In this chapter three types of excimer lasers will be discussed. These are the pure noble gas excimers, the rare gas oxides and the diatomic halogens. The closely related rare gas halide systems are examined in Chap. 4. The historical development of the noble gas excimer laser systems has already been discussed in Chap. 1, while the details of the potential curves relevant to the excimer systems appear in Chap. 2. Our discussion here, therefore, will be confined to a sketch of the present understanding of the kinetics and energy flow governing these systems, and also a brief summary of the more recent experimental efforts demonstrating the characteristics of these molecular systems. The mixed gas systems such as xenon/oxygen or argon/iodine, have the gas of molecular additive in very dilute concentrations. An essential feature of these systems is the fact that the dominant channel for populating the upper laser level is via neutral energy transfer from the excited rare gas atoms and directs. The rare gas energy donors (both atoms and dimers) are initially produced by excitation with an electron beam or an electron beam sustained discharge. Thus, the kinetics of the energy flow within pure rare systems are key elements of the behavior of mixed gas systems. Systems that are directly excited photolytically are exceptions, but even in these systems the light source is likely to be a rare gas laser with its internal kinetics. Historically, a major factor in the development of noble gas excimer lasers was the knowledge that direct e-beam excitation would efficiently produce high densities of metastable noble gas atoms. However, in contrast to the high values of computed laser efficiency for the rare gas dimer systems, the actual laser efficiencies and powers that were initially demonstrated were rather low. In addition, since the emission wavelengths were in the vacuum ultraviolet, several technical problems connected with optics and beam transport were associated with these systems. In these circumstances, there was motivation for the investigation of methods that would selectively transfer the energy from the noble gas excimer to an appropriate acceptor molecule. If the energy transfer is sufficiently specific, a sizable population inversion may be generated in the acceptor species. An optimistic view then suggests that the additive can be selected to have properties that match the requirements of particular applications. For example, it may be desirable to have a long upper state lifetime applicable to high energy storage systems for fusion lasers. A wide variety of additives have been studied recently (O 2 and oxygen bearing compounds, N~, CN, H20, mercury, the halogens and their compounds) and many, but not all, have been developed into successful lasers.

Laser action of HgCl* was observed using an electron-beam-controlled discharge to pump a gas mixture of Hg/Cl2/Ar. An order of magnitude in the fluorescence enhancement was obtained. This gives an excitation efficiency by controlled discharge to be a factor of about 2 higher than that by direct electron-beam pumping. In addition to the previously reported 5576-Å laser transition of HgCl*, a strong transition at 5584 Å was also observed.