Geoscience Reference
In-Depth Information
Table 1.1.
Color, typical corresponding wavelengths, frequency, and energy of light
Color
λ
(nm)
5 (10
−14
Hz)
E
(eV)
E
(kJ mol
−1
)
Infrared
>1000
<300
<1.24
<120
Red
700
428
1.77
171
Orange
620
484
2.00
193
Yellow
580
517
2.14
206
Green
530
566
2.34
226
Blue
470
638
2.64
254
Violet
420
714
2.95
285
Near ultraviolet
300
1000
4.15
400
Far ultraviolet
<200
>1500
>6.20
>598
Note
: Italics represent the visible spectrum.
may manifest itself electronically, vibrationally, or rotationally depending on the energy of
the incident photon. Due to the quantization of molecular energy levels, electronic excita-
tion of a particular species occur only if
E
corresponds to the difference in energy between
the ground electronic state and an electronically excited state of the absorber, which is
illustrated in
Figure 1.5
.
The absorption by a molecule of a photon of light, with energy
E
, equal to the difference
between energy states, can promote the transition of an electron from the ground electronic
state to a vibrationally and electronically excited state. This is illustrated in
Figure 1.3
. The
amount of incident radiation absorbed is proportional to the number of molecules (molec-
ular concentration) in the path (photons
-1
cm
-2
), and is expressed in Beer and Lambert's
Law (
Eq. [1.6]
):
I
=
I
exp
−
Î
cl
(1.6)
t
0
where
I
t
is the transmitted light intensity,
I
o
is the incident light intensity,
Î
is the molar
absorptivity (property of the absorbing species),
c
= the concentration of absorbing species,
and l = the path length of the sample. According to Beer and Lambert's Law, the optical
density of a solution should remain uniform as long as the product of the concentration and
the path length is constant. However, the molar absorptivity often varies appreciably with
the concentration of the solute and possible causes for this are molecular association of the
solute at high concentrations; ionization of the solute in the case of acids, bases, and salts;
and fluorescence of the solute.
1.3.1.3 Franck-Condon Principle
When molecules absorb energy that results in an electronic transition, molecular vibrations
are always observed. In the electronic ground state of a molecule, the locations of nuclei
in space are as a result of the Coulombic forces (electrostatic interaction between electri-
cally charged particles such as electrons) acting on them. During an electronic transition
