4. “when we look at the
universe in a different „light‟
i.e., at „non visible‟ wave
length, we probe different
kinds of physical conditions
and we can see new kinds of
objects”
6. Spectroscopic data is often represented by a spectrum, a plot of
the intensity of radiation as a function of wavelength or frequency.
Spectrum of Benzene molecule
8. Sir Chandrasekhara Venkata Raman
. November 7, 1888 - November 21, 1970
. Won the Nobel prize in 1930 for Physics
. Discovered the “Raman effect”
. Besides discovering the Raman effect he
studied extensively in X-ray Diffractions,
Acoustics, Optics, Dielectrics and Colloidal
solutions.
9. When a monochromatic radiation of frequency ʋ is passed
through a non absorbing medium,it is found that most of it is
transmitted without any change, and some of it is scattered. If
the scattered energy is analyzed by means of a
spectrometer, the bulk of the energy is found at the frequency
of the incident beam ʋ˳ but a small portion of the scattered
energy will be
found at frequencies ʋ =ʋ˳ . The scattering of radiation
with change of frequency is called Raman scattering.
10. In Raman spectroscopy, by varying the frequency of
the radiation, a spectrum can be produced, showing the
intensity of the exiting radiation for each frequency.
This spectrum will show which frequencies of
radiation have been absorbed by the molecule to raise
it to higher vibrational energy states.
11. When Light hits a sample, It is Excited, and is
forced to vibrate and move. It is these vibrations
which we are measuring.
12. Atoms are at a certain energy level at any
given time.
As a laser light hits the atom, it is excited
and reaches a higher level of energy, and
then is brought back down.
If an atom is at a given energy level, it can
be excited then fall below the original
level.
Anti-stokes spectrum are mirror spectrums
of Stokes Raman Spectrums
13. Energy Scheme for Photon Scattering
Virtual
State
h 0 h 0+h
h h h m
Energy
0 0 0
h m
E0+h m
E0
IR Rayleigh Stokes Anti-Stokes
Absorption Scattering Scattering Scattering
(elastic)
Raman
(inelastic)
The Raman effect comprises a very small fraction,
about 1 in 107 of the incident photons.
14. Raman Spectrum
A Raman spectrum is a plot of the intensity of Raman
scattered radiation as a function of its frequency
difference from the incident radiation (usually in units
of wavenumbers, cm-1). This difference is called the
Raman shift.
19. Days before Laser..
Commonly used sources were 435.8nm and 253.6nm
emission lines of mercury vapour
Disadvantages
the source is an extended one and the brightness available per unit area is very
small
the relatively high frequency of mercury radiation often causes the sample to
fluorescence
as colored samples absorb in this high frequency region, it is not possible to record
their spectra
20. With the discovery of lasers …
Advantages
excellent monochromaticity
good beam focusing capabilities and small
line widths
the second order Raman spectra can be
recorded
the broadening due to Doppler effect can be
minimized
22. Lasers using in Raman spectroscopy
Laser sources for Raman spectroscopy include laser
diodes, diode-pumped lasers and ion lasers.
The Innova 300C and 70C series of small-frame argon or
krypton ion lasers are also well suited for Raman
experiments in the visible region of the spectrum.
Innova 70C Spectrum is a mixed gas lasers that can generate
a number of laser lines from the UV to the near IR
23. Laser wavelengths ranging from ultra-violet through
visible to near infra-red can be used for Raman
spectroscopy.
Typical examples include,
Ultra-violet: 244 nm, 257 nm, 325 nm, 364 nm
Visible: 457 nm, 473 nm, 488 nm, 514 nm, 532
nm, 633 nm, 660 nm
Near infra-red: 785 nm, 830 nm, 980 nm, 1064 nm
24. The Invictus 785-nm NIR laser is the excitation laser
of choice for the majority of Raman spectroscopy
applications from pharmaceutical to polymers.
The Invictus 830-nm NIR laser has been developed
for biomedical applications of Raman spectroscopy
where sample absorption characteristics require longer
excitation wavelengths and reduced spectral range.
The Invictus 532-nm VIS laser is used for specific
classes of Raman spectroscopy including gas phase
measurements.
25. The choice of laser wavelength has an
important impact on experimental capabilities:
Sensitivity
Spatial resolution
Optimisation of resulting based on sample
behaviour.
26. Laser filters using in Raman spectroscopy
Optical filters
Edge
Holographic notch
27. Gratings using in Raman spectroscopy
Typical gratings used for Raman vary from perhaps 300gr/mm
(low resolution) through to 1800gr/mm (high resolution) – more
specialised gratings (including 2400gr/mm and 3600gr/mm) are
also available, but have certain limitations, and should not be
considered general purpose.
Raman spectrometers typically use holographic gratings, which
normally have much less manufacturing defects in their
structure than ruled gratings. Stray light produced by
holographic gratings is about an order of magnitude less intense
than from the ruled gratings of the same groove density.
28. Detectors used in Raman spectroscopy
Charge Coupled Device (CCD) detector is the “camera” used
to detect the Raman spectrum. A CCD detector is a two
dimensional array of very low noise, silicon detectors.
Typical CCD chip.
30. Applications of Raman spectroscopy
Raman spectroscopy is commonly used in chemistry, since vibrational
information is specific to the chemical bonds and symmetry of
molecules. Therefore, it provides a fingerprint by which the molecule
can be identified.
In solid-state physics, spontaneous Raman spectroscopy is used
to, characterize materials, measure temperature, and find the
crystallographic orientation of a sample.
Raman spectroscopy can be used to observe other low frequency
excitations of the solid, such as plasmons, magnons, and
superconducting gap excitations
31. Spatially-offset Raman spectroscopy (SORS), which is less
sensitive to surface layers than conventional Raman, can be used to
discover counterfeit drugs without opening their packaging, and for
non-invasive monitoring of biological tissue
Raman spectroscopy can be used to investigate the chemical
composition of historical documents such as and contribute to
knowledge of the social and economic conditions at the time the
documents were produced.
Raman spectroscopy is being investigated as a means to detect
explosives for airport security.
32. Raman spectroscopy can be used as a technique
for identification of seafloor hydrothermal and
cold seep minerals
Used to discriminate between healthy and
unhealthy tissues, or to determine the degree of
progress of a certain disease.
Used in medicine , aiming to the development of
new therapeutic drugs and in the diagnosis of
arteriosclerosis and cancer.
33. References
Colin N. Banwell, Elaine M.McCash, 1994.
Fundamentals of Spectroscopy, Tata McGraw-Hill
Publishing Company Limited, New Delhi, 308p.
B B Laud, 1991.Lasers and non linear
optics, New age International(P) Limited, New
Delhi,261p.
H S Randhawa, 2003. Modern Molecular
Spectroscopy, Macmillan India LTD, New
Delhi,584p.
