Xenon monochloride

Xenon monochloride (XeCl) is an exciplex which is used in excimer lasers and excimer lamps emitting near ultraviolet light at 308 nm. It is most commonly used in medicine. Xenon monochloride was first synthesized in the 1960s. Its kinetic scheme is very complex and its state changes occur on a nanosecond timescale. In the gaseous state, at least two kinds of xenon monochloride are known: XeCl and Xe2Cl, whereas complex aggregates form in the solid state in noble gas matrices. The excited state of xenon resembles halogens and it reacts with them to form excited molecular compounds.

Introduction Molecules that are only stable in electronically excited states are called excimer molecules, but may be called exciplex molecules if they are heteronuclear. The exciplex halides constitute an important class of rare gas halides with formula RgX. Rg is the noble gas, and X is the halogen. These molecules are de-excited by emitting a photon whose energy is some Electronvolts. Therefore, the wavelength of the light produced is in the visible or ultraviolet spectra. Gas or gaseous mixtures that may lead to the formation of these molecules is a quasi-ideal laser medium since the population inversion is directly obtained when the excimer is formed. The other consequence of the unstable ground state is that the excimer or exciplex species must be generated by an external excitation (either through a discharge, an electron beam, microwave, or radiation). At least two gases must be used to generate exciplexes: a halogen donor and a rare gas. However, as shown in Table 1, not all rare gas halide molecules lead to the development of lasers; some may not even exist. Multiple molecules and applications have been developed.

Several review articles related to xenon chloride laser technology and its applications have been published. Some authors stress the importance of accurately determining the kinetics of the laser medium when rare-gas halides are involved. Recent results have provided insight into the physical chemistry of the laser medium. Spectroscopic investigations are limited to the visible-near ultraviolet region where exciplex lasers operate. Only binary gas mixtures of xenon and a chlorine donor, or ternary mixtures that also include a buffer gas (a rare gas indicated by Rg) will be considered. The most interesting chlorine donors are CCl4 and HCl because of their use in laser technology, and Cl2 (see Figure 1). XeCl and Xe2Cl are most important in laser applications amongst the xenon chlorides. Although discharge lamps based on low-pressure mixtures of xenon and a chlorine donor emit incoherent light, they are reliable and easy to operate.

History The idea that the noble gases can form halides arose in the early 1920s: A. von Antropoff and Oddo suggested that krypton and xenon may form bromides and chlorides. In 1933, Yost and Kaye unsuccessfully tried to synthesize xenon chloride by illuminating a mixture of xenon (70 torr of pressure) and chlorine (225 torr) with a mercury-vapor lamp. Xenon monochlorides were first synthesized in 1965. Later, solid XeCl2 and XeCl4 compounds were synthesized at low temperatures. In 1991, Prosperio et al. demonstrated the existence of XeCl2 in the gaseous state, which is important for lasing kinetics, although it emits an uninteresting infrared light. In 1973 Riveros et al. synthesized XeCl− ions in the gaseous phase at a pressure of 10−4 torr. This ionic molecule attracted little interest. Systematic studies of XeCl were initiated in 1975 by Velazco and Setser, who demonstrated 304 nm emission from XeCl*. This emission was obtained by mixing xenon atoms (Xe3P2) with chlorine gas Cl2 or other chlorinated compounds (NOCl and SOCl2). The excitation was provided by a cold cathode discharge; the total pressure was a few torr. Months later, Ewing and Brau reported lasing from a XeCl film 2Σ1/2+ → 2Σ1/2+ at 308 nm, which was most promising for industrial applications. The preferred chlorine donor for XeCl laser is HCl. The reasons given are:

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