Glow discharge

NE-2 type neon lamp powered by alternating current

A glow discharge is a plasma formed by the passage of electric current through a low-pressure gas. It is created by applying a voltage between two electrodes in a glass tube containing the low-pressure gas. When the voltage exceeds a certain value called the striking voltage, the gas in the tube ionizes, becoming a plasma, and begins conducting electricity, causing it to glow with a colored light. The color depends on the gas used.

Glow discharge is widely used as a source of light in devices such as neon lights, fluorescent lamps, and plasma-screen televisions. Analyzing the light produced with spectroscopy can reveal much about the atomic interactions in the gas, so glow discharge is used in plasma physics and analytical chemistry. It is also used in the surface treatment technique called sputtering.

Electrical conduction in gas

Voltage-current characteristics of electrical discharge in neon at 1 torr, with two planar electrodes separated by 50 cm.
A: random pulses by cosmic radiation
B: saturation current
C: avalanche Townsend discharge
D: self-sustained Townsend discharge
E: unstable region: corona discharge
F: sub-normal glow discharge
G: normal glow discharge
H: abnormal glow discharge
I: unstable region: glow-arc transition
J: electric arc
K: electric arc
A-D region: dark discharge; ionisation occurs, current below 10 microamps.
F-H region: glow discharge; the plasma emits a faint glow.
I-K region: arc discharge; larges amounts of radiation produced.

Conduction in a gas requires charge carriers, which can be either electrons or ions. Charge carriers come from ionizing some of the gas molecules. In terms of current flow, glow discharge falls between dark discharge and arc discharge.

Below the ionization current or breakdown voltage there is no glow, but as the current increases to the ionization point the Townsend discharge happens just as glow discharge becomes visible; this is the start of the normal glow range. As the current is increased above the normal glow range, abnormal glow begins. If the current is increased to the point the cathode glow covers the entire cathode arc discharge begins.[2]

Glow discharge 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 1/10000th to 1/100th of atmospheric pressure). 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

A Crookes tube illustrating the different glowing regions that make up a glow discharge and a diagram giving their names.

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 (i.e., 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.[3][4]

Aston dark space

Electrons leave the cathode with an energy of about 1 eV, which is not enough to ionize or excite atoms, leaving a thin dark layer next to the cathode.[3]

Cathode glow

Electrons from the cathode eventually attain enough energy to excite atoms. These excited atoms quickly fall back to the ground state, emitting light at a wavelength corresponding to the difference between the energy bands of the atoms. This glow is seen very near the cathode.[3]

Cathode dark space

As electrons from the cathode gain more energy, they tend to ionize, rather than excite atoms. Excited atoms quickly fall back to ground level emitting light, however, when atoms are ionized, the opposite charges are separated, and do not immediately recombine. This results in more ions and electrons, but no light.[3] This region is sometimes called Crookes dark space, and sometimes referred to as the cathode fall, because the largest voltage drop in the tube occurs in this region.

Negative glow

The ionization in the cathode dark space result in a high electron density, but slower electrons, making it easier for the electrons to recombine with positive ions, leading to intense light, through a process called bremsstrahlung radiation.[3]

Faraday dark space

As the electrons keep losing energy, less light is emitted, resulting in another dark space.[3]

Anode layer

The anode layer begin with the positive column, and ends at the anode. The anode layer has a negative space charge and a moderate electric field.[3]

Positive column

With fewer ions, the electric field increases, resulting in electrons with energy of about 2 eV, which is enough to excite atoms and produce light. With longer glow discharge tubes, the longer space is occupied by a longer positive column, while the cathode layer remains the same.[3] For example, with a neon sign, the positive column occupies almost the entire length of the tube.

Anode glow

An electric field increase results in the anode glow.[3]

Anode dark space

Fewer electron results in another dark space.[3]

Striations

Bands of alternating light and dark in the positive column are called striations. Striations occur because only discrete amounts of energy can be absorbed or released by atoms, when electrons move from one quantum level to another. The effect was explained by Franck and Hertz in 1914. Striations can be very annoying when they occur in fluorescent lamps.[5]

Sputtering

In addition to causing secondary emission, positive ions can strike the cathode with sufficient force to eject particles of the material from which the cathode is made. This process is called sputtering and it gradually ablates the cathode. Sputtering is useful when using spectroscopy to analyze the composition of the cathode, as is done in Glow-discharge optical emission spectroscopy.[6]

However, sputtering is not desirable when glow discharge is used for lighting, because it shortens the life of the lamp. For example, neon signs have hollow cathodes designed to minimize sputtering, and contain charcoal to remove continuously remove undesired ions and atoms.[7]

Carrier gas

In the context of sputtering, the gas in the tube is called "carrier gas," because it carries the particles from the cathode.[6]

Color difference

Because of sputtering occurring at the cathode, the colors emitted from regions near the cathode are quite different than the anode. Particles sputtered from the cathode are excited and emit radiation from the metals and oxides that make up the cathode. The radiation from these particles combines with radiation from excited carrier gas, giving the cathode region a white or blue color, while in the rest of the tube, radiation is only from the carrier gas and tends to be more monochromatic.[6]

Electrons near the cathode are less energetic than the rest of the tube. Surrounding the cathode is a negative field, which slows electrons as they are ejected from the surface. Only those electrons with the highest velocity are able to escape this field, and those without enough kinetic energy are pulled back into the cathode. Once outside the negative field, the attraction from the positive field begins to accelerate these electrons toward the anode. During this acceleration electrons are deflected and slowed down by positive ions speeding toward the cathode, which, in turn, produces bright blue-white bremsstrahlung radiation in the negative glow region.[8]

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 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 identify the element and the number of ions that reflect the concentration. This method is referred to as glow discharge mass spectrometry (GDMS) and it has detection limits down to the sub-ppb range for most elements that are nearly matrix-independent.

