Geoscience Reference
In-Depth Information
Photodissociation event
S
1
S
1
Vibrational
Relaxation
Excitation
Emission
S
0
S
0
Internuclear Distance
Figure 1.7. Electronic transitions during excitation and emission of a fluorophore. This illustration
shows emission occurring from the lowest vibronic level of the excited state due to the high efficiency
of vibrational relaxation.
the fluorescence emission intensity as a function of the emission wavelength and this pro-
vides information pertaining to ground state vibrational levels. This concept is illustrated
in
Figure 1.7
.
1.3.2.2 Internal Conversion and Intersystem Crossing
Internal conversion (IC) is a form of radiationless deexcitation (relaxation), whereby a
molecule that is in the ground vibrational level of an excited electronic state migrates
directly
to a higher vibrational energy level of a lower energy electronic state with the
same spin multiplicity (Lakowitz,
2006
) as shown in
Figure 1.8a
. Through a combination
of internal conversion and vibrational relaxation the energy of the electronically excited
state is given off to vibrational modes of the molecule. Intersystem crossing (ISC) is also
a radiationless relaxation process whereby a molecule in the ground vibrational energy
level of an excited electronic state passes directly into a high vibrational energy level of
a lower energy electronic state (Lakowitz,
2006
). However, unlike in internal conversion,
the high vibrational energy level of the lower energy electronic state has a different spin
