Glow Discharges
Glow Discharges in low-pressure gases.
Plasma phenomena are very widespread in our surroundings. You can see them in nature, in everyday life, in your laboratory and in industry. Sun and other stars, aurora, neon lamps in the streets or plasma display of your TV set, ICP or laser in laboratory and so on. You may focus no attention on them. But all this is plasma phenomena. And it is very important and interesting thing to investigate.
They are very different and by their characteristics can be divided into many types. You can see a scheme, illustrating types of plasma with their parameters on the picture below. Left Y axis is logarithm of plasma temperature, evaluated in K. Right Y axis is also temperature, but it has energetic meaning (corresponds with particles energies) and evaluated in eV. High X axis shows so called “plasma frequency”(density of charges divided on density of total gas ).Low X axis shows logarithm of ions density (number of ions in a unit of volume). You can see here not only laboratory plasma (ICP, laser…) parameters, but also characteristics of your everyday plasma. For example, you can find the temperature of solar corona or temperature during supernova explosion, ions densities in flames or in ionosphere. You can compare parameters of solar corona, atmosphere and sun core. Even metals can be seen here! It seems strange, isn’t it? But remember, that metals have free electrons, forming so called “electron gas”.
Glow discharge plasma, the type we interested in most of all, is only the small part of all plasma phenomena.
The schematics above is from: R. Redmer, Phys. Reports 282, 35 (1997)
So what are the typical parameters of GD? Here they are:
| Pressure |
0,01 - 10 Torr |
| Dimensions |
0,1 – 10 cm |
| Voltage |
100 – 2000 V |
| Current |
0,1 – 100 mA |
| Temperature |
300 – 1000 K |
| Charged particle density |
106 – 1013 cm-3
|
| Plasma electrons (energy) |
0,1 – 1 eV |
| Plasma ions |
kT gas |
| Ions at cathode |
1 – 1000 eV |
| Low ionisation degree |
10-7 – 10-4 |
Different kinds of particles exist in glow discharge plasma: electrons, ions, metastables, excited atoms and so on. But if we interested in electrical effects in glow discharge, we should concern the charged particles most of all.
The particles move under the influence of electric and magnetic fields
and experience many collisions with the background gas.
Collisions.
- Elastic Total kinetic energy is conserved.
- e + Ar → e + Ar
- Inelastic Part of the kinetic energy is converted to internal energy.
- Excitation
- e + Ar → e + Ar* leading to light emission
- Ionisation
-
e + Ar → e + Ar+ + e leading to charge production
1. All types of collisions have their own characteristics – cross sections (σ)
Hard sphere cross section
But reality is more complicated
Cross sections give the probability of collision processes. Compare to electrons velocity, atoms are too slow that’s why they assumed stabile but not like a solid ball. Through strong electrostatic forces there are interactions between atoms, electrons and ions occur. Atoms have electrons, ions and also empty space if electrons approaching to collision. Electrons characteristics like directions, velocities effect these interactions and also caused probability of collisions with the gas density per unit volume (n) and effective collisions area of each molecule (q).
If ideal gas atom has 3 Å radius (or 2.8 10-15 cm2 area) and electron radius is 2.8179 × 10-13 cm, collision has also almost 3 Å radius and this will sweep out a volume of gas as the atom moves. Total probability of collisions has a value between 0-1 this means there is a collisions or not but there is also different collisions according to electrons energy so probability of that interactions depends on, number of the collisions in unit time and number of the interaction in the same time interval. In hard core model, collisions diameter is equal to summation of projectile and target spheres diameters (see figure above).
Cross sections give the probability of collision processes.
On this figure you can see the order of magnitude for cross sections in argon and its depending on energy. But also you can see what processes can be caused in these conditions. So another parameter that helps us to describe the motion of particles and processes in plasma is energy, gained between collisions. It defines not only by cross sections but also by electrical conditions (electric field intensity E) and density of particles (n).
2.Energy gain between collisions:
So we can characterize the motion of particles by E/n.
First published on the web: 09.12.2007
Authors: Anna Kravchenko and Hakan Candan. The text is based on a lecture given by Zoltan Donko, RISSP Budapest, at the first
Gladnet training course in Antwerp Sept. 2007
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Introduction to Glow Discharges
The development and application of glow discharge (GD) devices
and associated spectroscopies for chemical analysis began
as early as the 1930s and 1940s. However, it is since the
1960s and 1970s that Glow Discharge (GD) became a major focus
of research in many analytical chemistry laboratories. The
research work has been directed towards glow discharge source
development and the fundamental study of the physical characteristics
of the analytical glow discharge, and towards the development
and application of glow discharge spectroscopic techniques
for various analytical applications ranging from plasma etching
and deposition systems in the micro-electronics industry to
lasers or even plasma monitors. Hence, GD devices have become
well-known for their ability to directly analyse solid material.
