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Encyclopedia > Electric glow discharge
The central electrode of a plasma globe.
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The central electrode of a plasma globe.

Electric glow discharge is a type of plasma formed by passing a current at 100 V to several kV through a gas — usually argon or another noble gas. They are found in a variety of areas, including lighting (fluorescent lights), television (plasma-screen television), plasma physics, and analytical chemistry. Image File history File links Download high resolution version (978x768, 119 KB)Plasma Lamp PiccoloNamek July 8, 2005 22:21 (UTC) I, PiccoloNamek, took this picture. ... Image File history File links Download high resolution version (978x768, 119 KB)Plasma Lamp PiccoloNamek July 8, 2005 22:21 (UTC) I, PiccoloNamek, took this picture. ... A Plasma lamp, illustrating some of the more complex phenomena of a plasma, including filamentation A solar coronal mass ejection blasts plasma throughout the solar system. ... Josephson junction array chip developed by NIST as a standard volt. ... The volt is the SI derived unit for electric potential and voltage (derived from the ampere and watt). ... General Name, Symbol, Number argon, Ar, 18 Chemical series noble gases Group, Period, Block 18, 3, p Appearance colorless Atomic mass 39. ... For the musical band, see Noble Gas (band) The noble gases are the chemical elements in group 18 (old-style Group 0) of the periodic table. ...


See: fluorescent lamp, neon lamp, and plasma lamp To meet Wikipedias quality standards, this article or section may require cleanup. ... Lighting neon lamp, two 220/230 volt and 110 V neon lamps and a screwdriver with neon lamp inside A neon lamp is a gas discharge lamp containing neon gas (or in types with different colors also other noble gas) at low pressure. ... A plasma lamp The effect of a conducting object (such as a hand) in close proximity to the plasma globe glass The central electrode of a plasma globe Plasma lamps (also variously plasma globes, plasma balls, plasma domes, plasma spheres, or plasma orbs) are novelty items which were most popular...

Contents

Basic operating mechanism

The simplest type of glow discharge is a direct-current glow discharge. In its simplest form, it consists of two electrodes in a cell held at low pressure (1–10 torr). The cell is typically filled with argon. A potential of several hundred volts is applied between the two electrodes. A small population of atoms within the cell is initially ionized through random processes (collisions between atoms or with alpha particles, for example). The ions (which are positively charged) are driven towards the cathode by the electric potential, and the electrons are driven towards the anode by the same potential. The initial population of ions and electrons collides with other atoms, ionizing them. As long as the potential is maintained, a population of ions and electrons remains. The torr (symbol: Torr) or millimetre of mercury (mmHg) is a non-SI unit of pressure. ... An alpha particle is deflected by a magnetic field Alpha particles (named after the first letter in the Greek alphabet, α) are a highly ionizing form of particle radiation which have low penetration. ...


The ions strike the cathode, losing their kinetic energy to it. Collisions within the cathode redistribute this energy until a portion of the cathode is ejected, typically in the form of free atoms. This process is known as sputtering. Once free of the cathode, atoms move into the bulk of the glow discharge through drift and due to the energy they gained from sputtering. The atoms can then be collisionally excited. These collisions may be with ions, electrons, or other atoms that have been previously excited by collisions with ions or atoms. Once excited, atoms will lose their energy fairly quickly. Of the various ways that this energy can be lost, the most important is radiatively, meaning that a photon is released to carry the energy away. In atomic spectroscopy, the wavelength of this photon can be used to determine the identity of the atom (that is, which chemical element it is) and the number of photons is directly proportional to the concentration of that element in the sample. Some collisions (those of high enough energy) will cause ionization. In mass spectrometry, these ions are detected. Their mass identifies the type of atoms and their quantity reveals the amount of that element in the sample. Atomic spectroscopy is the determination of elemental composition by its electromagnetic or mass spectrum. ... The periodic table of the chemical elements A chemical element, often called simply an element, is a substance that cannot be decomposed or transformed into other chemical substances by ordinary chemical processes. ... Basic schematic of a mass spectrometer Mass spectrometry (also known as mass spectroscopy (deprecated)[1] or in common speech mass-spec) is an analytical technique used to measure the mass-to-charge ratio of ions. ...

