Biophotonics_Concepts_to_Applications

6.3.4 Inelastic (Raman) Scattering

A major type of inelastic scattering isRaman scatteringof photons by their
interaction with a molecule [ 28 – 30 ]. This effect is used in Raman spectroscopy and
is complementary to infrared absorption spectroscopy methods to obtain informa-
tion about molecular compositions, structures, and interactions in biological tissue
samples. As noted in Fig.6.6, molecules directly absorb photons during an infrared
absorption process. This effect excites the molecules to higher vibrational states. In
contrast, in a Raman inelastic scattering event two possible reactions can occur
during the interaction of a photon with a molecule. Either a small amount of energy

g = 0.9

g = 0.8

50

150

200

100

10 20 30 40

Probability function p(

θ)

Scattering angle θ

0

0

Fig. 6.16 Henyey-Greenstein function for two coefficients of anisotropic scattering

Anti-Stokes scattering

Stokes scattering

Incoming

photon

Stokes

photon

Anti-Stokes photon

Virtual state

Excited state

Ground state

Incoming

photon

= hR

h 0 h( 0 R) h^0 h( 0 + R)

Virtual

state

Fig. 6.17 Illustration of Stokes scattering (left) and anti-Stokes scattering (right); the thick
horizontal lines represent the vibrational energy levels of the material (e.g.,ν= 0 andν=1)

168 6 Light-Tissue Interactions