Transcript Three types of scattering - Jordan University of Science and

Jordan University Of Science and
Technology
Faculty of science
Department of physics
Seminar title
Raman scattering
Presented by :
Sakher Abed Al Razaq Hayajneh
Superviser name :
Dr. Fedda Alzoube
1st semester
2007/ 2008
. TABLE OF CONTENTS :
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Introduction
Types scattering of photon
Raman scattering explanation
Application for Raman scattering
Key feature of SERS
Mechanism of SERS
Application for SERS
Conclusion
References
INTRODUCTION :
The scattering of light may be thought of as
the redirection of light that takes place
when an Electromagnetic (EM) wave (i.e.
an incident light ray) encounters an
obstacle or nonhomogeneity, In our case,
the scattering material (solid, liquid or
gas).
As the EM wave interacts with the matter,
the electron orbits within the constituent
molecules are perturbed periodically with
the same frequency (no) as the electric
field of the incident wave.
The oscillation or perturbation of the
electron cloud results in a periodic
separation of charge within the
molecules, which is called an induced
dipole moment .
The oscillating induced dipole moment is
manifest as a source of EM radiation,
thereby resulting in scattered light.
The majority of light scattered is emitted
at the identical frequency (no) of the
incident light, a process referred to as
elastic scattering.
However, as explained below, additional light
is scattered at different frequencies, a
process referred to as inelastic scattering.
Raman scattering is one such example of
inelastic scattering.
In summary, the above comments describe
the process of light scattering as
a complex interaction between the incident
EM wave and the material’s
molecular/atomic structure, which is useful
to study the microscopic structure .
Three types photon of
scattering
1 - Thomson scattering
2 - Compton scattering
3 - Raman scattering
The last one will be discussed in details.
1.Thomson scattering:
Describe electromagnatic radiations in
a simple classical process scatter by the
electron of ionized gas.where thomson
cross-section
2
29
2
T8

6.65  10
m …eq (1)
 r 
Where ro = classical radius of electron
In Thomson scattering the radiation is not
absorbed but reappears as radiations
travelling in different directions .there is
no change in frequency in quantum term
the photon collides elastically with the
electrons .
In other words. The photon energy is
mush less than the rest mass of the
2
electron mc .
h
h
2.Compton scattering:
Compton scattering is observed in x-rays
passing through a solid or gas.
The essential interaction is between higher
photon energy and individual electrons.
wether or not that electron is bond to
atomic nucleus.
Under conservation of momentum and energy gives
compton scattering formula
  (   )  h / mc(1  cos  )
'
…eq(2)
Where λ΄ is the wavelength of the scattered photon
& λ is the wavelength of incident photon
. 
3 – Raman scattering :The spentaneous raman effect was discovered by
C .V Raman 1928 ,he was a distinguished
indian physist who was awarded the noble prize
in 1930 for his work on the scattering of light and
for the discovery of raman effect .
Raman scattering results from the interaction of
light with the vibrational modes of molecules
constituting the scattering medium raman
scattering can equivalently be described as the
scattering of light from optical phonons .
*Raman scattering can occur by change
vibrational , rotational or electronic energy
of the molecules .
The difference energy between the incident
photon and the raman scattered photon is
equal to the energy of a vibration
Of the scattering molecules and its
described is inelastic scattering .
If there no change in
frequency between
incident and
scattered photon
This scattering
called
“Reyliegh scattering’’

Figure
1 
Raman scattering has small fraction of light
such as 1 from 10^7 photon will be
inelastic scatter , the event of scattering
occurs in 10^-14 seconds or less .
In general the
scattered light
contains
frequencies
different from those
of excitation
source , those
components
shifted to lower
frequency are
called
“ Stokes lines ” .