An electric glow discharge tube featuring its most important characteristics: (a) An anode and cathode at each end (b) Aston dark space (c) Cathode glow (d) Cathode dark space (also called Crookes dark space, or Hittorf dark space) (e) Negative glow (Cathode glow) (f) Faraday space (g) Positive column (h) Anode glow (i) Anode dark space (Positive glow) Image File history File links Download high-resolution version (922x383, 20 KB) An electric glow discharge tube, featuring its most important characteristics: An anode and cathode at each end Aston dark space Cathode glow Cathode dark space (also called Crookes dark space, or Hittorf dark space) Negative glow (Cathode glow... Diagram of a zinc anode in a Daniells cell. ... Diagram of a copper cathode in a Daniells cell. ... When a cathode is bombarded by positive ions coming mainly from the negative glow region, where they are created by collisions with fast electrons. ...

Use in analytical chemistry

Glow discharges can be used to analyze the elemental, and sometimes molecular, composition of solids, liquids, and gases, but elemental analysis of solids is by far the most common. In this arrangement, the sample is used as the cathode. As mentioned earlier, gas ions and atoms striking the sample surface knock atoms off of it (a process known as sputtering). The sputtered atoms, now in the gas phase, can be detected by atomic absorption, but this is a comparatively rare strategy. Instead, atomic emission and mass spectrometry are usually used. Collisions between the gas-phase sample atoms and the plasma gas pass energy to the sample atoms. This energy can excite the atoms, after which they can lose their energy through atomic emission. By observing the wavelength of the emitted light, the atom's identity can be determined. By observing the intensity of the emission, the concentration of atoms of that type can be determined. Energy gained through collisions can also ionize the sample atoms. The ions can then be detected by mass spectrometry. In this case, it is the mass of the ions that identified the element and the number of ions that reflects the concentration


Operating modes

In analytical chemistry, glow discharges are most often operated in direct-current mode. For this mode, the cathode (which is the sample in solids analysis) must be conductive. The potential, pressure, and current are interrelated. Only two can be directly controlled at once, while the third must be allowed to vary. Typically, the pressure is held constant. The other constant parameter depends on the application.


Glow discharges may also be operated in radio-frequency mode. In this mode, a sine wave current of radio-frequency is applied to the cathode. Because alternating currents can pass through non-conductive materials, this allows sampling of such materials. In trigonometry, an ideal sine wave is a waveform whose graph is identical to the generalized sine function y = Asin[ω(x − α)] + C, where A is the amplitude, ω is the angular frequency (2π/P where P is the wavelength), α is the phase shift, and C...


Both radio-frequency and direct-current glow discharges can be operated in pulsed mode, where the potential is turned on and off. This allows higher instantaneous powers to be applied without excessively heating the cathode. These higher instantaneous powers produce higher instantaneous signals, aiding detection. Combining time-resolved detection with pulsed powering results in additional benefits. In atomic emission, analyte atoms emit during different portions of the pulse than background atoms, allowing the two to be discriminated. Analogously, in mass spectrometry, sample and background ions are created at different times.


See also


  Results from FactBites:
 
Electric glow discharge - Wikipedia, the free encyclopedia (1375 words)
Electric glow discharge is a type of plasma formed by passing a current at 100 V to several kV through a gas — usually argon or another noble gas.
The ions (which are positively charged) are driven towards the cathode by the electric potential, and the electrons are driven towards the anode by the same potential.
Glow discharges may also be operated in radio-frequency mode.
Atmospheric pressure gas glow discharge - Patent 5405514 (5398 words)
The glow discharge according to this invention may also be used to activate non-hydrocarbon gases, such as hydrogen, nitrogen, oxygen, ammonia, or silanes in molecular form to produce the corresponding atoms, ions, or excited species, such as free radicals.
Glow discharges provide much higher energy efficiencies than arcs since glow discharges are nonequilibrium devices operated at much lower temperatures with ionization produced by electric field acceleration of electrons rather than by thermal ionization, thereby consuming much less heat capacity energy and greatly reducing energy loss due to radiation.
The glow discharge may be initiated by igniting a low pressure air or nitrogen discharge, at less than about 0.1 atmosphere, and then adding the desired reactant gas and increasing the pressure to the desired pressure with flow of reactant gas and turning off the nitrogen flow.
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