raman spectroscopy 2003(last years)

8/2/2019 Raman Spectroscopy 2003(Last Years)

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RAMAN SPECTROSCOPY : PRINCIPLESAND APPLICATIONS

M. K. Panigrahi

Department of Geology &Geophysics


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Electromagnetic Spectrum and Phenomena causing absorption

Nuclear spin

resonance

Electron spin

resonance

Molecular

rotation-vibration

ValencyElectron

transitions

Core electron

transitions

Nuclear

transitions

Radio frequencymicrowave

Far IRIR

Vis

ultravioletX-rays

Rays

Energy in log E (eV) from -10 to 5 and beyond

Wavelength log (m) from 4 to -11 and beyond

Frequency log (Hz) from 4 to 20 and beyond


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What is Raman Scattering?

It is inelastic

It is weak

Quantum Theory

Interaction of a photon (of energy h) with a vibratingmolecule

Elastic no change in frequency of the emitted photon

Inelastic

1) the incident photon loses energy E and theemitted/scattered photons frequency changes to - (E /h)

2) the incident photon gains energy E and theemitted/scattered photons frequency changes to + (E /h)

Which vibrating bond will have an inelastic scattering with the incidentphoton is dictated by Selection Rule can not be explained here


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Classical Theory of Raman Spectroscopy


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E

Is the size of the induced dipole andE is the magnitude of the applied

Electric Field Is the polarizability

Classical Theory of Raman Spectroscopy

If sample of such a molecule is subjected to a beam of radiation offrequency the electric field experienced by each molecule variesaccording to

0 sin 2E E t

0sin 2

vibt

0sin 2E E t

0 0

0 0 0

( sin 2 ) sin 2

1sin 2 cos 2 cos 2

2

vib

vib vib

E t E t

E t E t t

Rayleigh Stokes

0 equilibriumpolarizability abd is the rate of changeof polarizability with

vibration

Anti Stokes


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the intensity of the induced dipole moment is different in differentdirection of the bond axis with respect to the direction of theapplied electric field (higher in b where the direction of theelectric field is same as the bond axis and lower in a where theyare mutually perpendicular)


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Anti Stokes Stokes

c

0r

R

0

R

r

Incident photon is 0 interacts with vibrating molecule and isannihilated a new photon with a lower frequency r than that of theincident photon is created and the molecule undergoes a Raman

active vibrational transition Rto a higher energy level. Alternatively.


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AntiStokesE + e

0e -e

StokesE - e

Rayleigh E

Raman Shift (cm-1)


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Calculation of Stokes and anti-Stokes intensity ratio

The Raman spectrum was taken at300 K using 1064 nm Nd-YAGradiation.

Check the intensity ratio of the 1features at 278.5 cm-1.What can you say about theintensity ratio of the band at 112cm-1?


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1

3625 cm-1 2

1595 cm-1 3

3756 cm-1

a


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FAQWhat parameters control the Raman scattering and

the intensity of Raman peaks? (Raman scattering

cross section, Concentration, Grain size andorientation etc.)

Are the peak position fixed? NO So?? (Manyreasons structural state : Raman Extensiometry)

What are the problems / interferences causedduring a scattering experiment? Fluorescence,Cosmic Ray, Detector characteristics etc.)

What are the limitations? (Weak scattering

10-6

)What is Resonance Raman scattering ?

What are Raman Imaging and Mapping?


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Raman spectra of a-quartz grainsin the 400-500 cm-1 region: (A) 210-250 mm; (B) 44-53 mm;(C) 63-74 mm; (D) 0-11mm.


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Resonance Raman

In the region of an absorption band of the sample, the intensity ofcertain vibration may be enhanced by orders of magnitude.

Further additional frequencies at 2 R, 3 R and 4 R may beobserved. This is known as resonance Raman effect and thoughthis is a disadvantage, is sometimes useful for identification.

Raman shift (cm-1)

Intensity

(a

rb

units)

Res onanant

Peak at ~1000

1st Order Peak at 524

Silicon Crystal


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Decreasing degree of crystallinity

D

O

S


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Laser induced transformation of FeOOH to Fe2O3

10%, 25%, 50% and 100% Laser power

Why this happens? How much energy isgenerated at the point of incidence of Laser bean?


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Stress Measurement by Raman spectroscopy

Equation shows a positive Raman shift, >0 , indicatescompressive stress, while a negative shift,


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An example of change inRaman Shift with Pressureimplying density of pure gas(methane in this case) andgas mixture (methane andnitrogen).

Surface Enhanced Raman Scattering (SERS)


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Surface Enhanced Raman Scattering (SERS)

Strong enhancementwhen molecules areadsorbed on noble

metal (nano) particles

Strong enhancementfor molecules with lonepairs and aromatics

raman spectroscopy 2003(last years)

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