Plasma electron density

When exposed to apower density of 10 W/cm, the tissue experiences an electric field of lO V/cm associated with the light. This field is considerably larger than the average Coulomb attraction between the electrons and the nuclei and causes a dielectric breakdown of the tissue to create a very large free electron density (plasma) of 10 cm in the focal volume of the laser beam in an extremely short period (less than hundreds of picoseconds). This high-density plasma strongly absorbs UV, visible and IR light which is called optical breakdown and leads to ablation. 

Above approximately 80 km, the prominent bulge in electron concentration is called the ionosphere. In this region ions are created from UV photoionization of the major constituents—O, NO, N2 and O2. The ionosphere has a profound effect on radio conmumications since electrons reflect radio waves with the same frequency as the plasma frequency, f = 8.98 x where 11 is the electron density in [147]. The... 

Figure C2.13.1. Electron energies and electron densities for different plasmas.

Central to the categorization of plasmas are electron temperature and electron density. Electrons have a distribution of energies, so it is useful to assume a MaxweUian distribution, in terms of electron energy, E, such that... 

Plasma Types. Eigure 1 (7—9) indicates the various types of plasmas according to their electron density and electron temperature. The colder or low electron energy regions contain cold plasmas such as interstellar and interplanetary space the earth s ionosphere, of which the aurora boreaUs would be a visible type alkaU-vapor plasmas some flames and condensed-state plasmas, including semiconductors (qv). 

Many sources of energy are used to excite samples to emit characteristic wavelengths for chemical identification and assay (91,92). Very high temperature sources can be employed but are not necessary. AH materials can be vaporized and excited with temperatures of only a few electron volts. The introduction of samples to be analyzed into high temperature or high density plasmas and thek uniform excitation often are problematic. 

Fig. 3.32. Basic principles of HF-plasma SNMS. Vhf, PHrarethe HF generator frequency and power, respectively, PArthe plasma gas (Ar) pressure 7e and the electron and plasma gas temperatures, respectively. Hpi = n is the plasma e, Ar" ) density. Bo the...

Flames are also plasmas, characterized by electron densities of about 10 /cm and electron energies of about 0.5 eV. Many excited species are present in the flame, namely free radicals, ions, excited atoms and molecules, and electrons [43]. Excited species that have been observed include O, OH, NH, NO, and CH [44]. 

Sawada, K., Brueckner, K. A., Fukuda, N., and Brout, R., Phys. Rev. 108, 507, "Correlation energy of an electron gas at high density plasma oscillations."... 

Microwave-Plasma Deposition. The operating microwave frequency is 2.45 GHz. A typical microwave plasma for diamond deposition has an electron density of approximately 10 electrons/m, and sufficient energy to dissociate hydrogen. A microwave-deposition reactor is shown schematically in Fig. 5.18 of Ch. 5.P ]P°]... 

Figure 12. Variation of the plasma parameters of a CH4/H2 plasma with pressure, (a) Plasma potential, (b) Electron temperature, (c) Electron density. Reprinted with permission from [88], K. Okada et al., /. Vac. Sci. TechnoL, A 17, 721 (1999). 1999, American Institute of Physics.

Figure 14. Variation of the plasma parameters of a CH4/CO/H2 plasma with [CO] content, (a) Plasma potential, (b) Electron temperature, (c) Electron density.

An external magnetic field has also been used to confine the plasma [143]. An arrangement where electromagnets are located under the cathode is known as the controlled plasma magnetron method [144]. The diffusion of electrons to the walls is prevented by the magnetic field between cathode and anode. This results in an increase in electron density, and consequently in a faster decomposition of silane and a higher deposition rate. At a deposition rate of 1 nm/s, device quality material is obtained [144]. In addition, a mesh is located near the anode, and the anode can by biased externally, both in order to confine the plasma and in order to control ion bombardment. 

The decrease of the silane partial pressure and the concomitant increase of the hydrogen partial pressure as a function of plasma power can be understood in terms of the increased electron density and electron energy. Both lead to a higher dissociation of silane and hydrogen. The silane radicals and atomic hydrogen thus... 

At the start of each modulation pulse, a sharp peak in optical emission is seen. Similar SiH emission peaks in pulsed plasmas have been found by Scarsbrook et al. [516] and Howling et al. [321]. The sharp peak was claimed to be caused by a pulse of high-energy electrons. Overzet and Verdeyen [517] measured electron densities at a 2.9-MHz excitation frequency and modulation frequencies up to 20 kHz. The optical emission of a SQWM argon plasma was measured by Booth et al. [518], who also performed particle-in-cell modeling. 

Figure 7. Schematic diagram of a flowing-afterglow electron-ion experiment. The diameter of flow tubes is typically 5 to 10 cm and the length is 1 to 2 meters. The carrier gas (helium) enters through the discharge and flows with a velocity of 50 to 100 m/s towards the downstream end of the tube where it exits into a fast pump. Recombination occurs mainly in the region 10 to 20 cm downstream from the movable reagent inlet, at which the ions under study are produced by ion-molecule reactions. The Langmuir probe measures the variation of the electron density in that region. A differentially pumped mass spectrometer is used to determine which ion species are present in the plasma.

The anomalous plasma decay suggests that the deionization coefficient is larger at higher electron densities and higher H2 densities. The second interpretation proposes that electrons and H2 molecules can act as stabilizing third bodies and that this process involves the same long-lived intermediate complexes that are observed in merged beam experiments (see Section IV.B). 

---