Geoscience Reference
In-Depth Information
S
1
S
1
S
0
T
1
(a) Internal Conversion
(b) Intersystem Crossing
Figure 1.8. Internal conversion and intersystem crossing between different vibrational levels of the
same energy (isoenergetic).
multiplicity (
Figure 1.8b
). Other processes that result in the loss of excitation energy as
heat instead of as emitted light include external conversion. This is a related form of radi-
ationless relaxation in which excess energy is transferred to a solvent molecule or another
component in the sample matrix via molecular collisions, and is sometimes referred to as
collisional quenching. This process is always present to some extent in solution samples
and ultimately the excitation energy is transformed into heat.
1.3.3 Radiative Decay
The process by which an electronically excited molecule undergoes relaxation by the
emission of a photon of radiation (light) is termed
radiative decay
(Lakowitz,
2006
). In
photoluminescence the two most relevant forms of radiative decay are fluorescence and
phosphorescence. Both of these processes are driven exclusively by the multiplicities
of the states from which the light-emitting transition occurs. For example, phosphores-
cence occurs when an electron in an excited triplet state relaxes to the singlet ground state
(dashed vertical lines in
Figure 1.1
). A molecule may undergo intersystem crossing from
an excited singlet state to a triplet excited state. Once in the lowest vibrational energy level
of an excited triplet electronic state phosphorescence occurs when the molecule relaxes to
the ground state (
Figures 1.1
and
1.6b
). Therefore, for the phosphorescence process, differ-
ent states of multiplicities are required. In comparison, the fluorescence process involves
a radiative transition between electronic states of the same spin multiplicity. For example,
when in an excited single state, a molecule may relax to a singlet ground state by emitting
