1. Assistant Professor Dr. Akram Raheem Jabur
Spectroscopy
FTIR
RAMAN By
Assistant Professor Dr. Akram Raheem Jabur

2. Spectroscopy
“seeing the unseeable”
Using electromagnetic radiation as a probe to obtain information about atoms and molecules that are too small to see.
Electromagnetic radiation is propagated at the speed of light through a vacuum as an oscillating wave.

3. electromagnetic relationships: λυ = c λ µ 1/υ E = hυ E µ υ
  E = hc/λ E µ 1/λ
λ = wave length
υ = frequency
c = speed of light
E = kinetic energy
h = Planck’s constant λ c

4. Two oscillators will strongly interact when their energies are equal.
E1 = E2
λ1 = λ2
υ1 = υ2
If the energies are different, they will not strongly interact!
We can use electromagnetic radiation to probe atoms and molecules to find what energies they contain.

5. some electromagnetic radiation ranges
Approx. freq. range Approx. wavelengths
Hz (cycle/sec) meters
Radio waves x x10-4
Infrared (heat) x x x10-7
Visible light x x x x10-7
Ultraviolet x x x
X rays x x
Gamma rays > 3x < 10-11

6. Two oscillators will strongly interact when their energies are equal.
E1 = E2
λ1 = λ2
υ1 = υ2
If the energies are different, they will not strongly interact!
We can use electromagnetic radiation to probe atoms and molecules to find what energies they contain.

7. Spectroscopy
λ = 2.5 to 17 μm
υ = 4000 to 600 cm-1
 These frequencies match the frequencies of covalent bond stretching and bending vibrations. Infrared spectroscopy can be used to find out about covalent bonds in molecules.
IR is used to tell:
1. what type of bonds are present
2. some structural information

8. IR source è sample è prism è detector
graph of % transmission vs. frequency
=> IR spectrum
100 %T
v (cm-1)

9. toluene

10. Some characteristic infrared absorption frequencies
BOND COMPOUND TYPE FREQUENCY RANGE, cm-1
C-H alkanes and
alkenes (m) and
RCH=CH and
R2C=CH
cis-RCH=CHR (v)
trans-RCH=CHR
aromatic rings (m) and
monosubst and
ortho-disubst
meta-disubst and (m)
para-disubst (m)
alkynes
O-H alcohols or phenols (b)
C=C alkenes (v)
aromatic rings and 1600 (v)
C≡C alkynes (v)
C-O primary alcohols (b)
secondary alcohols (b)
tertiary alcohols (b)
phenols (b)
alkyl ethers
aryl ethers (b) and (m)
all abs. strong unless marked: m, moderate; v, variable; b, broad

11. n-pentane 2850-2960 cm-1 CH3CH2CH2CH2CH3 sat’d C-H 3000 cm-1

12. IR of ALKENES
=C—H bond, “unsaturated” vinyl
(sp2) cm-1
RCH=CH &
R2C=CH
cis-RCH=CHR (v)
trans-RCH=CHR
C=C bond cm-1 (v)

13. The Raman Effect Raman Compared to IR
Induced Dipole
Sample in Equilibrium
Linear Molecule O C
Emission
Relaxation
Excitation
Laser
Excitation
Induced Dipole
Emission
Relaxation
There must be polarizability for Raman Effect to take place
Raman Compared to IR
H - Cl + -
Uneven distribution of charge = Dipole Moment
Dipole moments relate to IR absorption
Polarizability relates to Raman scattering
Polarizability  how “squishy” the electron cloud is 
No electric field
In the presence of an electric field
produces an induced dipole moment

14. Rayleigh Scattering
1000
2000
3000
4000
Rayleigh scattering is elastic and is indicated at zero wavenumbers
Can be a calibration aid if visible in the spectrum
Low Density Polyethylene
Wavenumbers (cm-1)

15. Raman Scattering Rayleigh scattering is elastic3
Rayleigh Scattering
Stokes Shift
Anti-Stokes Shift
100
200
300
-300
-200
-100
Raman Shift (cm-1)
Intensity
Lowest excited electronic state 2 1
Virtual states
Anti-Stokes DE
Stokes
Rayleigh scattering is elastic
Stokes and anti-Stokes scattering are inelastic
Stokes lines are more probable and therefore used most often
Anti-Stokes lines are not affected by fluorescence and occur more frequently at higher temperatures
Excitation
Rayleigh Scattering 3
Ground electronic state DE 2 1
More probable
Less probable

16. Application Areas Pharmaceuticals Food Science
Particulate Characterization
Process Monitoring (PAT)
Authentication
Method Development
Polymorphism
Grain Studies
Polymers
Crystallinity
Homogeneity
Earth Sciences
Geology
Mineralogy
Semiconductors
Phase Determination
Inclusion Detection
Biology
Medical Applications
Reaction Monitoring
Bacteria Characterization
Sensory Receptors
Cell Monitoring
Forensics
Fibers
Paints
Questioned Documents
Controlled Substances
Building Materials
Terrorism
Consumer Products
Particulate Contamination
Quality Control

17. Raman Instrumentation
Source Sample Illumination Spectrometer
Typically laser source
Raman intensity increases as the fourth power of source frequency
Raman signal is independent of laser wavelength
Longer wavelength sources tend to cause less laser induced fluorescence
Common Lasers
High frequency = Low wavelength = Higher Raman intensity
Low frequency = High wavelength = Lower Raman intensity

18. Raman Instrumentation
Source Sample Illumination Spectrometer
Confocal Set Up
Detector
Pinhole Aperture
Typically commercially available microscope platforms
Typically confocal configuration
Laser Spot sizes in 2-10 mm range
Barrier Filter
Out of Focus Light Rays
In Focus Light Rays
Dichroic Mirror
Laser
Objective
Pinhole Aperture
Band Pass Filter
Focal Planes
Sample

19. Raman Instrumentation
Source Sample Illumination Spectrometer
Widefield Set Up
Detector
Typically commercially available microscope platforms
Typically confocal configuration
Laser Spot sizes in mm range
Barrier Filter
Out of Focus Light Rays
In Focus Light Rays
Dichroic Mirror
Laser
Objective
Pinhole Aperture
Band Pass Filter
Focal Planes
Sample

20. Raman Instrumentation
Source Sample Illumination Spectrometer
Dispersive Spectrometer
Fourier Transform Spectrometer
Focusing Mirror
Collimating Mirror
Detector
Collimating Mirror
Dispersive Grating
Focusing Mirror
Spatial Filter
Dielectric Filters
Scattered Light from Sample
Scattered Light from Sample
Focusing Mirror
Detector

21. Raman Instrumentation
Raman Microprobe
End On View of Probe
Spectrometer
Input Fibers
Fiber Optic Cable
Collection Fibers
Focusing Objective
Probe
End On View of Collection Fibers
going to Spectrometer Slit
Laser
Sample

22. Sample Preparation Very little sample preparation needed
Solids, liquids and gasses can be analyzed
Gasses and liquids can be analyzed through a suitable container
Non-volatile liquids can be spotted onto a substrate provided slight evaporation is not critical
Samples not encompassing the entire laser spot should be placed on a suitable substrate
Aluminum is usually used as a substrate since it does not produce Raman information and does not produce fluorescence
Gold substrates can also be used
Quartz microscope slides produce a minimal amount of background fluorescence
Unsuitable Receptacle
Too little sample in receptacle, focal plane does not reach material
Suitable Receptacle
Sufficient amount of sample, focal plane reaches material

23. Analytical Comparison
Raman vs. Infrared