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):
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.