More precisely, GD devices have been used for more than bulk
analysis and are proving to be diverse tools for the in-depth
analysis of layered materials.
The glow discharge owes its name to the luminous
glow of the plasma. When a sufficiently strong electric field
reigns in a gaseous medium, atoms and molecules in the medium
will break down electrically, permitting current to flow.
The initial break down is created by free electrons generated
by collisions with cosmic particles constantly bombarding
the earth. These free electrons are accelerated in the electric
field. If they gain sufficient energy to cause ionisation
of neutral gas atoms, a chain reaction starts creating more
and more free charges.
The simplest glow discharge configuration consists of two
parallel elecetrode plates being held on different electrical
potential. One electrode is called cathode and is negatively
charged, the other is the anode; it is on positive
potential.Once the glow discharge is established the potential
drops rapidly close to the cathode, varies slowly in the plasma,
and changes again close to the anode. Consequently, the electric
field is strong in the vincinity of the cathode (Cathode Dark
Space, CDS) and the anode (Anode Zone).
The plasma, or more precisely the negative glow (NG) is
virutually field free. The electric fields in the system are
restricted to sheaths adjacent to each of the electrodes.
The sheath fields repel electrons, having a much higher mobilitw
then the ions, trying to reach either electrode. In fact,
the plasma potential is always higher then the adjacent walls,
thus reducing the electron loss rate towards the walls. Electrons
originating at the cathode will be accelerated, collide, ionise,
transfer energy, "dissapear" by recombination with
a positively charged particle. Some reach the anode and get
transferred into the outside circuit.
The luminous glow is produced because the electrons have
sufficient energy to generate visible light by excitation
collisions with the plasma carrier gas. Since there is a continuous
loss of electrons and ions there must be an equal degree of
ionisation going on to maintain the steady state. The energy
is being continuously transferred out of the discharge and
hence the energy balance must be satisfied as well. Simplistically,
the electrons absorb energy from the field by accelerating,
ionise some atoms, and the process becomes continuous. Additional
electrons must be produced by secondary
electron emission from the cathode. These are very important
to maintaining a sustainable discharge. Three major regions
can be distinguished in the discharge: the cathode region,
the glow regions, and the anode region.
The Aston Dark Space (A) is a thin region close to
the cathode. The electical field is strong in this region
accelerating the electrion away from the cathode. The Aston
dark space has a negative space charge, meaning that electrons
outnumber the positive ions in this region. The electron density
and energy is too low to efficiently excite the gas, it consequently
appears dark.
In the Cathodic Glow, (B) next to the Aston dark space,
the electrons are energetic enough to excite the neutral atoms
during collisions. The cathode glow has a relatively high
ion density. The cathodic glow sometimes masks the Aston dark
space as it approaches the cathode very closely. The axial
length of the cathode glow depends on the carrier gas, the
pressure and temperature.
The Cathode (Crooks, Hittorf) dark space (C) is a
relatively dark region that has a strong electric field, a
positive space charge and a relatively high ion density. Its
axial extension depends on the pressure and the applied voltage.
For discharges operating at a few hPa its lenght is about
0.5 mm. In this region the electrons are accelerated by the
electric field. Positive ions are accelerated towards the
cathode. They cause the pulverisation of the cathode material
and the emission of secondary electrons. These electrons will
be accelerated and cause the creation of new ions through
collision with neutrals. The majority potential difference
between the two electrodes is across a narrow region surrounding
the cathode. Hence, the CDS is also called 'cathode fall'.
The Negative Glow NG (D) is the brightest intensity
of the entire discharge. It extends typically for about 2-3
mm away from the sample. Electrons carry almost the entire
current in the negative glow region. Electrons that have been
accelerated in the cathode region to high speeds produce ionisation,
and slower electrons
that have had inelastic collisions already produce excitations.
The negative glow is predominantly generated by the slow electrons,
however other processes play a significant role. The NG is
the region where most exciting and ionising collision processes
occur because of the high density for both negative and positive
charged particelsin this area. Hence, this zone is the source
of light used in GD-OES and allows acquiring most analytical
information. In the presence of argon ions, electrons can
determine the space charge. Indeed, in this region, positive
and negative space charges are equal to each other, resulting
in charge neutrality. However, electrical current in NG is
predominantly carried by the electrons, due to their high
mobility. At the end of the negative glow, the electrons have
lost most of their energy, exctitaion and ionisation proceses
cease to exist. This is the start of the next dark region.
