Plasma from laser breakdown

Different analytical techniques are used for detection of the elemental composition of the solid samples. The simplest is direct detection of emission from the plasma of the ablated material formed above a sample surface. This technique is generally referred to as LIBS or LIPS (laser induced breakdown/plasma spectroscopy). Strong continuous background radiation from the hot plasma plume does not enable detection of atomic and ionic lines of specific elements during the first few hundred nanoseconds of plasma evolution. One can achieve a reasonable signal-to-noise ra-... 

The technique based on laser-induced breakdown coupled to mass detection, which should thus be designated LIB-MS, is better known as laser plasma ionization mass spectrometry (LI-MS). The earliest uses of the laser-mass spectrometry couple were reported in the late 1960s. Early work included the vaporization of graphite and coal for classifying coals, elemental analyses in metals, isotope ratio measurements and pyrolysis [192]. Later work extended these methods to biological samples, the development of the laser microprobe mass spectrometer, the formation of molecular ions from non-voIatile organic salts and the many multi-photon techniques designed for (mainly) molecular analysis [192]. 

A more suitable approach for on-line analysis is to view atomic emission directly from the small plasma formed above the sample surface when the sample is ablated using the laser. This approach, which is shown schematically in Fig. 20.12 has been applied to analyses of a variety of materials (e.g. [58-74]) and is known under a variety of names such as laser induced plasma spectrometry (LIPS), laser induced breakdown spectrometry (LIBS) and laser spark emission spectrometry (LASS). The laser used is typically a Nd YAG laser operating either at its fundamental, doubled or quadrupled frequency, although excimer and CO2 lasers have also been used. The important parameter of the laser is that it must be capable... 

Laser-induced breakdown is a phenomenon which occurs when the light is focused into a small volume and the electric field strength exceeds the breakdown threshold. From a quantum point of view, one may define the breakdown process as the formation of a plasma region due to multiphoton ionization processes. 

Principles and Characteristics Laser ablation is conceptually very simple, but mechanistically complicated. The process involves coupling of the photon energy of a laser pulse (typically about 20-30 ns wide, with an energy of 1-10 Jcm ) into the surface of a solid, resulting in evaporation and ejection of various species from the surface (the so-called plume ) within 10 to 10 s. The first experiments were carried out in 1962 [32]. When focused to a small area, a laser beam provides enormous power densities and electromagnetic fields. The plume , presumably a plasma, is accompanied by shock waves and electrical breakdown. The ejected material may eventually be deposited as a thin film. It is possible, by suitable selection of laser power and focus, to ablate a range of plastic materials in a controlled manner. For some matrices the polymer melts and diffuses away from the centre of the ablation site, leading to the forma-... 

Principles and Characteristics Simultaneous multi-element analysis based on emission from a plasma generated by focussing a powerful laser beam on a sample (solid, liquid, or gas) is known as laser-induced breakdown spectroscopy (LIBS) and under a variety of semantic variations time-resolved LIBS (TRELIBS), laser ablation emission spectroscopy (LAES), laser ablation atomic emission spectrometry (LA-AES), laser ablation optical emission spectrometry (LA-OES), laser plasma emission spectrometry (L-PES), laser-induced plasma spectroscopy (LIPS), laser spark spectroscopy (LSS), and laser-induced emission spectral analysis (LIESA ). Commercial LIBS analysers were already available in the 60/70s the technique now enjoys a renaissance. 

In the presence of a high-electric-fleld coldcathode, electron emission may be initiated by injecting a plasma into a vacuum gap from an external source or by bombarding either electrode with high-energy electrons, ions, or radiation from, for example, a laser beam. Any of these processes will lead to breakdown and the formation of a vacuum arc. 

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