Raman Spectroscopy Basics

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Raman Basics

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Introduction

Raman spectroscopy is a spectroscopic technique based on inelastic scattering of monochromatic light, usually from a

laser source. Inelastic scattering means that the frequency of photons in monochromatic light changes upon interaction

with a sample. Photons of the laser light are absorbed by the sample and then reemitted. Frequency of the reemitted

photons is shifted up or down in comparison with original monochromatic frequency, which is called the Raman effect.

This shift provides information about vibrational, rotational and other low frequency transitions in molecules. Raman

spectroscopy can be used to study solid, liquid and gaseous samples.

1. Origins of Raman

The Raman effect is based on molecular deformations in electric field E determined by molecular polarizability

α

. The laser

beam can be considered as an oscillating electromagnetic wave with electrical vector E. Upon interaction with the sample

it induces electric dipole moment P =

α

E which deforms molecules. Because of periodical deformation, molecules start

vibrating with characteristic frequency

υ

m

.

Amplitude of vibration is called a

nuclear displacement

. In other words, monochromatic laser light with frequency

υ

0

excites molecules and transforms them into oscillating dipoles. Such oscillating dipoles emit light of three different

frequencies (Fig.1) when:

1. A molecule with no Raman-active modes absorbs a photon with the frequency

υ

0

. The excited molecule returns back

to the same basic vibrational state and emits light with the same frequency

υ

0

as an excitation source. This type if

interaction is called an elastic

Rayleigh scattering

.

2. A photon with frequency

υ

0

is absorbed by Raman-active molecule which at the time of interaction is in the basic

vibrational state. Part of the photon’s energy is transferred to the Raman-active mode with frequency

υ

m

and the resulting

frequency of scattered light is reduced to

υ

0

-

υ

m

. This Raman frequency is called Stokes frequency, or just “

Stokes

”.

3. A photon with frequency

υ

0

is absorbed by a Raman-active molecule, which, at the time of interaction, is already in the

excited vibrational state. Excessive energy of excited Ramanactive mode is released, molecule returns to the basic

vibrational state and the resulting frequency of scattered light goes up to

υ

0

+

υ

m

. This Raman frequency is called Anti-

Stokes frequency, or just “

Anti-Stokes

”.

Raman