Raman Scattering. Ø A-term Resonance Raman Scattering. Ø B- term resonance Raman Scattering.

What is Resonance Raman Scattering? The wave number of

the exciting radiation is close or same with that of an electronic transition.

Advantages of Resonance Raman Spectroscopy. Enhancement of intensities of partticular

bands. High sensitivity…. Analytical tool for the study of chromophores. Detailed information about those excited states which raman active.

Why intensity is enhanced? i- is the initial state. f – is the final

Transition Polarisibillity tensor.

Irradianc Wave number of incident e. radiation. The above expression is the intensity of scattered radiation at 90• angle. What is transition polarizability tensor? What is irradiance? What is damping factor?

Theory of normal Raman Scattering  Adiabatic Born Oppenheimer

approximation……??

Formed by product of the pure vibrational and the pure electronic state.

ØElectronic state parametrically dependent on the normal co-ordinate of the molecule. ØElectronic transition moment also dependent on normal co-ordinate. The electronic and vibrational part of the integrals can be seperated.

When excitation far from any region of electronic absorption…..normal Raman.

The denominators in the transition polarizability

are large.

With all these approximation………..

What is the significance of each term? Rayleigh Scattering , Raman Scattering, overtone

and combination band.

Theory of Resonance Raman Scattering (1)Adiabatic Born Oppenheimer approximation is

assumed to valid.

H T perturbation description to vibronic coupling…….. 

Variation of transition moment Is related with the vatriation of Hamiltonian with normal coordinate can mix the state e

Four contribution to the transition polarizability…… What is vibronic coupling ? Is Born Oppenheimer Approx. applicable under

strong vibronic coupling? Brief discussion on each term

A-term Resonance Raman Scattering What are the conditions for A-term contribution to

RRS? What is Frank Condon factor? The technique requires the excitation within the contour of intense absorption band. Excitation with in the contour of weak band (like spin forbidden transition ) would not be expected to give significant A term.

Non-zero Vibrational overlap integral When Vibrational wave functions are non orthogonal. This condition arises when….there is either…

4) A difference in vibrational wave number i.e. change in shape of the potential surface. 6) A displacement of the potential energy minimum along normal co-ordinate. 8) This occurs only in totally symmetric modes of vibration…….why?

Orthogonality condition is not removed Produces non zero overlap integral For both totally symmetric and not

These two only applicable to the totally Symmetric mode .

B-term Resonance Raman Scattering

Conditions :

1)vibronic coupling . 2) both resonant transition and transitions are electric dipole

allowed. B-term is very much smaller than A –term

üwhen A term is more important than B-term? üWhen B term is more important than A-term?

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