Laser-induced fluorescence
Encyclopedia
- For other uses, see LIFLIFLIF, LiF or Lif may stand for:* Leukemia inhibitory factor, a cytokine that affects cell growth and development* Laser-induced fluorescence, a spectroscopic method* Low insertion force sockets* Leirvík ÍF a Faroese football club...
.
Laser-induced fluorescence (LIF) is a spectroscopic
Spectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...
method used for studying structure of molecules, detection of selective species and flow visualization and measurements.
The species to be examined is excited
Excited state
Excitation is an elevation in energy level above an arbitrary baseline energy state. In physics there is a specific technical definition for energy level which is often associated with an atom being excited to an excited state....
with a laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
. The wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
is often selected to be the one at which the species has its largest cross section
Cross section (physics)
A cross section is the effective area which governs the probability of some scattering or absorption event. Together with particle density and path length, it can be used to predict the total scattering probability via the Beer-Lambert law....
. 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 (PMT).
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
Absorption spectroscopy
Absorption spectroscopy refers to spectroscopic techniques that measure the absorption of radiation, as a function of frequency or wavelength, due to its interaction with a sample. The sample absorbs energy, i.e., photons, from the radiating field. The intensity of the absorption varies as a...
is that it is possible to get two- and three-dimensional images since fluorescence
Fluorescence
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. It is a form of luminescence. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation...
takes place in all directions (i.e. the fluorescence signal is isotropic). The signal-to-noise ratio
Signal-to-noise ratio
Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...
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
Combustion
Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame...
, plasma, spray and flow
Fluid mechanics
Fluid mechanics is the study of fluids and the forces on them. Fluid mechanics can be divided into fluid statics, the study of fluids at rest; fluid kinematics, the study of fluids in motion; and fluid dynamics, the study of the effect of forces on fluid motion...
phenomena (such as Molecular tagging velocimetry
Molecular tagging velocimetry
Molecular Tagging Velocimetry is a specific form of flow velocimetry, a technique for determining the velocity of currents in fluids such as air and water. In its simplest form, a single "write" laser beam is shot once through the sample space...
), in some cases visualizing concentrations down to nanomolar levels.
See also
- photomultiplierPhotomultiplierPhotomultiplier tubes , members of the class of vacuum tubes, and more specifically phototubes, are extremely sensitive detectors of light in the ultraviolet, visible, and near-infrared ranges of the electromagnetic spectrum...
- Planar laser-induced fluorescencePlanar laser-induced fluorescencePlanar laser-induced fluorescence is an optical diagnostic technique widely used for flow visualization and quantitative measurements. PLIF has been shown to be used for velocity, concentration, temperature and pressure measurements.- Working :...
- Ultra fast laser spectroscopyUltra fast laser spectroscopyUltra-fast laser spectroscopy is the study of molecules on extremely short time scales after their excitation with a pulsed laser. This method is used extensively to examine the energy states and electron dynamics of any molecule whose reaction to light is of interest...