Raman
Scattering
• Tyndall scattering – if small particles
are present
• During Rayleigh scattering
(interaction of light with relatively
small molecules) incident light is
scattered in all directions
• Some of the incident energy can be
converted into rotational or
vibrational energy- so wavelength of
scattered light is longer
In a fluorescence
experiment, the scattered
light will be collected along
with the fluorescence
• Thus we may see peaks in our
‘fluorescence’ spectrum that do not arise
by emission.
• Especially true with low levels of
fluorescence
• The Raman effect arises when a
photon is incident on a molecule and
interacts with the electric dipole and
causes perturbation
• In quantum mechanics the scattering
is described as an excitation to a
virtual state lower in energy than a
real electronic transition with nearly
coincident de-excitation and a change
in vibrational energy.
• At room temperature the thermal
population of vibrational excited
states is low
• Therefore the initial state is usually
the ground state and the scattered
photon will have lower energy (longer
wavelength) than the exciting photon.
• This Stokes shift is what is usually
observed in Raman spectroscopy.
• The selection rule for a Raman-active
vibration is that there be a change in
polarizability during the vibration
• If a molecule has a centre of
symmetry, vibrations which are
Raman-active will be silent in the
infrared, and vice versa.
Polarization effects
• Raman scatter from totally
symmetric vibrations will be strongly
polarized parallel to the plane of
polarization of the incident light.
• The scattered intensity from nontotally symmetric vibrations is ¾ as
strong in the plane perpendicular to
the plane of polarization of the
incident light as in the plane parallel
to it.
• Typical strong Raman scatterers are
moieties with distributed electron
clouds, such as carbon-carbon double
bonds.
• The pi-electron cloud of the double
bond is easily distorted in an external
electric field.
• Bending or stretching the bond changes
the distribution of electron density
substantially, and causes a large change
in induced dipole moment.
Instrumentation
Laser
Sample Cell
UV, vis, NIR
Monochromator
Detector
Fluorescence Interferes
• Just as a Raman peak can show up in
a fluorescence spectrum,
fluorescence can show up in – and
often swamps out – a Raman spectrum
• Change laser wavelength – to longer
wavelength
• Raman shifts are independent of the
wavelength of excitation
In summary:
• Raman spectroscopy gives
information about vibrations
• It uses UV, visible or NIR laser light
rather than IR
• The information is often
complementary to that obtaine by IR
– especially for molecules with a
centre of symmetry
• Water is less of a problem
Can use quartz cells and fibre
optics
Positions of the Raman bands of various
solvents when excited at selected wavelengths
Excitation wavelength
Solvents
313
366
405
436
water
350
418
469
511
acetonitrile
340
406
457
504
cyclohexane
344
409
458
499
chloroform
346
411
461
502
Raman peak maxima of water at
various wavelengths of excitation
Excitation/nm
200
250
300
350
400
450
500
Raman emission/nm
212
272
337
397
463
530
602
Distinguishing Raman from
Fluorescence peaks
• Raman – the scattering peak shifts
as the excitation wavelength shifts
• The amount of energy abstracted is
always constant (a vibrational energy)
• Fluorescence – changing the
excitation wavelength does not
affect the emission wavelength
• Fluorescence can be avoided by using
longer-wavelength lasers in the nearinfrared (NIR) region.
• The weaker Raman signal resulting
from NIR excitation requires
specialized components optimized for
maximum throughput and signal-tonoise ratio in the NIR region.