Transcript Basic principles - Resonance Raman scattering

Spectroscopy
FTIR
RAMAN
By
Assistant Professor Dr. Akram Raheem Jabur
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.
electromagnetic relationships:
λυ = c
λ 1/υ
E = hυ
E υ
E = hc/λ
E 1/λ
λ = wave length
υ = frequency
c = speed of light
E = kinetic energy
h = Planck’s constant
λ
c
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.
some electromagnetic radiation ranges
Approx. freq. range
Hz (cycle/sec)
Approx. wavelengths
meters
Radio waves
104 - 1012
3x104 - 3x10-4
Infrared (heat)
1011 - 3.8x1014
3x10-3 - 8x10-7
Visible light
3.8x1014 - 7.5x1014
8x10-7 - 4x10-7
Ultraviolet
7.5x1014 - 3x1017
4x10-7 - 10-9
X rays
3x1017 - 3x1019
10-9 - 10-11
Gamma rays
> 3x1019
< 10-11
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.
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
IR source  sample  prism  detector
graph of % transmission vs. frequency
=> IR spectrum
100
%T
0
500
1000
1500
v (cm-1)
2000
3000
4000
toluene
Some characteristic infrared absorption frequencies
BOND
COMPOUND TYPE
FREQUENCY RANGE, cm-1
C-H
alkanes
2850-2960 and 1350-1470
alkenes
3020-3080 (m) and
RCH=CH2
910-920 and 990-1000
R2C=CH2
880-900
cis-RCH=CHR
675-730 (v)
trans-RCH=CHR
aromatic rings
monosubst.
965-975
3000-3100 (m) and
690-710 and 730-770
ortho-disubst.
735-770
meta-disubst.
690-710 and 750-810 (m)
para-disubst.
810-840 (m)
alkynes
3300
O-H
alcohols or phenols
3200-3640 (b)
C=C
alkenes
1640-1680 (v)
aromatic rings
1500 and 1600 (v)
C≡C
alkynes
2100-2260 (v)
C-O
primary alcohols
1050 (b)
secondary alcohols
1100 (b)
tertiary alcohols
1150 (b)
phenols
1230 (b)
alkyl ethers
1060-1150
aryl ethers
1200-1275(b) and 1020-1075 (m)
all abs. strong unless marked: m, moderate; v, variable; b, broad
n-pentane
2850-2960 cm-1
3000 cm-1
sat’d C-H
1470 &1375 cm-1
CH3CH2CH2CH2CH3
IR of ALKENES
=C—H bond, “unsaturated” vinyl
(sp2)
3020-3080 cm-1
+ 675-1000
RCH=CH2
+ 910-920 & 990-1000
R2C=CH2
+ 880-900
cis-RCH=CHR
+ 675-730 (v)
trans-RCH=CHR + 965-975
C=C bond
1640-1680 cm-1 (v)
The Raman Effect
Induced Dipole
Linear Molecule
O
C
O
Sample in
Equilibrium
Laser
Induced Dipole
There must be polarizability for Raman Effect to take place
Raman Compared to IR
Dipole moments relate to IR absorption
+
H - Cl
Uneven distribution of
charge = Dipole Moment
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
Rayleigh Scattering
Rayleigh scattering is elastic and is indicated 
at zero wavenumbers
Can be a calibration aid if visible in the 
spectrum
Low Density Polyethylene
0
1000
2000
Wavenumbers (cm-1)
3000
4000
Raman Scattering
Lowest 3
excited 2
electronic 1
state
0
Intensity
Rayleigh Scattering
Virtual
states
DE
Stokes Shift
Anti-Stokes Shift
300 200 100
0
-100 -200 -300
Anti-Stokes
Rayleigh Scattering
Stokes
Excitation
Raman Shift (cm-1)
Rayleigh scattering is elastic
Stokes and anti-Stokes scattering are
3
Ground 2
electronic
state 1
0
DE
inelastic
Stokes lines are more probable and therefore
More probable
Less probable
used most often
Anti-Stokes lines are not affected by
fluorescence and occur more frequently at
higher temperatures
Application Areas
Pharmaceuticals
Particulate Characterization
Process Monitoring (PAT)
Authentication
Method Development
Polymorphism
Food Science
Grain Studies
Polymers
Crystallinity
Homogeneity
Earth Sciences
Geology
Mineralogy
Semiconductors
Phase Determination
Inclusion Detection
Biology
Medical Applications
Sensory Receptors
Cell Monitoring
Reaction Monitoring
Bacteria Characterization
Forensics
Fibers
Paints
Questioned Documents
Controlled Substances
Building Materials
Terrorism
Consumer Products
Particulate Contamination
Quality Control
Raman Instrumentation
Source
Sample Illumination
Spectrometer
 Typically laser source
Raman intensity increases as the fourth
power of source frequency

High frequency = Low wavelength = Higher Raman intensity
Low frequency = High wavelength = Lower Raman intensity
Raman signal is independent of laser
wavelength

Longer wavelength sources tend to cause
less laser induced fluorescence

Common Lasers
Type
Wavelength (nm)
Argon Ion
Krypton Ion
488.0 or 514.5
530.9 or 647.1
Helium/Neon
Diode
632.8
782 or 830
Nd/YAG
1064
Raman Instrumentation
Source
Sample Illumination
Spectrometer
Confocal Set Up
Detector
 Typically
commercially
available microscope
platforms
Pinhole Aperture
Barrier Filter
Out of Focus Light Rays
In Focus Light Rays
Laser
Dichroic Mirror
Typically confocal
configuration

Objective
Band Pass Filter
Laser
Spot sizes in
2-10 mm range
Focal Planes
Sample
Pinhole Aperture
Raman Instrumentation
Spectrometer
Sample Illumination
Source
Widefield Set Up
Detector
Typically 
commercially
available microscope
platforms
Barrier Filter
Out of Focus Light Rays
In Focus Light Rays
Laser
Dichroic Mirror
Typically confocal 
configuration
Objective
Band Pass Filter
Laser Spot sizes in 
50-500 mm range
Focal Planes
Sample
Pinhole Aperture
Raman Instrumentation
Source
Spectrometer
Sample Illumination
Dispersive Spectrometer
Focusing Mirror
Fourier Transform Spectrometer
Collimating Mirror
Detector
Collimating Mirror
Dispersive
Grating
Focusing Mirror
Spatial Filter
Dielectric Filters
Scattered Light
from Sample
Detector
Scattered Light
from SampleFocusing Mirror
Raman Instrumentation
Raman Microprobe
End On View of Probe
Spectrometer
Input Fibers
Fiber Optic Cable
Collection Fibers
Focusing Objective
Probe
Laser
Sample
End On View of Collection Fibers
going to Spectrometer Slit
Sample Preparation
Very little sample preparation needed
Aluminum is usually used as a
Solids, liquids and gasses can be
substrate since it does not produce
Raman information and does not
produce fluorescence
analyzed
Gasses and liquids can be analyzed
Gold substrates can also be used
through a suitable container
 Non-volatile liquids can be spotted
onto a substrate provided slight
evaporation is not critical
Quartz microscope slides produce a 
minimal amount of background
fluorescence
Samples not encompassing the entire
laser spot should be placed on a suitable
substrate
Unsuitable Receptacle
Too little sample in receptacle, focal
plane does not reach material
Suitable Receptacle
Sufficient amount of sample,
focal plane reaches material
Analytical Comparison
Raman vs. Infrared