Generally, LIF is a method that excites specific kinds of atoms or molecules and detects the resulting luminescence. The density of the atoms or molecules can be measured from the intensity of the excitation spectrum obtained using this procedure. In addition, the temperature can be measured based on the spectral distribution. LIF has become a major basic spectroscopic technology along with the development of tunable laser technology.
LIF makes it possible to perform high-sensitivity spatial and time-resolved measurements, and it has various practical applications, such as the analysis of reaction mechanisms by detecting intermediate products.
The spectrum that is obtained from observation of the light absorption when the wavelength of an incident laser light is changed is called the absorption spectrum. However, a precise observation of absorption spectra is very difficult, because light changes in the intensity should be detected. On the other hand, the spectrum that is obtained from observation of the fluorescence intensity caused by the radiative transition due to photoexcitation of atoms or molecules is called the fluorescence excitation spectrum.
LIF observes the fluorescence excitation spectrum and obtains the ground state distribution of the atoms or molecules
LIF observes the fluorescence emittedspontaneously from excited molecules that were promoted to the excited state throughthe resonant transition of atoms and molecules.
The chart on the left shows the excitation,
emission, and spontaneous emission
processes in a simple system consisting of a
ground state and one excited state. The
fluorescence intensity can be described as
in Eq .1.
In Eq.1, the meaning of each symbol is as follows:
A, B: Einstein A and B coefficients, respectively
Q: Nonradiative transition rate constant
c: Speed of light
IO: Intensity of the exciting light
NT: Number of atoms or molecules in the ground state prior to excitation
Eq.1 shows that NT can be calculated by observing the IFL. When the intensity of the exciting light is weak, Eq.2 can be adapted, and the ILF is proportional to the intensity of the exciting light.
If the exciting light is very intense, the intensity of the fluorescence is independent of IO. IFL in this state is called the saturated fluorescence. Except in special cases, LIF measurements are performed in a state without saturation.
As shown in Eq.1, the intensity of the fluorescence depends on the nonradiative transition rate constant, namely, the quantum efficiency (A/(A＋Q)). Therefore, the number of molecules in the ground state can be obtained from the intensity of the fluorescence, and the integration of the observed data at different excitation wavelengths gives the spectrum.
An excitation spectrum is obtained using this procedure. A nonradiative transition is the process of returning to the ground state from an excited state without emitting fluorescence. Various processes are involved in nonradiative transitions, e.g., collisions with coexisting gas atoms or molecules and the level structure of the molecule itself. Consequently, the excitation spectrum and the absorption spectrum don’t always coincide. In particular, OH is a common radical in combustion systems, and many practical examples of its measurements have been reported. However, most of these reports include only relative molecule density measurements. To perform absolute measurements, the specification of the measured area (the sheet light area in PLIF) is necessary.
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