PDF Padlet 6 Raman Spectroscopy Sept2014

8/18/2019 PDF Padlet 6 Raman Spectroscopy Sept2014

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Raman SpectroscopyChapter 18, pp 481


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Introduction

• C.V.Raman, Indian Physicist, 1928

• visible l of a small fraction of radiationscattered by certain molecules

– differ from the incident beam

• shifts in l depend on chemical structure ofthe molecules responsible for the scattering


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Introduction

• Raman spectroscopy –

– measurement of the wavelength and intensity of

inelastically scattered light from molecules.

• Raman scattered light occurs

– at wavelengths that are shifted from the incident

light by the energies of molecular vibrations.


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Introduction

•

the mechanism of Raman scattering is different fromthat of infrared absorption.

• Raman and IR spectra provide complementary

information.

• Typical applications are in structure determination,

multicomponent qualitative analysis, and quantitativeanalysis.


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Introduction

• compliments IR techniques

•

Advantages – water is a useful solvent

– signals usually in the visible or near IR region, can

use glass or quartz cells

• avoid working with NaCl or other atmospherically

unstable window materials


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Theory

• Raman spectra – irradiating a sample with a powerful laser source of visible or

near IR monochromatic radiation

• Irradiation (process) – the spectrum of the scattered radiation is measured at some

angle (often 90o) with a suitable spectrometer (figure 18-6,pp487)

• To avoid fluorescence – the excitation ls are usually well removed from an absorption

band of the analyte


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Inelastic Scattering

sample

Incident radiation

Po

scattered radiation

Ps

Figure 6-18, pp 149


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• Incident radiation of frequency, ex , impinges on thesample

– sample molecules are excited from one of their ground

vibrational states to a higher so-called virtual state (dashedline)

• When the molecule relaxes, it may return to the 1st

vibrational state & emit a photon of energy

E = h ( ex - v)

where v is the frequency of the vibrational transition


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• Alternatively, if the molecule is in the 1st excited

vibrational state, it may absorb a quantum of the

incident radiation, be excited to the virtual state, relaxback to the ground vibrational state.

E = h (ex + v)

In both cases emitted radiation differs in frequency from the

incident radiation by the vibrational frequency of the

molecule v

.


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• the intensities of the stokes and anti stokes peak give

quantitative information.

• the positions of the peaks give qualitative information

about the sample molecule.


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When scattered radiation is of lower frequency than the

excitation radiation – Stokes scattering

Scattered radiation of a higher frequency than the source

radiation – anti-Stokes scattering


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•

Elastic scattering can also occur with emission of a photonof the same energy as the excitation photon, hex

• Scattering radiation of the same frequency as the source –

Rayleigh scattering

Raman spectrum

of CCl4 excited by

laser radiation of

ex = 488 nm


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• Raman spectra, abscissa, wavenumber shift,

difference in wavenumbers (cm-1) between the

observed radiation & that of the source

definition

• Raman scattering require there be a change in

polarizability during vibration

polarizability – is a measure of deformability of the

bond in an electric filed


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• Magnitude of Raman shifts is independent of the l of theexcitation

• Raman shifts for CCl4 is identical regardless excitation withargon-ion laser (488.0 nm) or helium-neon laser (632 nm)

• Note: ratio of anti-stokes to stokes intensities increases

with temperature b’coz larger fraction of the molecules is inthe 1st vibrational excited state under these circumstances

Raman spectrum of CCl4

excited by laser radiation of

ex = 488 nm


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nstrumentation


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Instrumentation

•

Modern Raman spectroscopy – laser source

– sample illumination system

– suitable spectrometer

S

http://research.pbsci.ucsc.edu/chemistry/li/teaching/chem268/Spectroscopic%20techniques.pdf


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Source: http://research.pbsci.ucsc.edu/chemistry/li/teaching/chem268/Spectroscopic%20techniques.pdf

http://research.pbsci.ucsc.edu/chemistry/li/teaching/chem268/Spectroscopic%20techniques.pdfhttp://research.pbsci.ucsc.edu/chemistry/li/teaching/chem268/Spectroscopic%20techniques.pdf


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Instrumentation: Sources

• Sources – nearly always lasers

• High intensity necessary to produce Ramanscattering of sufficient intensity to be measured witha reasonable signal S/N ratio


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Instrumentation: Sample-illumination System

• Sample handling for Raman Spectroscopymeasurement is simpler than IR

•

Can use glass for windows, lenses and other opticalcomponents

• Common sample holder for non-absorbing liquid

samples – ordinary glass-melting-point capillary


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Optical diagram of an FT-Raman instrument

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PDF Padlet 6 Raman Spectroscopy Sept2014