Physical Chemistry Third Edition

23.7 Raman Spectroscopy 985

Substance Energy of First Triplet State/eV

Biphenyl 2.9

Toluene 3.6

Benzene 3.7

9,10-Diphenylanthracene 1.8

trans-1,3-Pentadiene 2.6

23.7 Raman Spectroscopy

In Raman spectroscopy, radiation is inelastically scattered by the sample substance,

either giving energy to or accepting energy from the substance. You can think of the

process as consisting of absorption of photons of one frequency accompanied by emis-

sion of photons of a different frequency, with the difference in photon energy being

absorbed or given off by the molecule. Figure 23.17 shows a schematic diagram of a

Raman spectrometer. The scattered radiation is observed at right angles to the incident

beam. It is generally much weaker than the incident beam, and lasers are now used to

provide intense incident beams.

Ramanspectroscopy was inventedby
Chandrasekhara VenkataRaman,
1888–1970,anIndianphysicist who
receivedthe 1930 Nobel Prize in
physics for this work.

The difference in the photon energies of the incident and scattered radiation must

equal the energy difference between two energy levels of the sample molecules. We

denote the frequency of the incident radiation byνand the frequency of the scattered

radiation byν′. If the radiation loses energy to the molecules,

hν−hν′Eupper−Elower (23.7-1)

Beam

attenuating shutter

Sector

Porro-prism

Depolarization

autorecording unit

Glan-Thompson prism

Interference

filter

Variable

iris Sample cell (liquid)

Photomultiplier

Shutter (manual)

Shutter

Phototube

Beamsplitter

Diffraction grating

Polarizer

Entrance slitField lens Exit slit

Polarization scrambler

Condensing lens

Iris

Iris

Beam

monitor

Laser

Intermediate slit

Figure 23.17 Schematic Diagram of a Laser Raman Spectrophotometer.In the dia-

gram, there are two distinct beams: the incident beam, and the scattered beam at right

angles to the incident beam. Courtesy of Jeol, Ltd.