Biophotonics_Concepts_to_Applications

Ianti-Stokesto the Stokes intensity IStokesfor a given molecular vibration can be
approximated by

IantiStokes

IStokes

expðhmR=kBTÞð 9 : 7 Þ

Here hνRis the Raman energy shift of the excited vibrational mode (that is, hνR
is the energy gained or given up by a photon through Raman scattering).

Example 9.10What is the ratio of the anti-Stokes intensity Ianti-Stokesto the

Stokes intensity IStokesfor a 218-cm−^1 Raman mode for CCl 4 at a temperature

of 20 °C?

Solution: First,

hmR¼hc=k¼hc 218= 10 ^2 m



¼ð 6 : 626  10 ^34 JsÞð 3  108 m=sÞ 218 = 10 ^2 m



¼ 4 : 33  10 ^21 J¼ 0 :027 eV

At 20 °C = 293 K, kBT=(1.38× 10 −^23 J/K)(293 K)/(1.6× 10 −^19 J/eV) =

0.025 eV

Using Eq. (9.7), the ratio is

IantiStokes

IStokes

expðhmR=kBTÞ¼exp½Š¼ð 0 : 027 = 0 : 025 Þ expð 1 : 08 Þ

¼ 0 : 340

This intensity difference can be seen in Fig.9.17, which shows the major

Raman spectral lines for CCl 4.

9.7 Surface Enhanced Raman Scattering Spectroscopy

Unlike Raman spectroscopy, which depends on evaluating weak spectral scattering
signatures from intrinsic molecular components of the sample under investigation,
surface enhanced Raman scattering(SERS) spectroscopy is based on using a large
array of efficient scattering molecules. These collections of molecules can be
conjugated to metallic substrates and then can produce distinct, optically strong
spectra upon illumination by a laser. Basically SERS combines Raman scattering
effects with surface plasmon resonance (see Sect.7.8) that takes place on a noble
metal surface (such as silver or gold), which has been roughened with nanoparticles
[ 41 – 46 ]. By applying an excitation wavelength that coincides with the plasmon
absorption resonance peak of the particular nanoparticle, surface plasmons are

9.6 Raman Spectroscopy 279