<s>
Raman	B-Architecture
scattering	I-Architecture
or	O
the	O
Raman	B-Architecture
effect	I-Architecture
(	O
)	O
is	O
the	O
inelastic	O
scattering	O
of	O
photons	B-Application
by	O
matter	O
,	O
meaning	O
that	O
there	O
is	O
both	O
an	O
exchange	O
of	O
energy	O
and	O
a	O
change	O
in	O
the	O
light	O
's	O
direction	O
.	O
</s>
<s>
Typically	O
this	O
effect	O
involves	O
vibrational	O
energy	O
being	O
gained	O
by	O
a	O
molecule	O
as	O
incident	O
photons	B-Application
from	O
a	O
visible	O
laser	O
are	O
shifted	O
to	O
lower	O
energy	O
.	O
</s>
<s>
This	O
is	O
called	O
normal	O
Stokes	O
Raman	B-Architecture
scattering	I-Architecture
.	O
</s>
<s>
The	O
effect	O
is	O
exploited	O
by	O
chemists	O
and	O
physicists	O
to	O
gain	O
information	O
about	O
materials	O
for	O
a	O
variety	O
of	O
purposes	O
by	O
performing	O
various	O
forms	O
of	O
Raman	B-Algorithm
spectroscopy	I-Algorithm
.	O
</s>
<s>
Many	O
other	O
variants	O
of	O
Raman	B-Algorithm
spectroscopy	I-Algorithm
allow	O
rotational	O
energy	O
to	O
be	O
examined	O
(	O
if	O
gas	O
samples	O
are	O
used	O
)	O
and	O
electronic	O
energy	O
levels	O
may	O
be	O
examined	O
if	O
an	O
X-ray	O
source	O
is	O
used	O
in	O
addition	O
to	O
other	O
possibilities	O
.	O
</s>
<s>
When	O
photons	B-Application
are	O
scattered	O
,	O
most	O
of	O
them	O
are	O
elastically	O
scattered	O
(	O
Rayleigh	O
scattering	O
)	O
,	O
such	O
that	O
the	O
scattered	O
photons	B-Application
have	O
the	O
same	O
energy	O
(	O
frequency	O
,	O
wavelength	O
and	O
color	O
)	O
as	O
the	O
incident	O
photons	B-Application
but	O
different	O
direction	O
.	O
</s>
<s>
An	O
even	O
smaller	O
fraction	O
of	O
the	O
scattered	O
photons	B-Application
(	O
approximately	O
1	O
in	O
1	O
million	O
)	O
can	O
be	O
scattered	O
inelastically	O
,	O
with	O
the	O
scattered	O
photons	B-Application
having	O
an	O
energy	O
different	O
(	O
usually	O
lower	O
)	O
from	O
those	O
of	O
the	O
incident	O
photons	B-Application
—	O
these	O
are	O
Raman	O
scattered	O
photons	B-Application
.	O
</s>
<s>
The	O
Raman	B-Architecture
effect	I-Architecture
is	O
named	O
after	O
Indian	O
scientist	O
C	O
.	O
V	O
.	O
Raman	O
,	O
who	O
discovered	O
it	O
in	O
1928	O
with	O
assistance	O
from	O
his	O
student	O
K	O
.	O
S	O
.	O
Krishnan	O
.	O
</s>
<s>
Raman	O
was	O
awarded	O
the	O
1930	O
Nobel	O
Prize	O
in	O
Physics	O
for	O
his	O
discovery	O
of	O
Raman	B-Architecture
scattering	I-Architecture
.	O
</s>
<s>
The	O
Raman	B-Architecture
effect	I-Architecture
was	O
first	O
reported	O
by	O
Raman	O
and	O
his	O
coworker	O
K	O
.	O
S	O
.	O
Krishnan	O
,	O
and	O
independently	O
by	O
Grigory	O
Landsberg	O
and	O
Leonid	O
Mandelstam	O
,	O
in	O
Moscow	O
on	O
21	O
February	O
1928	O
(	O
one	O
week	O
earlier	O
than	O
Raman	O
and	O
