Electric glow discharge

Electric glow discharge is a type of plasma formed by passing a current at 100V to several kV through a gas - usually argon or another noble gas. It is used in a variety of areas, including lighting (fluorescent lights), television (plasma-screen television), plasma physics, and analytical chemistry.

See: fluorescent lamp, neon lamp, and plasma lamp

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 (e.g. collisions between atoms or with alpha particles). 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 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 (i.e. what 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.

Operating modes

In science, glow discharges are most often operated in direct-current mode. For this mode, the cathode (which is the sample in analytical chemistry) 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, an sine wave of radio-frequency is applied to the cathode. Because alternating currents can pass through non-conductive materials, this allows sampling of such materials.

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. If time-resolved detection is used, it can also improve discrimination between analyte and background emission because they will peak at different points within or after the pulse.