Geoscience Reference
In-Depth Information
conserved and, more specifically, where some of the energy of the incident particle is lost
or gained.
Although this chapter specifically concentrates on photoluminescence, specifically fluo-
rescence spectroscopy, it is helpful, for completeness, to consider the other three main
methods in optical spectroscopy in the context of the instrumentation and measurement
requirements: absorption, emission and scattering (see also
Chapter 1
).
5.2.1 Absorption Spectroscopy
The absorption method involves measuring the ratio of two radiant powers (of the incident
and transmitted light), calculating the absorbance, and relating the absorbance to concen-
tration. For absorption to occur, the frequency of the incident light must correspond to
the energy difference between two states, which allows the species to be excited from the
ground state to some higher energy state. The energy absorbed is dissipated as lumines-
cence (radiant energy), photochemical reaction (chemical energy), or thermal energy. For
many experiments, the absorption of light follows the Beer-Lambert law (
Eq. [5.2]
) previ-
ously outlined in
Chapter 1
.
φ
φ
=
=−
( )
=−
log
T
log
ε
cl
(5.2)
0
where
A
is the absorbance,
T
is the sample transmittance;
I
0
and
I
represent the intensity
(or radiant power) of the incident light and the transmitted light,
ε
is the extinction coeffi-
cient of the sample,
c
is the concentration of the absorbing species, and
l
is the optical path
length through the sample (see
Figure 5.2
).
The absorption method is based on the following assumptions:
•
The absorbers act independently of each other.
•
The incident light intensity is not so high as to cause saturation or bleaching effects.
•
The incident light beam is perpendicular to the absorbing surface.
•
The path length is uniform, and the sample is homogeneous and does not scatter the
light.
I
o
I
t
c
,
α
l
Figure 5.2. The absorption (Beer-Lambert) of a beam of light through a sample cuvette.
