Section 4 Raman Spectroscopy

Section 4
Raman Spectroscopy
4-1 Introduction: Experimental Setup and Spectrum
4-2 Quantum Picture of the Raman Effect
4-3 Classical Description of the Raman Effect
4-4 Raman Selection Rules for Rotational Changes
4-5 Vibrational Raman and Polarisibility Change / Example CO 2
4-6 Vibrational Raman Spectroscopy (with rotational structure)
4-7 Symmetry Considerations and Rule of Mutual Exclusion
4-8 Some Specific Features of Raman Spectroscopy
4-9 Example:
Vibrations of a Polyatomic Molecule with D 4h Symmetry (XeF 4 )
4-10 Vibrational Selection Rules and Symmetry
4-11 Raman vs. IR Spectroscopy
4-12 Key Points --- Raman Spectroscopy

Raman Spectroscopy

Quantum Picture of the Raman Effect

Classical Description of the Raman Effect
Vibrations
Rotations

Raman Selection Rules for Rotational Changes

Vibrational Raman and Polarisability Change

Vibrational Raman Spectroscopy
(with rotational (sub)-structure)

Symmetry Considerations and
Rule of Mutual Exclusion
IR
For a vibration to be IR active, it must
transform in the same way as x, y, or z (translations)
Raman
For a vibration to be Raman active, it must transform in
the same way as x 2 , y 2 , z 2 , xy, xz or yz.
Rule of mutual exclusion
If a molecule has a centre of symmetry then IR active
vibrations are Raman inactive and vice versa.
Note that there may be modes inactive in both.
If a molecule has no centre of symmetry then some (but
not necessarily all) vibrations may be both IR
and Raman active.

Some Specific Features of Raman Spectroscopy
1) Resonance Raman
resonant
scattered
radiation
E el
-
… if incident radation nearly conincides with the
energy of an electronic transition of the sample.
-> intensity of the Raman scattered radiation is
strongly enhanced.
-> applications for pigments (β-carotene;
chlorophyll; phthalocyanines etc.) with strong
absorption in the UV and visible region of the
spectrum.
-> This leads to a certain degree of selectivity if
these pigments are bound to very large biological
molecules, and only certain vibrational excitations
in specific parts of the molecule are enhanced.
2) Polarisation characteristics
Incident
radiation
II II
Only transitions involving totally symmetric
vibrational modes give rise to polarised Raman
bands (i.e. for the scattered radiation).
All others are depolarised.
This is a considerable help in the assignment.
(see Haken/Wolf; Atkins; Davidson; Woodward
for details)
II I
Scattered
radiation

Example: Xe F 4

Vibrational Selection Rules and Symmetry

Raman vs IR Spectroscopy
1) Nature of the process:
--- IR absorption: “1-photon process”, Raman: “2-photon process”;
(thus Raman cross sections usually weak)
--- IR absorption: changes parity, Raman: conserves parity
2) Complementarity for vibrational modes
(Rule of mutual exclusion for centrosymmetric molecules)
3) Raman: Wavelength of primary radiation can be conveniently chosen
4) Specific polarisation features for Raman can help with identification
of nature of modes
5) Resonance Raman
In complex molecules vibrational excitations in specific parts of the
molecule can be studied by using primary radiation near the excitation
levels of this part (thus some degree of selectivity)
6) Raman can be applied with good spatial resolution
(more difficult for IR)
7) Raman scattering by water is weak (compared to IR), which is good
for studying samples surrounded by water (an important solvent)
8) Raman has an essentially frequency independent penetration depth
into the surrounding medium, IR penetration depth may vary more
strongly (normalisation easier for Raman)
9) For samples with significant fluorescence background (excited by the
primary radiation), it may be difficult to distinguish the Raman signal
from the background.

Key Points --- Raman Spectroscopy
1) Nature of the Raman process
--- incoming radiation hν -> outgoing radiation hν’
--- Raman: “2-photon process” (IR absorption: “1-photon process”)
(thus Raman cross sections usually weak)
--- Raman: “conserves parity” (IR absorption: changes parity)
2) Selection rules
--- rotational transitions
∆ J = +- 2 (or 0)
--- vibrational transitions
∆ v = +- 1
and anisotropic polarisability
and polarisability change upon vibration
3) IR vs Raman
a) Complementarity for vibrational modes
(Rule of mutual exclusion for centrosymmetric molecules)
b) Experimental considerations (Raman):
i) Wavelength of primary radiation can be conveniently chosen
ii) Polarisation features can help with identification of nature of modes
iii) Resonance Raman: stronger signal (and potentially selective)
iv) Raman can be applied with good spatial resolution
v) Raman scattering by water is weak (compared to IR)
vi) Raman has an essentially frequency independent penetration depth
vii) For samples with significant fluorescence background (excited by
the primary radiation), it may be difficult to distinguish the Raman
signal from the background.