It is convenient to generate excimer molecules in a
plasma. Electrons play an important role in a plasma and, in particular, in the formation of excimer molecules. To efficiently generate excimer molecules, the working medium (plasma) should contain sufficient concentration of electrons with energies that are high enough to produce the
precursors of the excimer molecules, which are mainly excited and ionized rare gas atoms. Introduction of power into a gaseous mixture results in the formation of excited and ionized rare gas atoms as follows:
Electron excitation :Rg + e− → Rg* + e−, Direct
electron ionization :Rg + e− → Rg+ + 2e−, Stepwise ionization :Rg* + e− → Rg+ + 2e−, where Rg* is a rare gas atom in an excited
electronic state, Rg+ is a rare gas ion, and e− is an electron. When there are enough excited rare gas atoms accumulated in a plasma, the excimer molecules are formed by the following reaction: :Rg* + Rg + M → Rg2* + M, where Rg2* is an excimer molecule, and M is a third particle carrying away the excess energy to stabilize an excimer molecule. As a rule, it is a rare gas atom of the working medium. Analyzing this three-body reaction, one can see that the efficiency of the production of excimer molecules is proportional to the concentration of excited rare gas atoms and the square of the concentration of rare gas atoms in the ground state. From this point of view, the concentration of rare gas in the working medium should be as high as possible. A higher concentration of rare gas is achieved by increasing gas pressure. However, an increase in the concentration of rare gas also intensifies the collisional quenching of excimer molecules, resulting in their radiationless decay: :Rg2* + Rg → Rg* + 2Rg. The collisional quenching of excimer molecules is negligible while the
mean time between collisions is much higher than the lifetime of an excimer molecule in an excited electronic state. In practice, the optimal pressure of a working medium is found experimentally, and it amounts to approximately one atmosphere. A mechanism underlying the formation of exciplex molecules (rare gas halides) is a bit more complicated than the mechanism of excimer molecule formation. The formation of exciplex molecules occurs in two main ways. The first way is due to a reaction of ion-ion recombination, i.e.,
recombination of a positive rare gas ion and a negative halogen ion: :Rg+ + X− + M → RgX* + M, where RgX* is an exciplex molecule, and M is a collisional third partner, which is usually an atom or molecule of a gaseous mixture or
buffer gas. The third particle takes the excess energy and stabilizes an exciplex molecule. The formation of a negative halogen ion results from the interaction of a low-energy electron with a halogen molecule in a so-called process of the dissociative electron attachment: :X2 + e− → X + X−, where X is a halogen atom. The pressure of a gaseous mixture is of great importance for efficient production of exciplex molecules due to the reaction of ion-ion recombination. The process of ion-ion recombination is dependent on three-body collisions, and the probability of such a collision increases with pressure. At low pressures of a gaseous mixture (several tens of
torr), the reaction of ion-ion recombination is of little efficiency, while it is quite productive at pressures above 100 Torr. The second way of the formation of exciplex molecules is a
harpoon reaction. In this case, a halogen molecule or halogen-containing compound captures a weakly bound electron of an excited rare gas atom, and an exciplex molecule in an excited electronic state is formed: :Rg* + X2 → RgX* + X. Since the harpoon reaction is a process of a two-body collision, it can proceed productively at a pressure significantly lower than that required for a three-body reaction. Thus, the harpoon reaction makes possible the efficient operation of an excimer lamp at low pressures of a gaseous mixture. The collisional quenching of exciplex molecules at low pressures of a gaseous mixture is much lower than at pressures required for productive proceeding the reaction of ion-ion recombination. Due to this, a low-pressure excimer lamp ensures the maximum efficiency in converting the pumping energy to UV radiation. It should be mentioned that both the harpoon reaction and reaction of ion-ion recombination proceed simultaneously. The dominance of the first or second reaction is mainly determined by the pressure of a gaseous mixture. The harpoon reaction predominates at low pressures (below 50
Torr), while the reaction of ion-ion recombination prevails at higher pressures (above 100 Torr). The
kinetics of reactions proceeding in a plasma is diverse and is not limited to the processes considered above. The efficiency of producing exciplex molecules depends on the composition of a gaseous mixture and conditions of its excitation. The type of halogen donor plays an important role. The most effective and widely used halogen-carriers are
homonuclear diatomic halogen molecules. More complex halogen compounds such as
hydrogen halides,
metal halides, and
interhalogens are also used as a halogen-carrier but to a lesser extent. A noteworthy halogen-carrier is
alkali halide. A feature of alkali halides is a similarity of their
chemical bond with that of exciplex molecules in excited electronic states. Exciplex molecules in excited electronic states are characterized by the
ionic bond as well as alkali halides in the ground state. It opens up alternative mechanisms for the formation of exciplex molecules, namely
substitution reactions: :Rg* + AX → RgX* + A, :Rg+ + AX → RgX* + A+, where AX is an alkali halide molecule, A is an
alkali metal atom, and A+ is an alkali metal ion. These mechanisms of the formation of exciplex molecules are fundamentally different from the reaction of ion-ion recombination and harpoon reaction. An exciplex molecule is formed simply by replacing an atom/ion of alkali metal from an alkali halide molecule by an excited atom/ion of rare gas. An advantage of using alkali halides is that both the substitution reactions can simultaneously proceed at low pressures with comparable productivity. Moreover, both excited atoms and ions of rare gas are effectively used in the production of exciplex molecules in contrast to excimer lamps using other halogen-carriers. It is of importance because the ionization and excitation of rare gas consume most of the introduced energy. Since the reaction of ion-ion recombination and harpoon reaction dominate depending on the pressure of a gaseous mixture, the generation of rare gas ions is unprofitable at low pressures, while the excitation of rare gas is unreasonable at high pressures. A drawback of using alkali halides is high temperatures required for providing the necessary concentration of alkali halide molecules in a gaseous mixture. Despite this, the use of alkali halides as a halogen-carrier is especially promising in the development of
exciplex lasers operating at low pressures. == Methods of excitation ==