Transcript Slide 1

j
1.1 Elastic scattering and inelastic scattering
Elastic scattering (Rayleigh scattering) : no change in energy of light
EMR induces oscillating electric field
in electron cloud around the moleculewhich is then re-emitted as a photon
j
If there is a vibration which results in a change in the net polarizability of the
electron cloud. (polarizability: a term describes a molecule properties having
to do with the deformability of a bond)
Inelastic scattering:
Scattered photon will be at
an energy either greater
than or lower than the
original energy by an
amount equal to the energy
of the vibrationa.
(only about 1 molecule in
108-109 will inelastically
scatter photons)
Stokes Lines: longer wavelength (more intense)
Anti-stokes Lines: shorter wavelength
 = Raman
Rayleigh line
1.2 Wave model
E  E0 cos(2 ext )
E interacts with an electron cloud of a bond, induced dipole moment
m  αE  E0 cos(2 ext )
a : polarizabilit y
a  a0  (r  req )(
a
)
r
r  req  rm cos(2 vibt )
m  a0 E0 cos(2 ext )  E0 rm (
a
) cos(2 vibt ) cos(2 ext )
r
(note: cos x cos y  [cos(x  y )  cos(x  y )] / 2)
m  a0 E0 cos( 2πνext) 
E0
a
rm(
) cos [ 2π(νex-νvib )t] 
2
r
E0
a
rm(
) cos [ 2π(νex  νvib )t]
2
r
Rayleigh scattering
Stokes line
Anti-Stokes line
Selection rule
1.
2.
There is a change in polarizability during the vibration
 =  1
For comparison, IR
1. There is a change in dipole moment
2.  = 1
Distortion of bonds becomes
easier as they lengthen and
more difficult as they shorten. >change in polarization
Distortion of one bond becomes
easier as it lengthen and another
more difficult as they shorten. >change in polarization cancel
off
1.3 Intensity of Raman Bands
concentration
-proportional to concentration
Source intensity
-directly to source intensity
Source wavelength
-1/4
2. Instrumentation
2.1 Block diagram
2.2 Sample handling
(pp488-490), simpler than for IR spectroscopy
glass can be used for windows, and can use aqueous solution.
Fig. 18-8, P489
2.3 Raman properties
Get vibrational spectrum
-Complementary with IR
-Aqueous solution are accessible to study
Problem:
-Low efficiency of effect = poor sensitivity
-Competition from fluorescence for high fluorescence species
Solving problems:
-Resonance Raman (selectivity and detectability)
-Surface-enhanced Raman (sensitivity and detectability)
-Multi-channel detection (detectability)
-Near-IR excitation (fluorescence rejection)
Resonance Raman *
Near-IR Excitation
3.1 Vibrational spectral analysis
- complementary with IR
- molecular structure info
- easy and fast
3.2 Quantitative Analysis
can be sensitive as fluorescence
more selective than fluorescence