SAMRAT PRITHVIRAJ CHAUHAN GOVERNMENT
Resonance Raman Spectroscopy
Department Of Chemistry
1 • Introduction
2 • Raman Effect
3 • Resonance Raman Effect
4 • Theory
5 • Instrumentation
6 • RRS of Pentacene
7 • Merits and Demerits of RRS
8 • Applications of RRS
9 • Conclusion
10 • References
Raman Spectroscopy is based on scattering of radiation
which is phenomenon discovered in 1928 by physicist Sir
C.V.Raman and he won the Noble Prize in 1930 for his work.
The field of Raman spectroscopy was greatly enhanced by
the advent of laser technology during 1960s.
Resonance Raman spectroscopy also helped to advance
1) The Resonance Raman Spectroscopy is a particular
application of the general Raman spectroscopy where the
incident laser radiation has a frequency that matches the
energy of an electronic transition in the sample.
2) This technique is more selective compared to non
resonance Raman spectroscopy.
3) It works by exciting the analyte with incident radiation
corresponding to the electronic absorption bands. This
Resonance Effect : When a beam of light is passed through
a transparent substance, a small amount of the radiation
energy is scattered, the scattering persisting even if all dust
particles or other extraneous matter are rigorously exclude
from the substance.
If monochromatic radiation, of narrow frequency band, is
used the scattered energy will almost entirely of radiation of
the incident frequency but in addition, certain discrete
frequencies above and below that of the incident beam will
be scattered, it is called Raman Scattering.
RESONANCE RAMAN EFFECT
The phenomenon in which Raman line intensities are
greatly enhanced by excitation with wavelengths that
closely approach that of an electronic absorption peak of an
analyte is known as Resonance Raman Effect.
Pre-resonance is the condition where the laser excitation is
around 100 wavenumbers below the electronic transition.
And the spectrum so obtained is called Resonance Raman
The theory of resonance Raman effect is rather
Kramers Heisenberg Dirac (KHD) equation
Normal modes with large displacement in
excited state are most intense
The Raman polarizability is given by the term (α
ρσ)GF where ρ and σ are the polarizations of
incident and Raman scattered light and the
terms G and F correspond to the ground and
final vibronic states of the molecule.
The excited state is indicated by E.
The polarization dependent dipole operator is
given by either rρ or rσ.
The summation symbol indicates that the
Raman polarizability is given by the sum of all
of the vibronic states of the molecule.
ν GE, νEF, and νL are the frequencies of the
ground to excited state transition, excited state
to final state transition, and laser.
The main components of a resonance raman spectroscopy
1) Light source
2) Optical components such as lenses and mirrors to focus
the light onto a sample and collect the scattered light
3) A spectrometer
4) A detector
The light source is typically a VIS,NIR laser emitting
monochromatic light. Types of lasers are gas lasers e.g.
Ar⁺(488 and 514.5nm), diode-pumped solid state lasers or
tunable lasers are most suitable for RRS.
Notch filters are used to filter the Rayleigh line intensity
before the scattered light is entering the spectrometer and
the detector (CCD camera).
RESONANCE RAMAN SPECTROSCOPY OF PENTACENE
Pentacene is a
is consisting of five
Pentacene is a
compound of great
interest in the world
electronics and is
solids at room
It absorb light in
the visible region of
the spectrum make
it good candidates
almost black. This
compound is an
A resonance Raman spectrum of pentacene acquired using
633 nm excitation.
The highest energy bands in the fingerprint region will be due to
aromatic ring vibrational modes at approximately 1600 cm-1.
The most prominent bands in the spectrum appear at
1158,1176,1371,1532 and 1597cm-1 . The first four of these bands
have been assigned to Ag symmetry species and the 1597 cm-1
band to the B3g symmetry species.
An expand view of the portion 1600 to 3000 cm-1 of the Raman
spectrum consisting of overtones and combination modes. The
peak at 3194 cm-1 can be attributed to aromatic C-H stretching.
A profile of Raman spectra of pentacene acquired using
405, 473, 532, 633, and 785 nm excitation.
All of the excitation wavelengths fall within the absorption
spectrum of pentacene and therefore yield resonantly
enhanced Raman spectra .
Although absorption occurs for all of the other laser excitation
wavelengths, they do not all couple to the same electronic
Using a longer excitation wavelength at 532 nm, we see
definite changes in the relative intensities of the bands.
At 633 nm excitation, the spectrum is similar to that obtained
with 532 nm excitation, but we observe a small recovery of
the strengths of the 1408 and 1456 cm-1 bands.
Finally, at 785 nm excitation we observe a very different
Raman spectrum with respect to relative intensities because
at this wavelength we are now out of resonance.
MERITS OF RRS
It improves the sensitivity, which allows detection of the
sample at micromolar concentration, whereas FTIR and
conventional Raman require millimolar concentration.
The state of the sample required for RRS can be solid,
fiber, gel, or solution.
The time scale is on the order of 10⁻15–10⁻14 s, which is
too short, and is on the order of molecular vibrations.
No additional probes are required for analysis.
DEMERITS OF RRS
The disadvantage of this technique is that the laser light
used for excitation can damage the sample, which can be
solved by either agitation of the sample or using flow
Fluorescence is a problem for Resonance Raman
techniques, particularly when using sources in the visible
range which can swamp the Raman signal.
APPLICATIONS OF RESONANCE RAMAN
RR Spectroscopy provides structural information.
•Active –site structure
•Relative changes across
•Excited state structure
•Nature of electronic transition
•Potential energy surface(s)
•Spectral/frequency changes as
a function of time
•Rapid freeze quenching
RRS is a powerful technique for monitoring the
structure and dynamics of proteins and peptides in
The ultraviolet resonance Raman spectra are
employed for the conformation analysis of proteins
and amyloid fibres.
RRS is employed in biomedical applications including
single blood cell detection by the trapping method.
The antioxidative capacity of the skin in vivo has been
studied using RRS.
RRS to identify carotenoids in lower epidermis and dermis in ca
Vibrational overtones and combination modes can appear in
resonance raman spectra whereas they are frequently
absent from non-resonant raman spectra.
A tunable laser is preferred for resonance Raman and can
be an advantage. That is because only one laser is
necessary to do analyses of multiples samples in which each
one requires a different excitation wavelength. This allows
the user to switch out samples without having to switch out
the lasers as well.
If the excitation wavelength is in resonance with an
electronic transition of the sample, then enhancement of the
signal strength of some Raman bands can occur.
Consequently, the resonance Raman spectrum can appear
quite different from the normal Raman spectrum because of
the sometimes very significant differences in relative
Molecular Structure and Spectroscopy
Fundamental of molecular spectroscopy
by COLIN N.BANWELL AND ELAINE M.McCASH
Spectroscopy solution for materials analysis