raman tutorial ppt

8/13/2019 Raman Tutorial ppt

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Raman Spectroscopy: Introductory Tutorial

Daniel T. Schwartz

Department of Chemical EngineeringBox 351750

University of WashingtonSeattle, WA 98195-1750

[email protected]

Goal of the tutorial is to answer the questions,

What is Raman spectroscopy and can the new

Raman microscope at NTUF help with my sample?


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Raman Spectroscopy: Some Sources

General Principles and Instrumentation:

Principles of Instrumental Analysis, by Douglas A. Skoog, F. James Holler, Timothy A. Nieman

Inorganic:

Infrared and Raman Spectra of Inorganic and Coordination Compounds : Theory and Applications

in Inorganic Chemistry (Volume A) by Kazuo Nakamoto

Infrared and Raman Spectra of Inorganic and Coordination Compounds : Applications in Coordination,

Organometallic, and Bioinorganic Chemistry (Volume B) by Kazuo Nakamoto

Organic:The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules

by Daimay Lin-Vien, et al


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Raman Spectroscopy: Overview

A vibrational spectroscopy

- IR and Raman are the most common vibrational spectroscopies for

assessing molecular motion and fingerprinting species

- Based on inelasticscattering of a monochromatic excitation source

- Routine energy range: 200 - 4000 cm1

Complementary selection rules to IR spectroscopy

- Selection rules dictate which molecular vibrations are probed

- Some vibrational modes are both IR and Raman active

Great for many real-world samples

- Minimal sample preparation (gas, liquid, solid)

- Compatible with wet samples and normal ambient

- Achilles Heal is sample fluorescence


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Raman Spectroscopy: General

IR and Raman are both useful for Fingerprinting

Symmetry dictates which are active in Raman and IR


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Group assignments identify characteristic vibrational energy

Raman Spectroscopy: General


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Raman Spectroscopy: Classical Treatment

Number of peaks related to degrees of freedom

DoF = 3N - 6 (bent) or 3N - 5 (linear) for N atoms

Energy related to harmonic oscillator

Selection rules related to symmetryRule of thumb: symmetric=Raman active, asymmetric=IR active

Raman: 1335 cm1

IR: 2349 cm1

IR: 667 cm1

CO2

or c

2

k(m1m2)

m1m2

Raman + IR: 3657 cm1

Raman + IR: 3756 cm1

Raman+ IR: 1594 cm1

H2O


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ElectronicGround State

1st ElectronicExcited State

ExcitationE

nergy,(

cm

1)

Vib.

states

4,000

25,000

0IR

2nd ElectronicExcited State

emit

fluorescence

Impurit

y

emit

fluorescence

UV/Vis

Fluorescence

emit

ElasticScattering(Raleigh)

Main Optical Transitions: Absorption, Scattering, and Fluorescence


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ExcitationE

nergy,(

cm

1)

Vib.

states

4,000

25,000

0 IR

emit


Raman

=emit

Resonance Raman

=emit

Raman Spectroscopy: Absorption, Scattering, and Fluorescence

Stokes Anti-Stokes


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Excitation Energy, (cm1)

Raman Spectroscopy: At NTUF, you pick the Laser Excitation

Intensity

11,000 13,000 15,000 17,000 19,000 21,000

Near IR

785 nm

Visible

514 nm

+ +

Stokes Anti-StokesStokes Anti-Stokes


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ExcitationE

nergy,(

cm

1)

Vib.

states

4,000

25,000

0

fluorescence

IR

emit


Raman

=emit-

fluorescence

Impurit

y

Fluorescence

= Trouble

Raman Spectroscopy: Absorption, Scattering, and Fluorescence

Stokes Anti-Stokes


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1000 2000 3000

Raman Shift (cm-1)

Ra

manIntensity

Without Bleaching

After 2 hours Bleaching

Poly (diallyl phthalate)

ex= 514.5 nm

Raman Spectroscopy: Coping w/ Fluorescence

1. Use NTUF 785 nm laser line (excites many fewer fluorophores)

2. Photobleach with long exposure laser irradiation.


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Raman Spectroscopy: Summary

1. Raman is a vibrational spectroscopy akin to IR

- Good for fingerprinting, probing molecular symmetry

2. Scattering-based, not transmission/reflection- Means no need for fancy sample preparationgas, liquid, or solid

- Virtually always use anti-Stokes lines due to stronger signal

3. You need to pick excitation energy (laser line)- 785 nm: Fluorescence less probable; Lower Raman signal

- 514 nm: Fluorescence more probable; Resonance more likely; Higher signal

4. Other things not talked about- SERS: Surface Enhanced Raman Spectroscopy

- Quantum origins of selection rules and scattering cross-section


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Raman Spectroscopy: Dans trip to NTUF

Fuel Cell Materials --- picked because I had little Raman experience

with these materials

From http://www.fueleconomy.gov/feg/fc_pics/fuel_cell_still.gif


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Flow Field Plate - Graphite

From early literature on graphitic materialsTuinstra and Koenig, J. Chem Phys. 53, 1126 (1970).

() 44 IgId

175

IgId

3.98

Nanocrystalline graphite has graphitic (g)

and disorder (d) peaks. The characteristic

dimension of graphitic domains is given by:


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Gas Diffusion Layers

(graphite paper)

() 76

() 161


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Gas Diffusion Layers

(Woven Fibers)

()

55


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Nafion

Fluorescence problems!

Maybe try photobleaching

as next option.

raman tutorial ppt

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