The Faraday dark space (E) separates the negative
glow from the positive column. The electron energy is low
in this region. The net space charge is very low, and the
axial electric field is small.
The Positive Column (F) is a luminous region that
prolongs the negative glow to the anode. It has a low net
charge density, only a small electric field of typically 1
V/cm. The electric field is just large enough to maintain
the degree of ionisation to reach the anode. As the length
of the discharge tube is increased at constant pressure, the
cathode structures do not change in size. It is the positive
column that lengthens to form a long, uniform glow region.
The uniformity can easily been perturbed and except when standing
or moving striations observed.
The Anodic glow (G)is slightly brighter than the positive
column. It is not always observed. The anode glow is the boundary
of the anode sheath.
The Anode dark space (H) or anode sheath is the space
between the anode glow and the anode itself.
It has anegative space net charge density due to electrons
traveling towards the anode. The electric field is higher
than in the positive column.
First published on the web: 17 October 2006.
Authors: Lydie Salsac & Thomas Nelis
This article is based on the master works of Lydie
Salsac and Anouar Kanzari. Both have gained their master degree
at INSTN,
Saclay, France, after performing their master work at EMPA Material
Science and Technology, Thun, Switzerland
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Glow Discharge Processes
When glow discharges emit light, this is a sign that many
atoms in the plasma are excited and possibly also ionised.
In a glow discharge used for analytical purposes, the excitation
of sputtered atoms is of particular interest, as these excited
atoms inform the observer that this material was present in
the analysed sample. In a glow discharge sputtered sample
atoms can be excited or ionised via different inelastic collisions,
either with electrons or with atoms and ions having sufficient
energy. When dealing with glow discharges, it is important
to understand that the collision processes occurring in the
GD can be complex. The complexity lies less in each of the
processes as in the number of possible processes that may
occur simultaneously, each varying the number densities of
the involved species. In the following paragraph only the
main ionisation processes will be presented . Other collision
processes such as excitation, de- excitation and recombination
processes are not presented on this page.
Depending on the way how energy is transferred between the
collision partners, collisions processes can be divided into
two groups:
In the first group of processes only kinetic energy is transferred
to an atom resulting in ionisation
The second kind processes in which potential energy in addition
to the kinetic energy is transferred to an atom resulting
in ionisation.
Both types of processes are inelastic collisions. Elastic
collision are also important processes in discharges as they
change the energy distribution of the involved particles.
Elastic collision are an important step in the thermalisation
process.
In the schematic representation , the main ionisation
processes are given.
e- is the electron
Ar is the argon atom. It is the discharge carrier gas.
Arm is the argon atom in its metastable electronic
state.
M is the analyte atom, a sputtered from the sample surface
and M0 represents the electonic ground state.
M+ the ionic state,
M* represents an excited atom
Collisions of the first kind: ionisation by electron impact
Electron impact ionisation occurs when an atom collides with
an electron (whose kinetic energy is higher than the ionisation
energy of the atom):
or
The electron collides with an atom, either a sputtered atom or a carrier gas atom. Under the shock of the collision, the atom is ionised and an additional electron is released. This process is crucial in a self-sustaining plasma because an additional electron liberated. Though the net charge has remained unchanged, the number of free charges in the has been increased during the process from one free electron to two electrons and an ion. These charged particles can then participate in further ionisation processes leading to electron multiplication.
Collisions of the second kind: Penning Ionisation
Penning ionisation occurs as the result of a collision between a metastable gas atom and an atomic species with ionisation energy below the energy of the excited metastable state of the gas atom. The metastable states must first be populated through other collision processes. Ultimately, the source for the energy required for these processes can be found in electron impact collisions.
In Penning ionisation a carrier gas atom, in many cases it will be argon, that has been excited to a metastable electronic state collides with an analyte atom. The energy of the metastable levels can be used to ionise the analyte atom if the ionisation potential of the latter is lower than the metastable energy.