Krishnan	O
)	O
.	O
</s>
<s>
In	O
1998	O
the	O
Raman	B-Architecture
effect	I-Architecture
was	O
designated	O
a	O
National	O
Historic	O
Chemical	O
Landmark	O
by	O
the	O
American	O
Chemical	O
Society	O
in	O
recognition	O
of	O
its	O
significance	O
as	O
a	O
tool	O
for	O
analyzing	O
the	O
composition	O
of	O
liquids	O
,	O
gases	O
,	O
and	O
solids	O
.	O
</s>
<s>
Modern	O
Raman	B-Algorithm
spectroscopy	I-Algorithm
nearly	O
always	O
involves	O
the	O
use	O
of	O
lasers	O
as	O
an	O
exciting	O
light	O
source	O
.	O
</s>
<s>
Because	O
lasers	O
were	O
not	O
available	O
until	O
more	O
than	O
three	O
decades	O
after	O
the	O
discovery	O
of	O
the	O
effect	O
,	O
Raman	O
and	O
Krishnan	O
used	O
a	O
mercury	O
lamp	O
and	O
photographic	B-Algorithm
plates	I-Algorithm
to	O
record	O
spectra	O
.	O
</s>
<s>
Early	O
spectra	O
took	O
hours	O
or	O
even	O
days	O
to	O
acquire	O
due	O
to	O
weak	O
light	O
sources	O
,	O
poor	O
sensitivity	O
of	O
the	O
detectors	O
and	O
the	O
weak	O
Raman	B-Architecture
scattering	I-Architecture
cross-sections	O
of	O
most	O
materials	O
.	O
</s>
<s>
The	O
most	O
common	O
modern	O
detectors	O
are	O
charge-coupled	B-Algorithm
devices	I-Algorithm
(	O
CCDs	O
)	O
.	O
</s>
<s>
The	O
following	O
focuses	O
on	O
the	O
theory	O
of	O
normal	O
(	O
non-resonant	O
,	O
spontaneous	O
,	O
vibrational	O
)	O
Raman	B-Architecture
scattering	I-Architecture
of	O
light	O
by	O
discrete	O
molecules	O
.	O
</s>
<s>
X-ray	O
Raman	B-Algorithm
spectroscopy	I-Algorithm
is	O
conceptually	O
similar	O
but	O
involves	O
excitation	O
of	O
electronic	O
,	O
rather	O
than	O
vibrational	O
,	O
energy	O
levels	O
.	O
</s>
<s>
Raman	B-Architecture
scattering	I-Architecture
generally	O
gives	O
information	O
about	O
vibrations	O
within	O
a	O
molecule	O
.	O
</s>
<s>
For	O
solids	O
,	O
phonon	B-Language
modes	O
may	O
also	O
be	O
observed	O
.	O
</s>
<s>
The	O
basics	O
of	O
infrared	O
absorption	O
regarding	O
molecular	O
vibrations	O
apply	O
to	O
Raman	B-Architecture
scattering	I-Architecture
although	O
the	O
selection	O
rules	O
are	O
different	O
.	O
</s>
<s>
This	O
number	O
arises	O
from	O
the	O
ability	O
of	O
each	O
atom	B-Language
in	O
a	O
molecule	O
to	O
move	O
in	O
three	O
dimensions	O
.	O
</s>
<s>
A	O
molecule	O
can	O
be	O
excited	O
to	O
a	O
higher	O
vibrational	O
mode	O
through	O
the	O
direct	O
absorption	O
of	O
a	O
photon	B-Application
of	O
the	O
appropriate	O
energy	O
,	O
which	O
falls	O
in	O
the	O
terahertz	O
or	O
infrared	O
range	O
.	O
</s>
<s>
This	O
is	O
called	O
Stokes	O
Raman	B-Architecture
scattering	I-Architecture
,	O
by	O
analogy	O
with	O
the	O
Stokes	B-Algorithm
shift	I-Algorithm
in	O
fluorescence	O
discovered	O
by	O
George	O
Stokes	O
in	O
1852	O
,	O
with	O
light	O
emission	O
at	O
