Geoscience Reference
In-Depth Information
Wavelength (
λ
)
Figure 1.6. How superimposed vibrational structure merge together to give a broad unresolved broad
electronic absorption band.
categorized into nonradiative and radiative decay (light emitting) processes. In this section
we consider some of the mechanisms that lead to nonradiative decay.
1.3.2.1 Vibrational Relaxation
Vibrational relaxation (VR) is a radiationless process in which a molecule in an excited
vibrational level returns to a lower vibrational energy level in the same electronic state
(Lakowitz,
2006
). Vibrational relaxation occurs by the transfer of energy from the excited
vibrating molecule to the surrounding environment such as colliding into nearby solvent
molecules. This energy transfer is very efficient and occurs over an average lifetime of
picoseconds (<10
-12
s). This energy transfer process is very quick and much shorter than
the average lifetime of an electronically excited state, which is on the order of nanosec-
onds. Molecules that are excited to different vibrational energy levels within the same
excited electronic state very quickly return to the lowest vibrational energy level of that
excited state. This is the main reason why, for most solvated molecules, fluorescence emis-
sion occurs only from the lowest vibrational energy level of an excited state. Therefore,
fluorescence emission occurs from the level (or state) which is thermally populated accord-
ing to the Boltzmann distribution. This is especially true for fluorophores in a condensed
phase where their proximity to solvent or coordinating molecules is close. Hence, appre-
ciable fluorescence yields can be obtained only from the
lowest
excited state of a given
multiplicity; consequently the fluorescence emission of a fluorophore is independent of
the excitation wavelength provided it is in a condensed phase. This concept is referred to
as Kasha's rule and this concept is important in helping to understand the significance of
excitation and emission spectra to investigate the vibrational structure of both the excited
and ground states. Excitation spectra can show the variation in the emission intensity as a
function of excitation wavelength, and this contains vibrational structural information rel-
evant to the excited state. Emission spectra, on the other hand, are obtained by monitoring
