A glow discharge is a plasma formed by the passage of electric current through a gas. It is often created by applying a voltage between two electrodes in a glass tube containing a low-pressure gas. When the voltage exceeds a value called the striking voltage, the gas ionization becomes self-sustaining, and the tube glows with a colored light. The color depends on the gas used. Glow discharges are used as sources of light in devices such as neon lights, cold-cathode fluorescent lamps, and plasma-screen televisions. Analyzing the light produced with spectroscopy can reveal information about the atomic interactions in the gas, so glow discharges are used in plasma physics and analytical chemistry. They are also used in the surface-treatment technique called sputtering.
Electrical conduction in gas
Conduction in a gas requires charge carriers, which can be either electrons or ions. Charge carriers come from ionizing some of the gas molecules. Depending on the amount of current passing through the gas, the ionization occurs by one of three different mechanisms: dark discharge, glow discharge, and arc discharge.
In a dark discharge, the gas is ionized (the carriers are generated) by a radiation source such as ultraviolet light or cosmic rays. At higher voltages across the anode and cathode, the freed carriers can gain enough energy so that additional carriers are freed during collisions; the process is a Townsend avalanche or multiplication. In a glow discharge, the carrier-generation process reaches a point where the average electron leaving the cathode allows another electron to leave the cathode. Thus the discharge becomes self-sustaining. For example, the average electron may cause dozens of ionizing collisions via the Townsend avalanche; the resulting positive ions head toward the cathode, and a fraction of those that cause collisions with the cathode will dislodge an electron by secondary emission. In an arc discharge, electrons leave the cathode by thermionic emission and field emission, and the gas is ionized by thermal means. Below the breakdown voltage there is little-to-no glow and the electric field is uniform. When the electric field increases enough to cause ionization, the Townsend discharge starts. When a glow discharge develops, the electric field is considerably modified by the presence of positive ions; the field is concentrated near the cathode. The glow discharge starts as a normal glow. As the current is increased, more of the cathode surface is involved in the glow. When the current is increased above the level where the entire cathode surface is involved, the discharge is known as an abnormal glow. If the current is increased still further, other factors come into play and an arc discharge begins.
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 (0.1–10 torr; about 1/10000 to 1/100 of atmospheric pressure). A low pressure is used to increase the mean free path; for a fixed electric field, a longer mean free path allows a charged particle to gain more energy before colliding with another particle. The cell is typically filled with neon, but other gases can also be used. An electric potential of several hundred volts is applied between the two electrodes. A small fraction of the population of atoms within the cell is initially ionized through random processes, such as thermal collisions between atoms or by gamma rays. The positive ions 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, exciting or ionizing them. As long as the potential is maintained, a population of ions and electrons remains.
Secondary emission Some of the ions' kinetic energy is transferred to the cathode. This happens partially through the ions striking the cathode directly. The primary mechanism, however, is less direct. Ions strike the more numerous neutral gas atoms, transferring a portion of their energy to them. These neutral atoms then strike the cathode. Whichever species (ions or atoms) strike the cathode, collisions within the cathode redistribute this energy, resulting in electrons ejected from the cathode. This process is known as secondary electron emission. Once free of the cathode, the electric field accelerates electrons into the bulk of the glow discharge. Atoms can then be excited by collisions with ions, electrons, or other atoms that have been previously excited by collisions.
Light production 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 optical 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 atomic 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.
Regions
The illustrations to the right shows the main regions that may be present in a glow discharge. Regions described as "glows" emit significant light; regions labeled as "dark spaces" do not. As the discharge becomes more extended (that is, stretched horizontally in the geometry of the illustrations), the positive column may become striated. That is, alternating dark and bright regions may form. Compressing the discharge horizontally will result in fewer regions. The positive column will be compressed while the negative glow will remain the same size, and, with small-enough gaps, the positive column will disappear altogether. In an analytical glow discharge, the discharge is primarily a negative glow with dark region above and below it.
Cathode layer The cathode layer begins with the Aston dark space, and ends with the negative glow region. The cathode layer shortens with increased gas pressure. The cathode layer has a positive space charge and a strong electric field.
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