Environmental Engineering Reference
In-Depth Information
principles of fluorescence spectroscopy have been summarized in earlier studies
(Senesi
1990a
; Hudson et al.
2007
; Guilbault
1990
; Grabowski et al.
2003
; Oheim
et al.
2006
). An organic molecule has a series of closely spaced energy levels, and
one of its electrons can be excited from a lower to a higher level upon absorption of
a discrete quantum of light that is equal in energy to the difference between the two
energy states (Fig.
1
) (Senesi
1990a
). Fluorescence can be simply defined as the
emission of a photon at a longer wavelength (lower energy, h
υ
F
) that occurs when
the electron returns to the ground state. Radiation absorption occurs at a timescale
of approximately 10
−
15
s, emission of fluorescence photons on a timescale of about
10
−
8
s, and internal conversion typically on a time scale of about 10
−
12
s or less
(Fig.
1
). Fluorescence is basically the reverse of absorption (Senesi
1990a
).
When a fluorophore or fluorescent molecule absorbs a photon with a
frequency
υ
, which corresponds to a photon energy h
υ
ex
(h
=
Planck's constant),
its fluorescence emission can simply be depicted by the wave function
ψ
as below
(Eqs.
2.1
,
2.2
):
Excitation
(
absorption
)
: ψ
0
+
h
υ
ex
→ ψ
e
(2.1)
FLUORESCENCE
(
EMISSION
): ψ
E
→ ψ
0
+
H
υ
F
+
HEAT
(2.2)
where
ψ
0
is termed the ground state of the fluorophore and
ψ
e
is its first electronically
excited state. The fluorescence emission energy, h
υ
F
, varies depending on the return
of the photon to the ground state level (
ψ
0
). A fluorophore in its excited state,
ψ
1
, can
lose its energy by internal conversion such as 'non-radiative relaxation', where the
excitation energy is dissipated as heat (vibrational relaxation) to the solvent.
Fig. 1
Schematic energy level diagram for a diatomic molecule illustrating principal excited-
state processes.
Data source
Senesi (
1990a
)
