Laser Induced Fluorescence (LIF)

Contact: Dr Guillaume de Sercey

Theory

A laser beam (or usually a laser sheet in automotive applications) is used to excite the specie (a molecule or atom) of interest (see figure 1). For this, the laser has to be selected or tuned so that its wavelength matches an absorption band of the specie (discrete for atoms, broadband for molecules). By absorbing the laser beam photons, the specie transition from a ground electronic state to a short-lived excited electronic state. In order to return to a stable ground state the molecule can release the extra energy through:

  • Laser induced emission, a weak phenomenon that only lasts as long as the excitation.
  • Thermal transfer to other molecules / atoms via collisions. This is usually called quenching as it competes with the fluorescence.
  • Emit a photon, the fluorescence.
  • Intersystem crossing (not represented). This is a forbidden transition (in quantum theory this means its probability of occuring is very small) that requires very tightly controlled conditions to occur and leads to phosphorensce, another luminescence process which is very slow (compared to fluorescence).

The fluorescence lifetime is dependent on the specie but is usually in the order of tens or hundreds of nanoseconds for useful species. Due to collisions with other molecules / atoms, the states involved in the fluorescence usually differs from the states involved in the absorption. As a result,the fluorescence wavelength differs from the laser wavelength. It is usually shifted towards the red. This is a useful property that allows for easy separation of the laser and fluorescence light with filters.

The fluorescence intensity is directly proportional to the number of molecules / atoms in the excited state. Therefore it provides information on the specie concentration. However the excited state population is temperature dependent (Boltzman law). Furthermore the fluorescence competes with quenching which, as it is due to collisions, is dependent on pressure, temperature, and concentration of other species. As a result, laser induced is usually used to provide qualitative or semi-quantitative concentration species distribution. A technique has been developed at the University of Brighton to provide quantitative information.

LIF mechanism
LIF mechanism

Fuel concentration measurements

One of the most popular use of LIF in automotive research is for fuel concentration measurement. Standard gasoline and diesel readily fluoresce when excited by UV light. They are however extremenly sensitive to quenching and are therefore confined to distribution visualisation.

For better results, standard gasoline or diesel is commonly replaced by a non-fluorescing one, iso-octane in the case of gasoline. A tracer of known property is then added to the non-fluorescing. The most popular tracers for gasoline studies belong to the ketone family, namely acetone and 3-pentanone,and aldehyde family, due to their low sensitivity to quenching. Conversely, toluene, which is very sensitive to quenching has also proved popular as it has been demonstrated that its dependency on quenching is actually proportional to oxygen concentration and thus indirectly would reprensent air-to-fuel ratios.

Another popular family of tracers are the exciplex (short for excited complex) tracers. These are actually a combination of two tracers which have the property of fluorescing at different wavelength depending on whether they are in a liquid or vapour phase, thus allowing to distinguish between liquid and vapour concentration. The most popular exciplexes are the TMPD (N,N,N',N'-tetramethyl-p-phenylene-diamine) / naphthalene combination for diesel and TEA (Triethylamine) / benzene for gasoline.

Temperature measurement

As stated before the fluorescence signal is directly proportional to the population of the excited state. This population is itself directly dependent on the specie temperature, thus LIF can be used to obtain 2-dimensional maps of temperature. In order to remove quenching dependency, and thus to calibrate the system, a two-line (also called two-colours) method is usally used. With the two colour method, two different transitions of a specie (usually OH or NO) are excited with two different laser wavelengths. As both transitions have the same dependency on quenching the ratio of the fluorescence emitted by each is only dependent on temperature.

Selected Publications: