Geoscience Reference
In-Depth Information
E
1
E
=
E
1
-
E
0
E
0
Figure 1.5. Absorption of radiation with energy
E
, The red arrow indicates an electronic transition
of an electron from the ground electronic state (with energy
E
0
) to an excited electronic state with
energy
E
1
.
electrons migrate to different parts of the molecule. This migration of electrons means that
the Coulombic forces acting on the nuclei are subject to change. In response to changes
in the Coulombic forces acting on the nuclei, molecules
vibrate
. Therefore, a proportion
of the energy required to stimulate electronic transitions within a molecule is also used to
excite
vibrations
of the absorbing molecule. This phenomenon explains why we don't usu-
ally observe pure electronic absorption lines. In fact, most observed absorption spectra are
a composite of many discrete vibrational lines, and these discrete lines are representative
of the vibrational structure of an electronic transition. In the case of gases, such vibrational
structure can be resolved; however, in liquid or solid states, it is more common to expe-
rience featureless, broad-band absorption spectra. The vibrational structure of electronic
transitions can be explained by the Franck-Condon principle (Lakowitz,
2006
) and the
concept of
vertical transitions
(
Figure 1.5
).
The Franck-Condon principle is perhaps best understood by first explaining that nuclei
are massive in comparison to electrons; therefore an electronic transition takes place faster
than the nuclei can actually respond to. As a result, an electronic transition is most likely
to occur without changes in the positions of the nuclei in the molecular entity and its envi-
ronment. In an electronic transition, changes in electron density are rapid as is the shift in
the distribution of charge around the nuclei. These changes impact on the nuclei as new
forces are created. The consequence of this is that nuclei begin to vibrate back and forth
from their initial electronic ground state to a final electronic ground state. This change
between electronic ground states represents a swinging (back and forth) in the molecular
potential energy. This characteristic is termed
vertical transition
in accordance with the
Franck-Condon principle. In practice the electronically excited molecule may manifest
itself in one of several excited vibrational states; therefore absorption occurs at several dif-
ferent frequencies. With the exception of some gases, these transitions merge together to
give a broad featureless band of absorption (
Figure 1.6
).
1.3.2 Nonradiative Decay
Any molecule in an excited state will return to the ground state, emitting the absorbed
energy via a number of processes. The main mechanisms by which this occurs can be
