Laser-Induced Fluorescence (LIF)
The interaction of a laser beam with an atom, ion or molecule may results in excitation to a higher quantum state. A process of excitation is more likely to occur when the laser is tuned to the energy difference between this original lower state and an upper (excited) state. Such an event is accompanied by absorption of a photon and this process can be monitored as a reduction in a total laser incident beam intensity after propagation of studying volume.* Another indication of that process is detection of subsequent spontaneous emission radiation as the resulting excited state undergoes to the lower state - the process is known as Laser Induced Fluorescence (LIF).
Since energy of species’ quantum states and their populations are affected by the environment of the species absorbing the laser photons, the use of spectrally narrow enough laser allows nonintrusive reconstruction of these environmental effects by laser scanning within certain spectral range and measurement of LIF signal. Recorded dependence of the LIF signal versus laser frequency is called LIF excitation spectrum. Several different phenomena may affect the shape of LIF excitation spectra leading, in general, to broadening and shift. If the mechanism of broadening and shift is known, accurate LIF spectrum line shape measurements can be used to reveal the value of environmental parameter responsible for the effect.
In LPN-PPPL the LIF technique will be used to probe various chemical species present in a gas phase during the nanomaterial synthesis. Density and temperature of selected species will be measured as a function of synthesis arc operation mode, distance from electrodes and composition of arc discharge plasma. For instance, carbon atoms and molecules (C2 and C3) which are involved in the very first steps of the carbon-based nanostructures growth, will be probed to map their density and temperature distributions. Similarly, metal catalysts in a gas phase will be also probed to understand its role in nucleation. Such measurements can be done with very high temporal and spatial resolution, high accuracy and selectivity.
To summarize, the LIF diagnostic, as an in-situ technique, is able to provide data about the environment needed for nanomaterial synthesis and the very early stage of species’ condensation and nucleation. These data will be further used as an input for modeling of the nanomaterial synthesis and verification of theoretical efforts.
* Indeed, there is a finite probability that more than one photon can be absorbed (even cross-section for such process becomes much much less compared to single photon absorption) leading to another technique known as a Two-photon Absorption Laser Induced Fluorescence (TALIF). TALIF can be the only possibility to probe species with excitation energies lying in deep UV region.