Hello friends!!!
Welcome to the next video in the analytical
instrumentation series.
In the previous few videos, we have learnt
about different components used in analytical
instruments like filters, monochromators as
well different detectors.
We have also covered different UV-Vis instruments
and fluorescence spectrometers in our previous
videos.
If you have missed these videos, check out
the link in the description for the playlist
on Analytical Instrumentation.
In this video, we will discuss the principle
of Raman effect and understand how it can
be used in Raman Spectroscopy to perform analysis
on a given sample.
Let us first understand the raman effect.
When a sample is exposed to a monochromatic
light in visible region, the sample absorbs
light and major portion of the light gets
transmitted through the sample.
However a minute part of the light is scattered
by the sample is all the directions.
One can observe the scattering at right angle
to the incident beam.
The incident light has a particular frequency.
If the scattered light has the frequency same
as the incident light, then scattering is
called as Rayleigh scattering.
However, it has been observed that about 1%
of total scattered intensity occurs at frequencies
different from the incident frequency.
This is called as Raman scattering.
Raman Scattering can be thought of as a two
photon process.
The electrons have different vibrational levels.
They are defined by specific energy differences.
When an incident monochromatic light interacts
with an electron in the sample, the electron
absorbs energy from the incident photon and
rises to a virtual state of energy.
The energy transferred is given by the formula
E=Hv, where v is the frequency of incident
photon.
The electron then falls back to an energy
level by losing energy.
If the energy lost equals to the energy of
incident photon, the electron falls back to
its initial vibrational level and in this
process emits another photon.
Since the energy lost is equivalent to the
energy of incident photon, the released photon
has same frequency as the incident photon.
As the frequency is same, Rayleigh scattering
occurs.
However, sometimes electrons when losing energy
from the virtual state, can fall back to a
different vibrational level.
In this case, the energy lost by the electron
is different than the energy absorbed from
the incident photon.
As a result, the photon emitted by the electron,
has energy, different than the incident photon.
This is possible when the frequency of the
emitted photon is different than the incident
photon.
This gives rise to raman scattering.
Depending upon the final energy of the electron
or final vibrational level of electron, raman
scattering can be separated into stokes lines
and anti-stokes lines.
If the frequency of the scattered photon is
less than the frequency of incident photon,
stokes lines are observed on Raman spectrum.
This happens when the electron absorbs energy.
Similarly, when frequency of emitted photon
is greater than incident photon, anti-stokes
lines are observed.This means that energy
is released by the electron.
The raman spectra gives the molecular fingerprint
and it is different for different molecules.
By studying the spectra one can identify the
rotational levels and thus a particular molecule.
This helps in performing qualitative analysis.
Similarly the intensity value of a particular
raman line helps to determine the concentration
of a molecule in a sample.
In this manner, quantitative analysis can
be done.
Thus, raman spectroscopy can be used to perform
both quantitative and qualitative analysis
on a sample.
Peace out
