1

Infrared and Raman Spectroscopy

Lecture 2Lecture 2

Historical Background

Sir William Hershel Sir Chandrasekhara Venkata Raman

In 1800 Sir William Hershel found that a thermometer placed in the region beyond theIn 1800 Sir William Hershel found that a thermometer placed in the region beyond the red end of the solar spectrum was heated even more than when placed in the visible position. Around 1900 infrared (IR) absorption investigations of molecules began.

In 1928, Venkata Raman discovered that the scattered radiation contained photons not only of the same frequency as incident light, but also a very small number of photons with changed or shifted frequency (1 photon out of a million).

Energy UnitsPlank constant h = 4.1357 · 10-15 [eV·s]:

Speed of light c = 2.9979 · 108 [m·s-1]:

W b [ 1]1 νν

Wavelength [m]:cλν

=

Frequency [Hz]: ν

Wavenumber [cm-1]: c

νλ

= =

Energy [eV]: cE hv h hcνλ

= = =

Energy levels of a diatomic molecule

2

Raman and Infrared:

Raman andRaman and Infrared:

ΔE = 0.01 – 1 eV

Raman and Infrared

Raman and Infrared:Infrared:

ΔE = 0.01 – 1 eV

ΔE = 0.01 – 1 eV

Raman and Infrared:

ΔE = 0.01 – 1 eV

Transitions between

vibrational levelslevels

(Change of configuration)

Theory of Molecular Vibrations (Classical Description)

Harmonic oscillator. Harmonic means that its motion is governed by a restoring force directly proportional andgoverned by a restoring force directly proportional and opposite in sign to displacement of the body from equilibrium position.

kxF −= Hooke’s law. k – is stiffness of the spring

2

2

dtxdmF = Newton’s law. m - is mass of the body

dt

kxdt

xdm −=2

2

02

2

=+ kxdt

xdm Differential equation of motion of harmonic oscillator

02

2

=+ kxdt

xdm ⎟⎟⎠

⎞⎜⎜⎝

⎛+= ϕt

mkAx cos

mk

πν

21

=

3

Theory of Molecular Vibrations (Selection Rules)

Active in IR Active in Raman

To be visible in IR To be visible in

IR – spectroscopy of functional groups and polar bonds: C=O, O-H etc.Raman – spectroscopy of carbon allotropes and polarizable bonds: C-C, C=C etc.

vibration must change the dipole moment of the molecule

Raman vibration must change the polarizability of

the molecule

p py p p ,

IR and Raman are both vibration spectroscopy techniques.

There is only one single common source of information which can be obtained by both techniques. This source of information is vibrations of atoms in molecules.

Example (CO2) of the Difference Between Raman and IR Spectroscopyp py

Symmetric

C

Asymmetric

O CO O O

In the symmetric mode, there is no change in dipole moment therefore IR inactive whiledipole moment, therefore, IR inactive, while polarizability fluctuates, Raman active.In the asymmetric, change in dipole moment, thus IR active, but polarizability remains the same, Raman inactive.

Raman vs IR spectra Raman Versus IRRaman IR

• IR spectroscopy isABSORPTION spectroscopy

• Raman spectroscopy isSCATTERING spectroscopy ABSORPTION spectroscopy

•Requires the vibrational mode ofthe molecule to have a change inthe dipole moment or chargedistribution associated with it.Only then, radiation of samefrequency interact with the

SC NG spect oscopy

• Scattering involves amomentary distortion of theelectrons distributed around amolecular bond. Thus, themolecule is temporarilypolarized i e a momentarily frequency interact with the

molecule, and promote it to theexcited state.

• Sample preparation is requiredin most cases.

polarized, i.e., a momentarilyinduced dipole that disappearsupon relaxation and reemission.

• For example, H2 and N2 can bedetected.

4

IR and Raman Experiments Schematics

IR sourceSample DetectorIR source

Laser

Detector

Sample

Absorption of Light by Matter (Lambert-Beer Law)

lCIT ε−101 lC

IT ε−== 10

0

1

T - is transmission (or transmissivity) of light through a substance, ε - is molar absorptivity of the absorber, C – is concentration of absorbing species in the material

lCIA ε=⎟⎟⎞

⎜⎜⎛

−= 1logCalibration Curve

lCI

A ε=⎟⎟⎠

⎜⎜⎝

−=0

10log

A - is absorption. A is a linear function of C

C

A

In wide concentration range Lambert-Beer law is not always valid

Scattered Light IntensityVibrational Transition m to n

24 )()( ∑±∝ ENI ανν,

0 )()( ∑±∝←ji

jmnimnmn ENI ανν

0ν Incident light frequency

N Number of molecules

mnν

mn)( ρσαE

vibration frequency

Polarizability

Electric field strength

Spectroscopic Transitions in a Diatomic Molecule

5

Schematic of a Raman Experiment

How to Measure the Raman Shift?