Metastable electronic state are particular electronic states having long radiative lifetime, because electric dipol transitions to lower-energy levels are forbidden. They play an important role in glow discharges, because they can reach significant number densities. For inert gases the energy levels for these metastable states are high (Arm: 11.548 eV) compared to the excitation and ionisation potentials of many elements. They are an efficient carrier of potential energy. Some elements, however, can not be ionised through Penning ionisation with argon. These elements include H, N, O, F, Cl and Br. Their ionisation energy is higher than potential energy of the meta-stable Ar atom. The ionisation energy for these atoms are H:13.598 eV, F:17.422 eV, Cl 12.967 eV, Br 11.894 eV, O: 13.614 eV, N: 14.534 eV. Penning ionisation again increases the number of free charged particles, necessary to compensate the constant loss of charged particles in a discharge.
Asymmetric Charge Transfer (ACT)
During asymmetric charge transfer an ionised atom hit a neutral atom of different element. During the collision, one electron is transferred from the neutral atom to the ion. As a result the atom that was initially neutral has been ionised, most likely in an excited ionic state. The initially charged atom, Ar+, has turned into a neutral atom, Ar0. This process can have considerable cross section when the electronic states of the atoms and ions involved match, i.e. when the energy difference is small (~0,2 eV) and positive.
A typical example of this is the ionisation of copper in an argon discharge. The ionisation energy of copper is 7.726 eV. The excited state of the Cu ion (3d94p 3P2 ) lies 8.234 eV above the ionic ground state. This is 0.165 eV above the ionisation energy of Ar (15.795 eV). However, if the Cu atom meet an Argon ion in the meta stable P1/2 state, 0.18 eV above the P3/2 ground state of the Ar+ ion, the energy difference for an asymmetric charge transfer reaction will only be 0.015 eV. The reaction is a little exothermic. As a consequence the spectral line of Cu II, at 224.7 nm, originating from the 3d94p 3P2 state has surprisingly high intensities in argon discharges when copper is sputtered. The discharge is obviously not in thermal equilibrium, a quite normal situation for low pressure discharges. A different example for the importance of asymmetric charge transfer reactions are inelastic collisions between the hydrogen ion, the proton, and metal atoms in the glow discharge. These reaction will be discussed on a different page, dedicated to the effect of hydrogen in analytical glow discharges.
Asymmetric charge transfer reaction also play an important role in metal vapour lasers, as they participate in obtaining the population inversion necessary for laser operation.
First published on the web: 17 October 2006.
Author: Thomas Nelis
This page is based on the master works of Lydie
Salsac and Anouar Kanzari. Both have gained their master degree
at INSTN, Saclay, France, after performing their master work at
EMPA Material Science and Technology, Thun, Switzerland.
We also owe much information on ACT reactions to the work of Prof. Edward
B.M. Steers, London Metropolitan University, UK.
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Discharge Regimes
All discharges have in common that free ions and electrons
in a gaseous atmosphere are involved. We can distinguish different
kinds of discharges:
Arcs frequently observed during thunder
storms,
Glow discharges as in Neon
tubes
Dark dischages less frequently
observed in day to day live
Discharges are distinguished not only by their luminescence
but also by their Current-Voltage characteristics, the current
density and breakdown voltage. These main characteristics
depend on the geometry of the electrodes and the vessel, the
gas used, the electrode material. By changing the discharge
current, we move from one discharge type to the next. The
observed voltage current characteristic will be highly non-linear.
Dark Discharges
The regime between A and E on the voltage-current characteristic
is termed a dark discharge because, except for corona discharges
and the breakdown itself, the discharge remains invisible
to the eye.
During the background ionisation stage of the process (A-B
) The charge density is extremely low. The electric field
applied along the axis of the discharge tube only sweeps out
the ions and electrons created by ionisation from background
radiation. The background radiation originates from cosmic
rays, radioactive minerals, or other sources. It produces
a constant and measurable degree of ionisation in air at atmospheric
pressure. Under the force of the electrical field the ions
and electrons migrate to the electrodes producing a weak electric
current. When voltage is increased more charged particles
are drawn to the electrodes and the current increases. An
avalanche reaction does not occur.
If the potential difference between the electrodes is further
increased, eventually all the available electrons and ions
are swept away, the current consequently saturates. In this
saturation region (B - C), the current remains constant while
the voltage is increased. This amplitude of the current depends,
however, linearly on the radiation source strength This property
makes the regime useful for the design of radiation counters.
When further increasing the voltage across the low pressure
discharge tube; beyond point C, the current rises exponentially.
The electric field is now high enough for the electrons initially
present in the gas to acquire sufficient energy. They are
now able to ionise a neutral atom creating more free charged
particles. The avalanche process has started as these newly
generated secondary electrons may gain enough energy to take
their turn and ionise yet another neutral gas atom. This region
of exponentially increasing current is called the Townsend
discharge (C-E).