longer	B-Algorithm
wavelength	I-Algorithm
(	O
now	O
known	O
to	O
correspond	O
to	O
lower	O
energy	O
)	O
than	O
the	O
absorbed	O
incident	O
light	O
.	O
</s>
<s>
An	O
increase	O
in	O
photon	B-Application
energy	O
which	O
leaves	O
the	O
molecule	O
in	O
a	O
lower	O
vibrational	O
energy	O
state	O
is	O
called	O
anti-Stokes	O
scattering	O
.	O
</s>
<s>
Raman	B-Architecture
scattering	I-Architecture
is	O
conceptualized	O
as	O
involving	O
a	O
virtual	O
electronic	O
energy	O
level	O
which	O
corresponds	O
to	O
the	O
energy	O
of	O
the	O
exciting	O
laser	O
photons	B-Application
.	O
</s>
<s>
Absorption	O
of	O
a	O
photon	B-Application
excites	O
the	O
molecule	O
to	O
the	O
imaginary	O
state	O
and	O
re-emission	O
leads	O
to	O
Raman	O
or	O
Rayleigh	O
scattering	O
.	O
</s>
<s>
In	O
all	O
three	O
cases	O
the	O
final	O
state	O
has	O
the	O
same	O
electronic	O
energy	O
as	O
the	O
initial	O
state	O
but	O
is	O
higher	O
in	O
vibrational	O
energy	O
in	O
the	O
case	O
of	O
Stokes	O
Raman	B-Architecture
scattering	I-Architecture
,	O
lower	O
in	O
the	O
case	O
of	O
anti-Stokes	O
Raman	B-Architecture
scattering	I-Architecture
or	O
the	O
same	O
in	O
the	O
case	O
of	O
Rayleigh	O
scattering	O
.	O
</s>
<s>
When	O
the	O
exciting	O
laser	O
energy	O
corresponds	O
to	O
an	O
actual	O
electronic	O
excitation	O
of	O
the	O
molecule	O
then	O
the	O
resonance	B-Algorithm
Raman	I-Algorithm
effect	O
occurs	O
.	O
</s>
<s>
A	O
classical	O
physics	O
based	O
model	O
is	O
able	O
to	O
account	O
for	O
Raman	B-Architecture
scattering	I-Architecture
and	O
predicts	O
an	O
increase	O
in	O
the	O
intensity	O
which	O
scales	O
with	O
the	O
fourth-power	O
of	O
the	O
light	O
frequency	O
.	O
</s>
<s>
Oscillations	O
at	O
the	O
external	O
field	O
frequency	O
are	O
therefore	O
observed	O
along	O
with	O
beat	B-Application
frequencies	I-Application
resulting	O
from	O
the	O
external	O
field	O
and	O
normal	O
mode	O
vibrations	O
.	O
</s>
<s>
The	O
spectrum	O
of	O
the	O
scattered	O
photons	B-Application
is	O
termed	O
the	O
Raman	B-Algorithm
spectrum	I-Algorithm
.	O
</s>
<s>
It	O
shows	O
the	O
intensity	O
of	O
the	O
scattered	O
light	O
as	O
a	O
function	O
of	O
its	O
frequency	O
difference	O
Δν	O
to	O
the	O
incident	O
photons	B-Application
,	O
more	O
commonly	O
called	O
a	O
Raman	O
shift	O
.	O
</s>
<s>
In	O
contrast	O
to	O
IR	O
spectroscopy	O
,	O
where	O
there	O
is	O
a	O
requirement	O
for	O
a	O
change	O
in	O
dipole	O
moment	O
for	O
vibrational	O
excitation	O
to	O
take	O
place	O
,	O
Raman	B-Architecture
scattering	I-Architecture
requires	O
a	O
change	O
in	O
polarizability	O
.	O
</s>
<s>
A	O
Raman	B-Algorithm
transition	I-Algorithm
from	O
one	O
state	O
to	O
another	O
is	O
allowed	O
only	O
if	O
the	O
molecular	O
polarizability	O
