Laser-induced fluorescence (LIF) or laser-stimulated fluorescence (LSF) is a spectroscopic method in which an atom or molecule is excited to a higher energy level by the absorption of laser light followed by

spontaneous emission Spontaneous emission is the process in which a quantum mechanical system (such as a molecule, an atom or a subatomic particle) transits from an excited energy state to a lower energy state (e.g., its ground state) and emits a quantized amount of ...

of light. It was first reported by

Zare The Zare is a river of Mecklenburg-Vorpommern, Germany. It flows into the Sude near Walsmühlen. See also *List of rivers of Mecklenburg-Vorpommern A list of rivers of Mecklenburg-Vorpommern, Germany: A * Aubach * Augraben, tributary of t ...

and coworkers in 1968. LIF is used for studying structure of molecules, detection of selective species and flow visualization and measurements. The wavelength is often selected to be the one at which the species has its largest cross section. The excited species will after some time, usually in the order of few nanoseconds to microseconds, de-excite and emit light at a wavelength longer than the excitation wavelength. This fluorescent light is typically recorded with a

photomultiplier tube Photomultiplier tubes (photomultipliers or PMTs for short) are extremely sensitive detectors of light in the ultraviolet, visible, and near-infrared ranges of the electromagnetic spectrum. They are members of the class of vacuum tubes, more specif ...

(PMT) or filtered photodiodes.

Types

Two different kinds of spectra exist, disperse spectra and excitation spectra. The disperse spectra are performed with a fixed lasing wavelength, as above and the fluorescence spectrum is analyzed. Excitation scans on the other hand collect fluorescent light at a fixed emission wavelength or range of wavelengths. Instead the lasing wavelength is changed. An advantage over absorption spectroscopy is that it is possible to get two- and three-dimensional images since fluorescence takes place in all directions (i.e. the fluorescence signal is usually isotropic). The

signal-to-noise ratio Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to the noise power, often expressed in deci ...

of the fluorescence signal is very high, providing a good sensitivity to the process. It is also possible to distinguish between more species, since the lasing wavelength can be tuned to a particular excitation of a given species which is not shared by other species. LIF is useful in the study of the electronic structure of molecules and their interactions. It has also been successfully applied for quantitative measurement of concentrations in fields like combustion,

plasma Plasma or plasm may refer to: Science * Plasma (physics), one of the four fundamental states of matter * Plasma (mineral), a green translucent silica mineral * Quark–gluon plasma, a state of matter in quantum chromodynamics Biology * Blood pla ...

, spray and

flow Flow may refer to: Science and technology * Fluid flow, the motion of a gas or liquid * Flow (geomorphology), a type of mass wasting or slope movement in geomorphology * Flow (mathematics), a group action of the real numbers on a set * Flow (psych ...

phenomena (such as molecular tagging velocimetry), in some cases visualizing concentrations down to nanomolar levels. LED-induced fluorescence has been used in situ to delineate aromatic hydrocarbon contamination as a cone penetrometer add on module and also as a percussive capable asset.

Applications

* Detection of purity * Optical tumor diagnosis * Imaging of paleontological specimens *Detection and quantification of

biomolecule A biomolecule or biological molecule is a loosely used term for molecules present in organisms that are essential to one or more typically biological processes, such as cell division, morphogenesis, or development. Biomolecules include large ...

s and biological processes (e.g.

DNA sequencing DNA sequencing is the process of determining the nucleic acid sequence – the order of nucleotides in DNA. It includes any method or technology that is used to determine the order of the four bases: adenine, guanine, cytosine, and thymine. Th ...

, trace protein analysis, polymerase chain reaction products, and single-cell analysis) *Measurement of ion distribution functions and velocity space diffusion and convection in a plasma

References

{{Lasers Fluorescence