Wavenumber Δν shift is defined as the difference inb ( 1) b t th b d di ti d th twavenumbers (cm-1) between the observed radiation and that

of the source.

Δν Raman shift = ν Laser - ν Scattered

Notice That:Shifts in wavelength depend on the chemical structure of theShifts in wavelength depend on the chemical structure of the molecule responsible for the scatteringAnti-Stokes lines are less intense than Stokes lines, as a result, the Stokes part is generally usedThe intensities of the Rayleigh and Raman scattered light are proportional to the number of molecules being excited.

Raman Spectrum Using Different Excitation Wavelength to Eliminate Fluorescence

6

FluorescenceFluorescence is the emission of a photon by a molecule following its excitation by an electromagnetic radiation of a precise wavelength.

This excitation induces an energy jump from a fundamental electronic state S0 to an upper electronic energy level S1. The molecule comes back to its fundamental energy level while a photon is emitted. The photon energy corresponds to the energy difference between the two levels S0 and S1. Some small energy losses are due to internal conversion among the different vibrational and rotational levels of each electronic state of the molecule.

That's why E1 (excitation energy) is always greater than E2 (emission energy).

1 2 1 21 2

and , so c cE h E h λ λλ λ

= ⋅ = ⋅ <

Fluorescence is common in many organic materials, but also in impure inorganic materials. Fluorescence does no affect the Raman effect. However, since the fluorescence spectrum is much stronger and broader than the Raman spectrum, the Raman spectrum may disappear under the fluorescence background.

Fluorescence vs. Raman

60

70

Fl

20

30

40

50

Inte

nsity

Fluorescence

Raman shift

400 300

0

10

20

Wavelength

lase

r

lase

r

lase

r

Raman Spectra of Crystalline Solids: Relation to Structure

Χ

Brillouin zone (BZ) TA si

on

Γ

u.)

GaAs

correspond-ing to the

FCC unit cell

with high-symmetry

points in k-space

LO

TO

LA

GaAs rum

Phon

on d

ispe

rs

0 50 100 150 200 250 300 350 Wavenumber (cm-1)

Inte

nsity

(a.GaAs

unit cell(zincblende structure –

FCC Bravaislattice with2 atoms in the basis)

LOTO

GaAs

Ram

an s

pect

rRaman SpectrometersRaman spectrometers basically employ one of two technologies for the collection gof spectra1- Dispersive Raman2- Fourier Transform Raman (FT- Raman)Each technique has unique advantages q q gand each is ideally suited to specific analyses but dispersive Raman dominates the market

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Dispersive Raman Spectroscopy

To separate the collected Raman scattered light into individualwavelengths, the Raman signal is focused onto a grating that separatesthe light into the different frequencies. Then, the dispersed beam isdirected to the CCD (charged-coupled devices) detector to be collected.

Wavelength Dispersion by Diffraction Grating

sind mθ λ=Diffraction angles:

FT- Raman (Nondispersive Raman)Consists of an excitation laser (longer wavelength), an interferometer and high sensitivity near IR detector.

Interferometer produces interferogram which encodes the unique frequencies of the Raman scattering into a single signal.

How does FT-Raman work?

Vibrational spectra are presented as frequency spectra, which are thendecoded using Fourier transformation mathematical techniques, andfinally the desired spectral information is presented.

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Dispersive vs. FT-Raman Spectrometers

F t Di i (FT R )Feature Dispersive (FT-Raman)

Available Wavelength <200nm to 850nm 1064nm

FluorescenceMore fluorescence

(Except UV)Better fluorescence

avoidance

Detector CCD Ge or GaAs

Best Spectral Resolution Typically 1-4 cm-1 ~0.5cm-1

Couple Raman Spectroscopy with a Microscope

Why?Why?-Allows analysis of very small samples

-Distinguish the substance of interest from its surrounding.

How does it work?By focusing the laser beam onto the sample, and passing the returning beam into the system for analysis and detection.

Renishaw 1000 Raman Microspectrometer

D B C

A

D B C

F

E

A – Microscope objectives & Motorized XYZ stageB – Spectrograph entrance slit assemblyC – Diffraction grating assembly E – Beam expanderD – Holographic notch filter F – CCD detector

Raman Spectroscopy in Materials Research

CorrosionCorrosion• Analysis of corrosion products and anticorrosioncoatings, as well as the identification of micron-sizedcontaminants within the coatings can be measured.

• In-situ electrochemical studies concerned withcorrosion product formation are now possible.