of	O
those	O
states	O
is	O
different	O
.	O
</s>
<s>
A	O
selection	O
rule	O
relevant	O
only	O
to	O
ordered	O
solid	O
materials	O
states	O
that	O
only	O
phonons	B-Language
with	O
zero	O
phase	O
angle	O
can	O
be	O
observed	O
by	O
IR	O
and	O
Raman	O
,	O
except	O
when	O
phonon	B-Language
confinement	O
is	O
manifest	O
.	O
</s>
<s>
Monitoring	O
the	O
polarization	O
of	O
the	O
scattered	O
photons	B-Application
is	O
useful	O
for	O
understanding	O
the	O
connections	O
between	O
molecular	O
symmetry	O
and	O
Raman	O
activity	O
which	O
may	O
assist	O
in	O
assigning	O
peaks	O
in	O
Raman	O
spectra	O
.	O
</s>
<s>
The	O
symmetry	O
of	O
a	O
vibrational	O
mode	O
is	O
deduced	O
from	O
the	O
depolarization	O
ratio	O
,	O
which	O
is	O
the	O
ratio	O
of	O
the	O
Raman	B-Architecture
scattering	I-Architecture
with	O
polarization	O
orthogonal	O
to	O
the	O
incident	O
laser	O
and	O
the	O
Raman	B-Architecture
scattering	I-Architecture
with	O
the	O
same	O
polarization	O
as	O
the	O
incident	O
laser	O
:	O
Here	O
is	O
the	O
intensity	O
of	O
Raman	B-Architecture
scattering	I-Architecture
when	O
the	O
analyzer	O
is	O
rotated	O
90	O
degrees	O
with	O
respect	O
to	O
the	O
incident	O
light	O
's	O
polarization	O
axis	O
,	O
and	O
the	O
intensity	O
of	O
Raman	B-Architecture
scattering	I-Architecture
when	O
the	O
analyzer	O
is	O
aligned	O
with	O
the	O
polarization	O
of	O
the	O
incident	O
laser	O
.	O
</s>
<s>
The	O
Raman-scattering	O
process	O
as	O
described	O
above	O
takes	O
place	O
spontaneously	O
;	O
i.e.	O
,	O
in	O
random	O
time	O
intervals	O
,	O
one	O
of	O
the	O
many	O
incoming	O
photons	B-Application
is	O
scattered	O
by	O
the	O
material	O
.	O
</s>
<s>
This	O
process	O
is	O
thus	O
called	O
spontaneous	O
Raman	B-Architecture
scattering	I-Architecture
.	O
</s>
<s>
On	O
the	O
other	O
hand	O
,	O
stimulated	B-Architecture
Raman	I-Architecture
scattering	I-Architecture
can	O
take	O
place	O
when	O
some	O
Stokes	O
photons	B-Application
have	O
previously	O
been	O
generated	O
by	O
spontaneous	O
Raman	B-Architecture
scattering	I-Architecture
(	O
and	O
somehow	O
forced	O
to	O
remain	O
in	O
the	O
material	O
)	O
,	O
or	O
when	O
deliberately	O
injecting	O
Stokes	O
photons	B-Application
(	O
"	O
signal	O
light	O
"	O
)	O
together	O
with	O
the	O
original	O
light	O
(	O
"	O
pump	O
light	O
"	O
)	O
.	O
</s>
<s>
In	O
that	O
case	O
,	O
the	O
total	O
Raman-scattering	O
rate	O
is	O
increased	O
beyond	O
that	O
of	O
spontaneous	O
Raman	B-Architecture
scattering	I-Architecture
:	O
pump	O
photons	B-Application
are	O
converted	O
more	O
rapidly	O
into	O
additional	O
Stokes	O
photons	B-Application
.	O
</s>
<s>
The	O
more	O
Stokes	O
photons	B-Application
that	O
are	O
already	O
present	O
,	O
the	O
faster	O
