Application of IR
Raman Spectroscopy
• 3 IR regions
• Structure and Functional Group
•
•
•
•
•
Absorption IR
Reflection IR
Photoacoustic IR
IR Emission
Micro
10-1
Mid-IR
• Mid-IR absorption
 Samples
 Placed in cell (salt)
 Combined with oil
 Need cell that does not absorb IR
 KBr, NaCl
* Tends to absorb water
 Gases
 Solutions
 Solvent issues
* Dissolution of cell
10-2
Analysis
• Can calculate group
frequencies

C-H, C=O, C=C, O-H
 Variations of
frequencies for
group
• Fingerprint region

Compare to standards

Absorption of
inorganics
 Sulphate,
phosphate, nitrate,
carbonate
• Search spectra against library
10-3
Mid-IR
10-4
10-5
10-6
10-7
Interpretation
•
•
•
•
•
•
Alcohols and amines display strong broad O-H and N-H stretching bands in the
region 3400-3100 cm-1

bands are broadened due to hydrogen bonding and a sharp 'non-bonded'
peak can around 3400 cm-1 .
Alkene and alkyne C-H bonds display sharp stretching absorptions in the region
3100-3000 cm-1

bands are of medium intensity often obscured (i.e., OH).
Triple bond stretching absorptions occur in the region 2400-2200 cm-1

Nitriles are generally of medium intensity and are clearly defined

Alkynes absorb weakly unless they are highly asymmetric
 symmetrical alkynes do not show absorption bands
Carbonyl stretching bands occur in the region 1800-1700 cm-1

bands are generally very strong and broad

Carbonyl compounds (acyl halides, esters) are generally at higher wave
number than simple ketones and aldehydes

amides are the lowest, absorbing in the region 1700-1650 cm-1
Carbon-carbon double bond stretching occurs in the region around 1650-1600 cm-1

bands are generally sharp and of medium intensity

Aromatic compounds will display a series of sharp bands
Carbon-oxygen single bonds display stretching bands in the region 1200-1100 cm-1

bands are generally strong and broad
10-8
Quantitative IR
• Difficult to obtain reliable quantitative data based on
IR
 Deviations from Beer’s law
 Narrow Bands and wide slit widths required
* Require calibration sources
 Complex spectra
 Weak beam
 Lack of reference cell
 Need to normalize refraction
* Take reference and sample with same cell
10-9
Other methods
• Reflectance IR
 Measurement of absorbance from reflected IR
 Surface measurement
• Photoacoustic IR
 can use tunable laser
• Near IR
 700 nm to 2500 nm
 Quantitative analysis of samples
* CH, NH, and OH
 Low absorption
• Emission IR
10-10
Raman Spectroscopy
• Scattering of light
 Fraction of scattered light in the visible differs
from incident beam
 Difference based on molecular structure
* Based on quantized vibrational changes
* Difference between incident and scattered
light is in mid-IR region
 No water interference
 Can examine aqueous samples
 Quartz or glass cells can be used
 Competition with fluorescence
10-11
Raman Spectroscopy
• Theory
• Instrumentation
• Application
• Method
 Excitation with UV or NIR
 Measurement of scatter at 90 °
Measurement 1E-5 of incident beam
10-12
Theory
• 3 types of scattered radiation

Stokes
 Lower energy than AntiStokes
* Named from
fluorescence behavior
 More intense
 Used for Raman
measurements

Anti-Stokes
 No fluorescence
interference

Rayleigh
 Most intense
 Same as incident radiation
• Shift patterns independent of
incident radiation wavelength
10-13
Theory
• Excitation
 From ground or 1st vibrationally excited state
 Population of excited state from Boltzmann’s
equation
* Molecule populates virtual states with
energy from photon
* Can be effected by temperature
 Elastic scattering is Rayleigh
 Energy scattered=energy incident
 Energy difference due to ∆ ground and 1st excited
state
 hn-DE is Stokes scattering
 Hn+DE is anti-Stokes scattering
10-14
10-15
Theory
• Variation in polarizability of bond with length
• Electric field (E) due to excitation frequency
with E0
E  E0 cos( 2nex t )
• Dipole moment (m) based on polarizability of
bond (a)
m  aE  aE0 cos( 2next )
• For Raman activity a must vary with distance
a
along bond
a  a 0 + (r - req )( )
r
 a0 is polarizability at req
r - req  rmax cos( 2nv t )
10-16
Theory
E0
a
m  a 0 E0 cos( 2nex t ) +
rm ( ) cos( 2 (n ex -n n )t ) +
2
r
E0
a
rm ( ) cos( 2 (n ex +n n )t )
2
r
• Equation has Rayleigh, Stokes, and Anti-Stokes
component
• Complementary to IR absorbance
 Overlap not complete
10-17
10-18
Instrumentation
• Laser source
 Ar (488 nm, 514.5 nm)
 Kr (530.9 nm, 647.1 nm)
 He/Ne (623 nm)
 Diode (782 nm or 830 nm)
 Nd/YAG (1064 nm)
 Tunable lasers
Intensity proportional to n4
* Consider energy and chemical effect
of absorbing energy
10-19
Instrumentation
• Sample holder
 Glass
 Laser focusing allows small sample size
 Liquid and solid samples can be examined
 Use of fiber optics
10-20
Applications
• Laser microprobes
 Use of laser permits
small sampling area
• Resonance Raman
 Use electronic
absorption peak
 Low concentrations can
be examined
 Lifetimes on 10 fs
• Surface enhanced Raman
 Increase of sensitivity by
1000 to 1E6
10-21
10-22
10-23