On the other hand, when electromagnetic radiation is shone on a substance (rather than being emitted from it), several processes may take
place (see Figure 6.2). Typically, the radiation is transmitted through the
matter at a velocity that depends upon the refractive index (n) of the
medium, which we previously discussed in Chapter 5. The consequences
of light traveling at different velocities through different materials will be
further discussed in the context of surface analysis techniques in Chapter 8.
However, if the frequency (or energy) of the incoming light happens to
match the exact difference between two of the allowed energy states in a
Triplet state
Intersystem
crossing
Fluorescence
(a)
(b)
Absorption
Nonradiative
relaxation
ΔE 2
Phosphorescence
Nonradiative
relaxation
ΔE 1
ΔE 1
ΔE 2
Absorption
E incoming light = ΔE 1
E incoming light = ΔE 1
E emitted light = ΔE 2
E emitted light = ΔE 2
Ground electronic state
and associated
vibrational states
Excited electronic state
and associated
vibrational states
Ground electronic state
and associated
vibrational states
Excited Electronic State
and associated
vibrational states
Figure 6.2 Energy diagrams
depicting the transitions that
occur during fluorescence
(a) and phosphoresence (b).
Note that the energies of
the photons (either those
being absorbed or those being
emitted) are identical to the
corresponding difference in
energy states of the molecule.
CHAPTER 6: Bulk Characterization Techniques for Nanomaterials
184