Chapter 15 Molecular Luminescence Spectrometry

Molecular Fluorescence  Optical emission from molecules that have been excited to higher energy levels by absorption of electromagnetic radiation. Dkkdj

Photoluminescence  Light is directed onto a sample, where it is absorbed and imparts excess energy into the material in a process called "photo- excitation." 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 to s. 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       15%20- %20Molecular%20Luminescence%20Spectrometry.pdf   nce2.html