Download Molecular Luminescence Spectroscopy

Chapter 15
Molecular Luminescence
Spectrometry
Molecular Fluorescence

Optical emission
Dkkdj
from molecules that
have been excited
to higher energy
levels by absorption
of electromagnetic
radiation.
Photoluminescence

Light is directed onto a
sample, where it is
absorbed and imparts
excess energy into the
material in a process
called "photoexcitation." One way
this excess energy can
be dissipated by the
sample is through the
emission of light, or
luminescence.

The intensity and
spectral content of this
photoluminescence is a
direct measure of
various important
material properties.
Photoluminescence

Band gap determination. The most common
radiative transition in semiconductors is between
states in the conduction and valence bands, with the
energy difference being known as the band gap.

Impurity levels and defect detection. Radiative
transitions in semiconductors also involve localized
defect levels. The photoluminescence energy
associated with these levels can be used to identify
specific defects.
Photoluminescence

Recombination mechanisms. The return to
equilibrium, also known as "recombination," can
involve both radiative and nonradiative processes.
The amount of photoluminescence and its
dependence on the level of photo-excitation and
temperature are directly related to the dominant
recombination process.

Material quality. In general, nonradiative processes
are associated with localized defect levels. Material
quality can be measured by quantifying the amount of
radiative recombination.
Photoluminescence
Electron Spin

The Pauli exclusion principle states that no
two electrons in an atom can have the same
set of four quantum numbers. This restriction
requires that no more than two must have
opposed spin states. Because of spin pairing,
most molecules exhibit no net magnetic field
and are thus said to be diamagnetic. In
contrast, free radical, which contain unpaired
electrons, have a magnetic moment are said
to be paramagnetic.
Singlet/Triplet Excited States

A molecular electrons
state in which all
electron spins are
paired is called a singlet
state and no splitting of
electronic energy levels
occurs when the
molecule is exposed to
a magnetic field.

The ground state for a
free radical, on the
other hand, is a doublet
state because the odd
electron can assume
two orientations in a
magnetic field, which
imparts slightly different
energies to the system
Rates of Absorption and
Emission...
 The
rate at which a photon of radiation
is absorbed is enormous, the process
requiring on the order o f 10-14 to 10-15s.
Fluorescence emission, on the other
hand, occurs at a significantly slower
rate. Here, the lifetime of the excited
state is inversely related to the molar
absorptivity of the absorption peak
corresponding to the excitation process.
Deactivation Processes

An excited molecule
can return to its ground
state by a combination
of several mechanistic
steps. The deactivation
steps, indicated by
wavy arrows, are
radiationless processes.
The favored route to the
ground state is the one
that minimizes the
lifetime of the excited
state.

Vibration Relaxation
 Internal Conversion
 External Conversion
 Intersystem
Crossing
 Phosphorescence
Variables That Affect Fluorescence
and Phosphorescence

Both molecular
structure and chemical
environment influence
whether a substance
will or will not
luminesce; these
factors also determine
the intensity of emission
when luminescence
does occur.

Quantum Yield
 Transition Types in
Fluorescence
 Quantum Efficiency
and Transition Type
 Fluorescence and
Structure
Effect of Concentration on
Fluorescence Intensity
Components of Fluorometers and
Spectrofluorometers



Sources: A more intense source in needed than the
tungsten of hydrogen lamp.
Lamps: The most common source for filter
fluorometer is a low-pressure mercury vapor lamp
equipped with a fused silica window. For
spectrofluorometers, a 75 to 450-W high-pressure
xenon arc lamp in commonly employed.
Lasers: Most commercial spectrofluorometers utilize
lamp sources because they are less expensive and
less troublesome to use.
Components of Fluorometers and
Spectrofluorometers



Filters and Monochromators: Both interface and
absorption filters have been used in fluorometers for
wavelength selection of both the excitation beam and
the resulting fluorescence radiation. Most
spectrofluorometers are equipped with at least one
and sometimes two grating monochromators.
Transducers: Photomultiplier tubes are the most
common transducers in sensitive fluorescence
instruments.
Cell and Cell Compartments: Both cylindrical and
rectangular cell fabricated of glass or silica are
employed for fluorescence measurements.
Fluorometer Schematic
Fluorometer Figure
Spectrofluormeter Schematic
Spectrofluormeter Figure
References
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http://www.acs.org
http://www.cas.org
http://www.chemcenter/org
http://www.sciencemag.org
http://www.kerouac.pharm.uky.edu/asrg/wave/wavehp.html
http://www.chemistry.msu.edu/courses/cem333/Chapter%20
15%20%20Molecular%20Luminescence%20Spectrometry.pdf
http://elchem.kaist.ac.kr/vt/chem-ed/spec/molec/mol-fluo.htm
http://www.shsu.edu/~chm_tgc/chemilumdir/chemiluminesce
nce2.html