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 a source of light in devices such as neon
lights, 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.
1 Electrical conduction in gas
Glow discharge mechanism
2.1 Secondary emission
2.2 Light production
2.4.1 Aston dark space
Cathode dark space
2.4.4 Negative glow
2.4.5 Faraday dark space
2.5.1 Positive column
Anode dark space
2.8 Carrier gas
2.9 Color difference
3 Use in analytical chemistry
3.1 Depth analysis
4 Powering modes
5 Application to analog computing
6 Application to voltage regulation
7 See also
9 Further reading
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
F-H region: glow discharge; the plasma emits a faint glow.
I-K region: arc discharge; large 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.
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. 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
Below the breakdown voltage there is no or little 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
increase 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
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). A low pressure is used to increase the mean
free path; for the 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.
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
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.
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
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
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
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 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. 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
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.
Faraday dark space
As the electrons keep losing energy, less light is emitted, resulting
in another dark space.
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
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. For example, with a neon sign, the positive column occupies
almost the entire length of the tube.
An electric field increase results in the anode glow.
Anode dark space
Fewer electrons result in another dark space.
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
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.
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 continuously remove undesired ions and atoms.
In the context of sputtering, the gas in the tube is called "carrier
gas," because it carries the particles from the cathode.
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
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.
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
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.
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
DC powered neon lamp, showing glow discharge surrounding only the
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
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
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
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. According to a Nature
news article describing the work, 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
Electric arc discharge
Fluorescent lamp, neon lamp, and plasma lamp
List of plasma (physics) articles
^ Fridman, Alexander (2011).
Plasma physics and engineering. Boca
Raton, FL: CRC Press. ISBN 978-1439812280.
^ Principles of Electronics By V.K. Mehta ISBN 81-219-2450-2
^ a b c d e f g h i j Fridman, Alexander (2012). Plasma chemistry.
Cambridge: Cambridge University Press. p. 177.
^ 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.
doi:10.1051/jp4:1997420. ISSN 1155-4339. Retrieved June 19,
^ Csele, Mark (2011). "2.6 The Franck–Hertz Experiment".
Fundamentals of Light Sources and Lasers. John Wiley & Sons.
pp. 31–36. ISBN 9780471675228.
^ a b c Mavrodineanu, R. (1984). "Hollow
Cathode Discharges -
Analytical Applications" (PDF). Journal of Research of the National
Bureau of Standards. 89 (2): 147. doi:10.6028/jres.089.009.
ISSN 0160-1741. Retrieved June 14, 2017.
^ Claude, Georges (November 1913). "The Development of Neon Tubes".
The Engineering Magazine: 271–274. LCCN sn83009124.
^ Whitaker, Jerry (1999). Power vacuum tubes handbook, Second Edition.
Boca Raton: CRC Press. p. 94. ISBN 1420049658.
^ 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. doi:10.1039/B200589A.
^ Mini-map gives tourists neon route signs:
S. Flügge, ed. (1956). Handbuch der Physik/Encyclopedia of Physics
band/volume XXI - Electron-emission • Gas discharges I.
Springer-Verlag. First chapter of the article Secondary effects
by P.F. Little.
R. Kenneth Marcus (Ed.) (1993). Glow Discharge Spectroscopies. Kluwer
Academic Publishers (Modern Analytical Chemistry).
ISBN 0-306-44396-1. CS1 maint: Extra text: authors list
Quadrupole mass filter
Quadrupole ion trap
Microchannel plate detector