Environmental Engineering Reference
In-Depth Information
The key processes that compete with fluorescence emission from the lowest
excited singlet to the ground state are internal conversion, intersystem crossing
and photodegradation (Fig.
1
) (Senesi
1990a
). The internal conversion depends
on several factors such as increasing solvation, increasing temperature and
molecular flexibility. Such factors increase the interaction of a molecule with
its medium and accelerate the rate of internal conversion by collisional deacti-
vation. Intersystem crossing involves the transition from the lowest vibrational
level of an excited singlet state to an upper vibrational level of a triplet state, or
vice versa. A change of spin occurs in intersystem crossing, but its rate is usu-
ally slower compared to that of internal conversion. Photodegradation causes
a decrease in fluorescence intensity, and its effect might be higher for light-
sensitive organic substances. The probability of a photodegradation process
depends primarily on the energy difference between the ground state and the
first excited singlet, i.e. it increases when the energy content of the excited state
is increased.
The fluorescence quantum yield (or efficiency) (
f
) is defined as the ratio of
the number of emitted fluorescence photons to the number of photons absorbed
(Senesi
1990a
).
Ф
number of photons as fluorescence
number of photons absorbed
Φ
f
=
The fluorescence efficiency determines the effectiveness with which the
absorbed energy is re-emitted. It depends on several factors such as the molecular
structure of the fluorescent molecule and its absorption nature, the non-radiative
processes, the temperature and the wavelength used for excitation (Senesi
1990a
;
Wehry
1973
).
The probability of finding a molecule in the excited state at a time
t
after
the excitation source is turned off can be expressed as exp(
−
t
/
τ
) where
τ
is the
fluorescence lifetime (Senesi
1990a
). The fluorescence lifetime of a molecule is
defined as the mean lifetime of the excited state before photon emission. The
fluorescence intensity (
F
) typically follows a first-order kinetics and can be written
as follows (Senesi
1990a
):
F
(
t
) =
F
0
exp
(−
t
/τ )
(2.3)
where
F
(
t
) is the fluorescence intensity at the time
t
,
F
0
the initial maximum inten-
sity with the excitation source on, i.e. during excitation,
t
is the time elapsed after
the excitation source is turned off, and
τ
is the fluorescence lifetime or the decay
rate of fluorescence.
Fluorescence lifetimes or decay rates (
τ
) depend on the overall rates at which
the excited state is deactivated with both radiative and non-radiative processes.
It is
τ
tot
−
1
. Fluorescence lifetimes for commonly used fluo-
rescent molecules are typically of the order of nanoseconds. The intrinsic or
natural lifetime (
τ
0
) corresponds to an absolute quantum efficiency (
−
1
=
τ
rad
−
1
+
τ
nrad
0
) equal
to 1. It happens when fluorescence is the only mechanism by which the excited
Ф