Depth analysis

Both bulk and depth analysis of solids may be performed with glow discharge. Bulk analysis assumes that the sample is fairly homogeneous and averages the emission or mass spectrometric signal over time. Depth analysis relies on tracking the signal in time, therefore, is the same as tracking the elemental composition in depth.

Depth analysis requires greater control over operational parameters. For example, conditions (current, potential, pressure) need to be adjusted so that the crater produced by sputtering is flat bottom (that is, so that the depth analyzed over the crater area is uniform). In bulk measurement, a rough or rounded crater bottom would not adversely impact analysis. Under the best conditions, depth resolution in the single nanometer range has been achieved (in fact, within-molecule resolution has been demonstrated).

The chemistry of ions and neutrals in vacuum is called gas phase ion chemistry and is part of the analytical study that includes glow discharge.

Powering modes

DC powered neon lamp, showing glow discharge surrounding only the cathode

In analytical chemistry, glow discharges are usually operated in direct-current mode. For direct-current, the cathode (which is the sample in solids analysis) must be conductive. In contrast, analysis of a non conductive cathode requires the use of a high frequency alternating current.

The potential, pressure, and current are interrelated. Only two can be directly controlled at once, while the third must be allowed to vary. The pressure is most typically held constant, but other schemes may be used. The pressure and current may be held constant, while potential is allowed to vary. The pressure and voltage may be held constant while the current is allowed to vary. The power (product of voltage and current) may be held constant while the pressure is allowed to vary.

Glow discharges may also be operated in radio-frequency. The use of this frequency will establish a negative DC-bias voltage on the sample surface. The DC-bias is the result of an alternating current waveform that is centered about negative potential; as such it more or less represent the average potential residing on the sample surface. Radio-frequency has ability to appear to flow through insulators (non-conductive 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. 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.

Application to analog computing

An interesting application for using glow discharge was described in a 2002 scientific paper by Ryes, Ghanem et al.[9] According to a Nature news article describing the work,[10] researchers at Imperial College London demonstrated how they built a mini-map that glows along the shortest route between two points. The Nature news article describes the system as follows:

To make the one-inch London chip, the team etched a plan of the city centre on a glass slide. Fitting a flat lid over the top turned the streets into hollow, connected tubes. They filled these with helium gas, and inserted electrodes at key tourist hubs. When a voltage is applied between two points, electricity naturally runs through the streets along the shortest route from A to B - and the gas glows like a tiny neon strip light.

The approach itself provides a novel visible analog computing approach for solving a wide class of maze searching problems based on the properties of lighting up of a glow discharge in a microfluidic chip.

Application to voltage regulation

A 5651 voltage-regulator tube in operation

In the mid-20th century, prior to the development of solid state components such as Zener diodes, voltage regulation in circuits was often accomplished with voltage-regulator tubes, which used glow discharge.

See also

References

  1. ↑ Fridman, Alexander (2011). Plasma physics and engineering. Boca Raton, FL: CRC Press. ISBN 978-1439812280.
  2. ↑ Principles of Electronics By V.K. Mehta ISBN 81-219-2450-2
  3. 1 2 3 4 5 6 7 8 9 10 Fridman, Alexander (2012). Plasma chemistry. Cambridge: Cambridge University Press. p. 177. ISBN 978-1107684935.
  4. ↑ Konjevic, N.; Videnovic, I. R.; Kuraica, M. M. (1997). "Emission Spectroscopy of the Cathode Fall Region of an Analytical Glow Discharge". Le Journal de Physique IV. 07 (C4): C4–247–C4–258. ISSN 1155-4339. doi:10.1051/jp4:1997420. Retrieved June 19, 2017.
  5. ↑ Csele, Mark (2011). "2.6 The Franck–Hertz Experiment". Fundamentals of Light Sources and Lasers. John Wiley & Sons. pp. 31–36. ISBN 9780471675228.
  6. 1 2 3 Mavrodineanu, R. (1984). "Hollow Cathode Discharges - Analytical Applications" (PDF). Journal of Research of the National Bureau of Standards. 89 (2): 147. ISSN 0160-1741. doi:10.6028/jres.089.009. Retrieved June 14, 2017.
  7. ↑ Claude, Georges (November 1913). "The Development of Neon Tubes". The Engineering Magazine: 271–274. LCCN sn83009124.
  8. ↑ Whitaker, Jerry (1999). Power vacuum tubes handbook, Second Edition. Boca Raton: CRC Press. p. 94. ISBN 1420049658.
  9. ↑ Reyes, D. R.; Ghanem, M. M.; Whitesides, G. M.; Manz, A. (2002). "Glow discharge in microfluidic chips for visible analog computing". Lab on a Chip. ACS. 2 (2): 113–6. PMID 15100843. doi:10.1039/B200589A.
  10. ↑ Mini-map gives tourists neon route signs: http://www.nature.com/news/2002/020527/full/news020520-12.html

Further reading

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