Corona Discharge |
A Corona discharge occurs in Townsend dark discharges
in regions of high electric field near sharp points, edges,
or wires in gases prior to electrical breakdown. It is important
to point out that the electrical field is here the dominant
parameter, rather then the potential difference between the
electrodes . If the coronal currents are high (D-E) enough,
corona discharges can be visible to the eye and resemble a
glow discharge For low currents, the entire corona is dark,
as appropriate for the dark discharges. Corona discharges
are also called partial discharges as they do not occupy the
entire distance between the two electrodes, but are present
only in the region of high electrical field. Corona discharges
in air emit light mainly in the UV spectral region with a
small portion in the blue. They are therefore not always visible
to the eye. Related phenomena include the silent electrical
discharge. They are called silent because they are inaudible
form of filamentary discharges. A Brush discharge,
a luminous discharge in a non-uniform electric field, consists
of many corona discharges which are active at the same time
and form streamers through the gas.
Brush Discharge |
The electrical breakdown occurs in Townsend regime
when the ions reaching the cathode have sufficient energy
to generate secondary electrons(E). Photon impact is a different
possible process for generating secondary electrons. At the
breakdown, or sparking potential VB, the current
might increase by a factor of 104 to 108.
It is usually limited only by the internal resistance of the
power supply connecting the two electrodes. If the current
supply is to low, discharge tube cannot draw enough current
to break down the gas, and the tube will remain in the corona
regime with small corona points or brush discharges being
visible at the electrodes. If power supply delivers enough
current the gas will break down at the voltage VB,
avalanche processes weill ocurre and the discharge will move
into the normal glow discharge regime. The breakdown voltage
for a particular gas and electrode material depends on the
product of the pressure and the distance between the electrodes,
as expressed in Paschen’s law (1889).
Glow Discharge
The glow discharge regime owes its name to the typical luminous
glow. The plasma gas emits light because the electron energy
and number density are high enough to generate excited gas
atoms by collisions. These excited gas atoms will eventually
relax to their ground state by emission of photons. The applications
of glow discharge include fluorescent lights (Neon tubes),
dc parallel plate plasma reactors, used for depositing thin
films. Glow discharges are also extensively used in plasma
chemistry.
After a discontinuous transition from E to F, the gas enters
the normal glow region (F - G), in which the voltage
is almost independent of the current over several orders of
magnitude in the discharge current. This current-voltage behaviour
is very different from a normal Ohm type resistance. The electrode
current density does not change with the total current in
this regime. Only a small part of the cathode surface at low
currents is in contact with the plasma. As the current increases
from F to G, the fraction of the cathode occupied by the plasma
increases, until plasma covers the entire cathode surface
at point G. The discharge voltage remains constant over a
large range of current variation( 2 or 3 magnitudes).
Once the whole surface of the cathode is covered by the discharge,
the only way the total current can increase further is to
drive more current through the cathode by increasing the current
density. This requires more energy, applying more voltage
moving away from the Paschen minimum. This regime where the
voltage increases significantly with the increasing total
current (G-H) is named the abnormal glow regime. The
discharge now behaves here more like a normal resistance.
Starting at point G and decreasing the current, a form of
hysteresis is observed in the voltage-current characteristic.
The discharge maintains itself at considerably lower currents
and current densities than at point F and only then makes
a transition back to Townsend regime.
In the abnormal discharge the cathode fall potential
increases rapidly, and the dark space shrinks. Except for
being brighter, the abnormal glow discharge resembles the
normal discharge. The structures near the cathode may blend
into one another and a rather uniform glow can be observed.
At the same time as voltage and cathode current density increase
the average ion energy bombarding the cathode surface also
increases. Due to the high current density abnormal discharges
are commonly used as sputter sources. The bombardment with
ions ultimately heats the cathode causing thermionic emission.
Once the cathode is hot enough to emit electrons thermionically,
the discharge will change to an arc regime.
Arc Discharges
At point H, the electrodes become sufficiently hot that the
cathode emits electrons thermionically. If sufficient current
is supplied to the discharge it will undergo a glow-to-arc
transition, (H-I). The arc regime, from I through K is one
where the discharge voltage decreases as the current increases,
until large currents are achieved at point J, and after that
the voltage increases again slowly with increasing current.
In spectro chemistry arc discharges are used in Spark OES
and DC arc spectroscopy.