Figure 2 
and those shifted
to higher
frequency are
called
“Anti stoke lines”
.
Figure 3 
The stockes line are typically orders of
magnitude more intense than anti
stockes lines because at thermal
equilibrium and normal temperatures the
population of electrons in level n state is
smaller than the population in ground
state result from Boltzman energy
distribution ( N proportional
 w / kT)
to
e
Figure 4 
Stokes scattering result when molecules are
in their ground state when it is interact with
the beam of light some of energy from the
colliding photon is channeled into the
vibrational mode of the molecules this
causes the light to absorbed and then reemitted at lower frequency.
Figure 5 
Anti stockes scattering occurs when
molecules is in avibrationally excited state
when it interact with the radiations the
interaction can cause to drop to ground
state and lose some of it is vibrational
energy to the re-emitted a higher
frequency light.
Figure 6 
. Diagram illustrate raman shift
Figure 7 
Raman scattering explanation :
Light is treated as a electromagnatic wave
and the molecules is modeled as small
spheres connected by spring.
The incident light can be described by
following equations :
E (x, t) = E0 Cos (wℓ t - k’x),
…eq (3)
The induced dipole is
µ=α . E ,
…eq (4)
α is the polarizability tensor , substitute
in the electric field of light
µ = α E0 Cos (wℓ t) ,
…eq (5)
The polarizability tensor depend on
the conformation of the molecule,
it changes as the molecule vibrate .
But α = α (Q ) ,
…eq (6)
Q is the normal vibrational coordinates .
We can expand α as tayler series ,
α = αo + [ ∂ (α)/ ∂ (Q )](Q - 0) +… …eq (7)
where Q = Q0 Cos (wm t),
…eq (8)
Wm is the frequency for the molecule vibrate
By substitute α we calculate the induced
dipole ,
µ = αo E0 Cos (wℓ t) + [∂ (α)/ ∂ (Q )]. Q0 E0
Cos (wℓ t) .Cos (wm t) ,
…eq (9)
µ = αo E0 Cos (wℓ t) + [∂ (α)/ ∂ (Q )] .
Q0 E0 [Cos ((wℓ - wm )t) +
Cos ((wℓ + wm )t)] ,
…eq (10)
1st term { αo E0 Cos (wℓ t) }
mean the incident light frequency is the same
for the scattered light frequency which was
mentioned ( Reyliegh scattering ) .
2nd term L.H.S
[∂(α)/ ∂ (Q )]. Q0 E0 [Cos ((wℓ - wm )t) ,
mean lower shift of frequency which was
mentioned (Raman stokes shift )
2nd term R.H.S
[∂ (α)/ ∂ (Q )]. Q0 E0 [Cos ((wℓ + wm )t) ,
mean higher shift of frequency which was
mentioned (Raman anti stokes shift ) .
Note that ,the Raman scattering intensity
proportional to
[∂ (α)/ ∂ (Q) ]^2 .
- The selection rule for raman scattering
is
∂ (α)/ ∂ (Q) ≠ 0
Applications for raman
scattering :
Surface enhanced raman scattering
spectroscopy (SERS).
This review covers from the basic principles
of raman spectroscopy to the advanced
technique of surface enhanced raman
scattering (SERS) spectroscopy.
SERS was accidentally discovered while people
tried to do raman on the electrode in 1974 .the
original idea to generate a high surface area on
the roughed metal.
In 1977 found the rough sliver electrode produce
raman spectrum that is a million fold more
intense than was expected .
This enormously strong signal debuted surface
enhanced raman scattering (SERS) .
Key features of SERS:
- SERS occurs when molecules are brought to the
surface of metal in a variety of
morphologies .
- large enhancement are observed from silver ,gold
and copper ,the particles size for
enhancement raman to happen ranges from
20 nm - 300nm.
- Molecules adsorbed in the first layer on the surface
show largest enhancements (large range effect 10
nanometer ) .
Figure 8 
Mechanism of SERS :
- several years ago few were fully convinced
that SERS enhancement could be as high
as 15 orders of magnitude and SERS
active materials would include
a variety of transition metals and probably
semi-conductors .

The enhancement most probably comes
from the increasing of the electric field.
When the shape of particle was sharp
The electric field near the sharp tip would be
greatly enhanced , then if we collect two or
more particles gather together the electric
field could be collective resonance of free
electron of the surface of the metal
particles which provides great
enhancement .
Application for SERS :
SERS used in detection of DNA & RNA
- Procedure :
the gold particles were designed in special
way to make use of SERS the dye CY3 was
attached to the gold particles and the target
was attached to the CY3 then the gold
particle thrown onto the DNA chip After that
add some silver particles to the solution
which gather around the gold particles which
give SERS effect .
Now we are in a good shape to obtain
Raman signals from the target and locate
it with high resolution and will get
“ fingerprint ” of the DNA & RNA .
Conclusion
As previous overview the important factor
for raman scattering that it is more
sensitive to different vibrational modes this
is the reason why its called
“
fingerprint of molecules” after SERS was
discovered .
SERS is among the most sensitive
techniques available to surface science .
its capability to delivering specific chemical
identification and to couple this with a wide
range of instruments, has led to its
continuing use in both new and traditional
areas of surface science .
.
such industrial fields related to surface
oxidation ,adhesion, corrosion and
catalytic processes and in advanced
materials ,biology and sensor research .
Finally , SERS became a good tool in all
advanced techniques .
Physicists study to increase surface
enhancement using nano wires
technology .
REFERENCES :
- MIT course 6.975 handout ,Katrin Kneipp(200)
- Introductory Raman Spectroscopy,R.LFerraro,academic
prees USA(1994).
- Introduction to modern optics, Grant R.Fowles,2nd edition,
New Work1975
- The new interfacial ubiquity of surface enhanced Raman
spectroscopy, NM ,J.Weaver (2000)
- Surface enhanced Raman spectroscopy
,G,C.Weaver,K,Norrod,J.Chem.75,5,621(1998)
- Electromagnetic Mechanism of surface enhanced
Spectroscopy
,G,C,Schatz,759-774 (2002)
- M.Moskovits ,D.P.Dilella,in surface enhanced Raman
Scattering, R.K.Chang new work, 243-273(1982)
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