• Measure the composition of corrosion products overa large surface with a 1μm sample analysis area canbe done

9

F OOH ( k b 245 300 390 420

Raman Bands of the Main Corrosion Products of Steel

• α-FeOOH (peaks at about 245, 300,390, 420, 480, 550 and 685 cm-1)

• γ- FeOOH (peaks at about 250 and 380 cm-1)• Fe3O4 (peaks at about 540 and 665 cm-1)• γ-Fe O (peaks at about 265 300 345 395 515• γ-Fe2O3 (peaks at about 265, 300, 345, 395, 515,

645, 670, 715 and 1440 cm-1)• α-Fe2O3 (peaks at about 225, 245, 295, 415,

500, 615, and 1320 cm-1)

1377

γ-Fe2O3

Raman Analysis of Corrosion Products at SCC (Stress Corrosion Cracking)

345

680

1345

385

496

700

800

900

Crack

600

500 1000

Raman Shift (cm-1)Polished cross-section, tip of an

SCC crackZhang, Gogotsi, Chudnovsky, Teitsma 1998

Polymers

Conducting Polymers

Polyaniline sample obtained from Dr. Ko

Many sharp bands are seen in the Raman spectrum of collagen

U. Kentucky

10

Carbon Allotropes

http://cnst.rice.edu/images/allotropes.jpg

1332

Large Synthetic Diamond and Its Raman Spectrum

400

600

800

1000

nten

sity

(a.u

.)

1000 1200 1400 16000

200

Wavenumber (cm-1)

In

mm

Raman Analysis of Carbon• Plane displacement vibration that occur at high frequency(1582 cm-1)

a) Highly oriented pyrolytic graphite

• Finite crystals size effects(nanocrystals) and lattice defects (incarbon fibers) introduce a break in thetranslational symmetry as in disordercarbon.

• Disorder gives rise to the disorderinduced (D) line with a peak near

M.Pelletier 1999

a) Highly oriented pyrolytic graphite (No D-band) at 1582 cm-1

b) Activated Charcoal (D and G bands at 1360, 1580cm-1)

c) Amorphous Carbon (a very broad peak)

( ) p1360 cm-1.

• The ratio between disorder induces(D) and Raman allowed (G) is ameasure of disorder in carbon.

Disordered CarbonsYou can measure the carbon disorder by varying the heat treatment temperature THT of vapor grown carbon fibers.

As THT ,D band intensitySpectra taken with 488nm for a Benzene derived carbon fibers treated with various temperatures

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Graphite Intercalation CompoundsGraphite intercalation compounds are formed by the insertion of atomic or molecular layers of guest into Graphite.

R.L. McCreery 2000

• GICs show high degree of ordering.

• Ordering depends on periodic arrangement of n graphite layers between sequential intercalate layers.

Spectra of Various Graphite Structures

First Order Spectra Second Order Spectra

Bamboo Filament

Hydrothermal Nano-Pipe

Hydrothermal Micro-Pipe

Hydrothermal GraphiteGraphite

Highly Oriented Natural Graphite

Libera, Gogotsi Carbon1307-1318 (2001)

Raman Spectra of Disordered Carbon

800900

1000

D1350

G

D G

0100200300400500600700

0 500 1000 1500 2000

Inte

nsity

(arb

itrar

y)

G1595

Ar+, 514.5 nm

UV, 325 nm

0 500 1000 1500 2000

Raman Shift (cm-1)

bUV, 244 nm

FullerenesDepending on their symmetry, they have different shapes

(a) Icosahedral soccer ball C60 (b) Rugby ball C70 (D5h)

(c) Extended Rugby ball C80 (D5d) (d) Truncated icosahedron (Ih)

12

Raman Spectroscopy of Fullerene

70

80

15691427

1245

1103

771

709

495

431

271

as received

30

40

50

60

70

400°C

1191

1608

1335

as received

tens

ity (a

rb. u

nits

)

1465

400 800 1200 1600 2000

10

20

500°C

800 °C

Int

Raman Shift (cm-1)

600 °C

Gogotsi, J.Mater. Res vol. 14(2) 1999

Normal Modes of Carbon Nanotubes

• A1g mode at 1587cm-1 allA atoms are moving in theA atoms are moving in theopposite directions to thenearest neighbor Batoms.

• A1g mode at 165cm-1 isthe radial breathing mode

ibl hi h h

A1g modes have no nodes

E1g modes have 2 nodes

E2g modes have 4 nodes

responsible which showsa strong dependence ondiameter.

Carbon Nanotubes

The distribution of nanotubes diameter and symmetries (armchair, zigzag and chiral) are responsible for the details of their spectra.

Raman Spectroscopy is Useful For Various Carbon Materials

-Carbyne (sp bondedCarbyne (sp bonded carbon)

-Graphite(sp2 bonded carbon)

-Disordered sp2 bonded graphite

-Fullerene C60Fullerene C60

-Carbon Nanotubes

-Diamond (sp3 bonded carbon)