3.4. Lasers
Gas discharges are also used for laser applications, more specifically as gas lasers. Several kinds
of gas lasers exist (see below), but they have one common characteristic, i.e. the mechanism of population inversion, necessary for laser action, always occurs via gas discharges. The gas, at reduced pressure, is contained within a glass discharge tube with mirrors at the ends of the tube. Anode and cathode can be placed at both ends of the tube. Alternatively, the cathode can have a hollow cathode geometry (see also below), with anode rings at the ends. Generally, three classes of gas lasers can be distinguished, depending on whether the lasing transition occurs between energy levels of atoms, ions or molecules w155x.
3.4.1. Atomic lasers
3.4.1.1. He–Ne laser. The He–Ne laser is one of the most-used lasers w155x. As predicted by the
name, the active medium is a mixture of helium and neon in a typical ratio of 10:1. It operates in
a glow discharge tube of a few mm diameter and a length of 0.1–1 m. The total pressure is approximately 1000 Pa. The lasing transitions take place between the energy levels of neon. Several different transitions are possible, all starting from the 3s and 2s levels. Excitation of these upper laser levels occurs in two steps (see Fig. 25):
1. electron impact excitation of helium: eyq He™eyqHe*; and
2. followed by energy exchange between He* and Ne: He*qNe™HeqNe* (in 2s or 3s levels).
This is a typical example of combined electron excitation as creation process (step 1) and heavy
particle kinetics to realize the application (step 2) as discussed in the Introduction. The three main transitions are at 3.39 mm, 1.15mm and 632.8 nm. Because these transitions have either the same upper or lower laser levels, they are competing with each other. Therefore, the mirrors should be selective, by being highly reflective only at the desired wavelength.A disadvantage of the He–Ne laser is that it gives a low power output (typically 0.5–10 mW),because higher power gives saturation due to population in the glow discharge of the lower laser levels. Advantages are the excellent beam quality,the very narrow laser linewidth and the relatively small size w155x. The He–Ne laser is used for optical alignment, by producing a visible line which can be used for
positioning an object, for guidance of equipment in construction (aircrafts, ships), or for the alignment of other (e.g. IR) lasers, as well as for
3.4.1.2. Copper vapor laser. The copper vapor laser (CVL) is based on a glow discharge at high
temperature (wall temperature typically 1400–1500 8C). The discharge tube is typically almost
1 m long with a diameter of 2 cm w156x. It is filled with neon at a pressure of 3000–7000 Pa.
Metallic copper is present in a reservoir, and it evaporates due to the high temperature. Lasing occurs at two wavelengths, i.e. at 510.6 and 578.2 nm. The upper levels of both laser lines
are the Cu0 3d10 4p levels, and the lower laser levels are the Cu0 3d9 4s2 levels w156x. Because
the latter are metastable levels with a relatively long lifetime, the lasing will be short, until the
population inversion is destroyed. Therefore, the laser is also called ‘self-terminating CVL’. Typically, it takes 25 ms to deactivate the lower laser levels, and to produce new laser action. Hence, the CVL is operated in the pulsed mode, with a pulse repetition frequency in the order of 1–100kHz. The average power output is 10–100 W. The CVL has a high gain (1% per mm), and the overall efficiency is rather high (up to 2%) w155x. The CVL has found application in medicine (dermatology and oncology w157x), as pumping sources for dye and Tiysapphire lasers for tunable output w158x, and for industrial materials processing w159x.
3.4.2. Ion lasers
3.4.2.1. Argon ion laser. In conventional argon ionlasers, the active medium is the positive column region of a high current density argon glow discharge w160x. The mechanism for laser activation typically occurs in two steps: (i) ionization of argon, and (ii) excitation of Arq w155x. Because of the two-step process, and because of the high energies required for ionization and excitation (i.e. 15.76 eV for ionization, and typically 19.68 eV for excitation of the ions to the upper laser level), the positive column argon ion laser has a low power efficiency (typically 0.05%) w160x. Hence, the glow discharge must be very intense, with an electrical dissipated power in the kW range w155x. This requires considerable engineering skills. The laser efficiency can, however, be increased by applying a magnetic field along the tube axis w155x. Alternatively, an electron beam with energy close to the peak of the direct excitation cross
section has been used to directly excite the argon ion upper laser levels w161x.Several laser lines are possible between 454 nm and 529 nm, corresponding to 4p–4s transitions. Hence, the laser can be tuned to one specific wavelength, if needed. The two most intense lines are at 488 nm and at 514.5 nm. Argon ion lasers are used for laser printers, optical disks and for Raman spectroscopy. Moreover, they are also used in medicine, for the treatment of detached retinas (ophtalmology). The radiation is strongly absorbed by red blood cells and the resulting thermal effects lead to a reattachment of the retina w155x. A variation to the argon ion laser is the krypton ion laser, which has a lower gain and is less powerful, but on the other hand, it has an even broader wavelength range (between 337 and 800 nm, with the most intense laser line being at 647nm), and it is often attractive for this reason w155x.
3.4.2.2. Metal-vapor ion lasers. The metal-vapor
ion lasers (MVILs) are probably most similar to analytical glow discharges w162–165x. They operate in a rare gas (mostly helium or neon), at a pressure of 100–1000 Pa. The metal vapor is
traditionally introduced by thermal evaporation. For some metals (e.g. Cu, Ag, Au, etc.) the
temperature has to be very high, which may lead to technical difficulties for the development of
MVILs. Therefore, the metal vapor is sometimes obtained from volatile compounds of the metal
(e.g. CuBr, CuCl) which are dissociated in the plasma by electron impact, after which the metal
atoms can be excited. In the case of Cu, the temperature can in this way be reduced from 1500
8C to 400–600 8C w163x. Another way to introducethe metal vapor into the discharge is by sputtering, as in analytical glow discharges. This typically occurs in hollow cathode discharge lasers w166x. The main advantage is that lower temperatures can be used, which simplifies the experimental design.
3.5. Ozone generation
Ozone (O3) generation is also a typical application of DBDs or high pressure GDs w41,42,45,48,49x. Ozone can be generated from oxygen, air or from other N2yO2 mixtures. The
first step towards ozone formation in gas discharges is the dissociation of O2 molecules by electron impact and by reactions with N atoms or excited N2 molecules, if nitrogen is present. Ozone is then formed in a three-body reaction involving O and O2. In recent years, considerable progress was made with respect to attainable ozone concentrations and energy consumption. Ozone concentrations up to 5 wt.% from air and up to 18 wt.% from technical
oxygen can now be reached w42x. Large ozone generating facilities produce several hundred kg
ozone per hour at a power consumption of several megawatts. With modern technology, ozone can be produced at a price less than 2 US$ykg w42x. The main applications are in water treatment and in pulp bleaching. Applications in organic synthesis include the ozonation of oleic acids and the production of hydroquinone, piperonal, certain hormones, antibiotics, vitamins, flavors and perfumes w42x.