more	O
of	O
them	O
are	O
added	O
.	O
</s>
<s>
Effectively	O
,	O
this	O
amplifies	O
the	O
Stokes	O
light	O
in	O
the	O
presence	O
of	O
the	O
pump	O
light	O
,	O
which	O
is	O
exploited	O
in	O
Raman	B-Algorithm
amplifiers	I-Algorithm
and	O
Raman	B-Algorithm
lasers	I-Algorithm
.	O
</s>
<s>
Stimulated	B-Architecture
Raman	I-Architecture
scattering	I-Architecture
is	O
a	O
nonlinear	O
optical	O
effect	O
.	O
</s>
<s>
Generally	O
,	O
as	O
the	O
exciting	O
frequency	O
is	O
not	O
equal	O
to	O
the	O
scattered	O
Raman	B-Algorithm
frequency	I-Algorithm
,	O
the	O
corresponding	O
relative	O
wavelengths	O
and	O
are	O
not	O
equal	O
.	O
</s>
<s>
In	O
Impulsive	O
Stimulated	B-Architecture
Raman	I-Architecture
Scattering	I-Architecture
(	O
ISRS	O
)	O
,	O
the	O
length	O
of	O
the	O
pulses	O
must	O
be	O
shorter	O
than	O
all	O
relevant	O
time	O
constants	O
.	O
</s>
<s>
The	O
inverse	O
Raman	B-Architecture
effect	I-Architecture
is	O
a	O
form	O
of	O
Raman	B-Architecture
scattering	I-Architecture
first	O
noted	O
by	O
W	O
.	O
J	O
.	O
Jones	O
and	O
Boris	O
P	O
.	O
Stoicheff	O
.	O
</s>
<s>
This	O
phenomenon	O
is	O
referred	O
to	O
as	O
the	O
inverse	O
Raman	B-Architecture
effect	I-Architecture
;	O
the	O
application	O
of	O
the	O
phenomenon	O
is	O
referred	O
to	O
as	O
inverse	O
Raman	B-Algorithm
spectroscopy	I-Algorithm
,	O
and	O
a	O
record	O
of	O
the	O
continuum	O
is	O
referred	O
to	O
as	O
an	O
inverse	O
Raman	B-Algorithm
spectrum	I-Algorithm
.	O
</s>
<s>
In	O
the	O
original	O
description	O
of	O
the	O
inverse	O
Raman	B-Architecture
effect	I-Architecture
,	O
the	O
authors	O
discuss	O
both	O
absorption	O
from	O
a	O
continuum	O
of	O
higher	O
frequencies	O
and	O
absorption	O
from	O
a	O
continuum	O
of	O
lower	O
frequencies	O
.	O
</s>
<s>
They	O
note	O
that	O
absorption	O
from	O
a	O
continuum	O
of	O
lower	O
frequencies	O
will	O
not	O
be	O
observed	O
if	O
the	O
Raman	B-Algorithm
frequency	I-Algorithm
of	O
the	O
material	O
is	O
vibrational	O
in	O
origin	O
and	O
if	O
the	O
material	O
is	O
in	O
thermal	O
equilibrium	O
.	O
</s>
<s>
For	O
high-intensity	O
continuous	O
wave	O
(	O
CW	O
)	O
lasers	O
,	O
stimulated	B-Architecture
Raman	I-Architecture
scattering	I-Architecture
can	O
be	O
used	O
to	O
produce	O
a	O
broad	O
bandwidth	O
supercontinuum	O
.	O
</s>
<s>
This	O
process	O
can	O
also	O
be	O
seen	O
as	O
a	O
special	O
case	O
of	O
four-wave	O
mixing	O
,	O
in	O
which	O
the	O
frequencies	O
of	O
the	O
two	O
incident	O
photons	B-Application
are	O
equal	O
and	O
the	O
emitted	O
spectra	O
are	O
found	O
in	O
two	O
bands	O
separated	O
from	O
the	O
incident	O
light	O
by	O
the	O
phonon	B-Language
energies	O
.	O
</s>
<s>
The	O
initial	O
Raman	B-Algorithm
spectrum	I-Algorithm
is	O
built	O
up	O
with	O
