PDF Padlet 6 Raman Spectroscopy Sept2014
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Transcript of 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