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absorption (which also investigates vibrations), with the advantage of
much narrower peaks,
2
the width of which can be used to characterize
amorphous and low-crystallinity solids. However, as Raman scattering
only affects one out of a million incident photons, its study could not
generalize before the advent of lasers— powerful monochromatic exci-
tations — in the 1970s. The availability of high - sensitivity charge - coupled
devices (CCDs) designed for multichannel detection later made it rea-
sonable, timewise, to work with very low laser powers (a necessary
condition to analyze colored materials without burning them). It also
reduced the acquisition time for a given power and allowed tight surface
mappings or real-time monitoring of the effect of an external perturba-
tion (such as temperature, stress, voltage)[4]. The latest trend in Raman
spectroscopy is “hyperspectral” imaging, where a Raman mapping of a
surface is used to image a physical or chemical property, after due
processing of the spectral features by dedicated programs.
Rama
n
spectroscopists normally re
fe
r to vibrations by their wave
number,
in cm
− 1
unit), and the classical
electromagnetic theory of oscillating dipoles predicts Raman peaks
should have a
νν
=
vib
c
(
c
, the light speed;
ν
0
- wide Lorentzian shape:
3
Γ
1
()
∝
I
ν
.
(2.4)
2
Γ
+
⎛
⎝
⎞
⎠
[
]
2
0
νν
−
vib
2
The signal intensity is predicted with the following formula [5] :
2
I
∝ν
I
4
e
α
e
s
d
Ω
.
(2.5)
Raman
laser
laser
0
In Equation 2.5 ,
e
o
and
e
s
are unit vectors indicating the laser polariza-
tion and direction of observation, respectively, whereas
d
Ω
represents
the solid angle of light collection.
As polarizability changes drastically from one bond to another,
Raman intensity may not be used to measure the relative amounts of
different phases. This limitation can sometimes be an advantage when
secondary phases like an enamel pigment [6] or carbon in SiC fi bers
[7] can be detected in very small quantities (even traces). From a
2
The probes of infrared spectroscopy, the instantaneous dipole moments, are subject to
long - distance interactions.
3
The experimental bands are a convolution between this natural line shape, the instru-
mental transfer function, and the disorder-induced distribution of vibrators. It is
often taken as a Gaussian or Voigt function (a perfectly symmetric convolution of a
Lorentzian function with a Gaussian function).
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