spontaneous	O
emission	O
and	O
is	O
amplified	O
later	O
on	O
.	O
</s>
<s>
At	O
high	O
pumping	O
levels	O
in	O
long	O
fibers	O
,	O
higher-order	O
Raman	O
spectra	O
can	O
be	O
generated	O
by	O
using	O
the	O
Raman	B-Algorithm
spectrum	I-Algorithm
as	O
a	O
new	O
starting	O
point	O
,	O
thereby	O
building	O
a	O
chain	O
of	O
new	O
spectra	O
with	O
decreasing	O
amplitude	O
.	O
</s>
<s>
Since	O
this	O
technology	O
easily	O
fits	O
into	O
the	O
fast	O
evolving	O
fiber	B-Device
laser	I-Device
field	O
and	O
there	O
is	O
demand	O
for	O
transversal	O
coherent	O
high-intensity	O
light	O
sources	O
(	O
i.e.	O
,	O
broadband	O
telecommunication	O
,	O
imaging	O
applications	O
)	O
,	O
Raman	B-Algorithm
amplification	I-Algorithm
and	O
spectrum	O
generation	O
might	O
be	O
widely	O
used	O
in	O
the	O
near-future	O
.	O
</s>
<s>
Raman	B-Algorithm
spectroscopy	I-Algorithm
employs	O
the	O
Raman	B-Architecture
effect	I-Architecture
for	O
substances	O
analysis	O
.	O
</s>
<s>
Raman	B-Algorithm
spectroscopy	I-Algorithm
is	O
used	O
to	O
analyze	O
a	O
wide	O
range	O
of	O
materials	O
,	O
including	O
gases	O
,	O
liquids	O
,	O
and	O
solids	O
.	O
</s>
<s>
Highly	O
complex	O
materials	O
such	O
as	O
biological	O
organisms	O
and	O
human	O
tissue	O
can	O
also	O
be	O
analyzed	O
by	O
Raman	B-Algorithm
spectroscopy	I-Algorithm
.	O
</s>
<s>
For	O
solid	O
materials	O
,	O
Raman	B-Architecture
scattering	I-Architecture
is	O
used	O
as	O
a	O
tool	O
to	O
detect	O
high-frequency	O
phonon	B-Language
and	O
magnon	O
excitations	O
.	O
</s>
<s>
Stimulated	O
Raman	B-Algorithm
transitions	I-Algorithm
are	O
also	O
widely	O
used	O
for	O
manipulating	O
a	O
trapped	O
ion	O
's	O
energy	O
levels	O
,	O
and	O
thus	O
basis	O
qubit	O
states	O
.	O
</s>
<s>
Raman	B-Algorithm
spectroscopy	I-Algorithm
can	O
be	O
used	O
to	O
determine	O
the	O
force	O
constant	O
and	O
bond	O
length	O
for	O
molecules	O
that	O
do	O
not	O
have	O
an	O
infrared	O
absorption	O
spectrum	O
.	O
</s>
<s>
Raman	B-Algorithm
amplification	I-Algorithm
is	O
used	O
in	O
optical	O
amplifiers	O
.	O
</s>
<s>
The	O
Raman	B-Architecture
effect	I-Architecture
is	O
also	O
involved	O
in	O
producing	O
the	O
appearance	O
of	O
the	O
blue	O
sky	O
(	O
see	O
Rayleigh	O
Scattering	O
:	O
'	O
Rayleigh	O
scattering	O
of	O
molecular	O
nitrogen	O
and	O
oxygen	O
in	O
the	O
atmosphere	O
includes	O
elastic	O
scattering	O
as	O
well	O
as	O
the	O
inelastic	O
contribution	O
from	O
rotational	O
Raman	B-Architecture
scattering	I-Architecture
in	O
air	O
 '	O
)	O
.	O
</s>
<s>
Raman	B-Algorithm
spectroscopy	I-Algorithm
has	O
been	O
used	O
to	O
chemically	O
image	O
small	O
molecules	O
,	O
such	O
as	O
nucleic	O
acids	O
,	O
in	O
biological	O
systems	O
by	O
a	O
vibrational	O
tag	O
.	O
</s>
