Abstract
Recent developments in plasma science and technology have opened new areas of research both for fundamental purposes (e.g., description of key physical phenomena involved in laboratory plasmas) and novel applications (material synthesis, microelectronics, thin film deposition, biomedicine, environment, flow control, to name a few). With the increasing availability of advanced optical diagnostics (fast framing imaging, gas flow visualization, emission/absorption spectroscopy, etc.), a better understanding of the physicochemical processes taking place in different electrical discharges has been achieved. In this direction, the implementation of fast (ns) and ultrafast (ps and fs) lasers has been essential for the precise determination of the electron density and temperature, the axial and radial gradients of electric fields, the gas temperature, and the absolute density of ground-state reactive atoms and molecules in non-equilibrium plasmas. For those species, the use of laser-based spectroscopy has led to their in situ quantification with high temporal and spatial resolution, with excellent sensitivity. The present review is dedicated to the advances of two-photon absorption laser induced fluorescence (TALIF) techniques for the measurement of reactive species densities (particularly atoms such as N, H and O) in a wide range of pressures in plasmas and flames. The requirements for the appropriate implementation of TALIF techniques as well as their fundamental principles are presented based on representative published works. The limitations on the density determination imposed by different factors are also discussed. These may refer to the increasing pressure of the probed medium (leading to a significant collisional quenching of excited states), and other issues originating in the high instantaneous power density of the lasers used (such as photodissociation, amplified stimulated emission, and photoionization, resulting to the saturation of the optical transition of interest).
Highlights
Following the construction of the first operating laser in 1960 by Theodore H
For laser intensities well–below than 18 MW·cm−2, Kr atom can be used as a calibrating species for measuring by means of ps–two-photon absorption laser induced fluorescence (TALIF) the absolute densities and quenching coefficients of H and N atoms in plasmas and flames
To infer the quenching rate and effective lifetime of species from the fluorescence signal, the use of adequate models can be helpful on the correction of the TALIF signal [32,65,106]
Summary
Following the construction of the first operating laser in 1960 by Theodore H. Commercial lasers are widely used as multimode light sources in light shows for public entertainment. They have been wellestablished in metallurgical industry for welding and cutting of different metals [4,5]. Their implementation in many other aspects of everyday life is well-known, including laser projectors, printers, pointers, lithography, micromachining, medical lasers for aes-. The reliable use of fast (ns) and ultrafast (ps/fs) lasers for the measurement of atomic species absolute densities in plasmas and flames is demonstrated in Section 3 (representative examples from the literature are discussed).
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