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