Excimer Lasers

Similar to CO2 , CO, and N2 lasers, these gas lasers also use molecular transitions for their lasing operation. What makes them especially different is that the molecular gas used for these lasers has no ground state! Typically these molecules include an atom belonging to the inert gas family (argon, xenon, krypton) and one from the halide group (chlorine, fluorine, and bromine). The inert gas atoms (also known as the rare gases) do not want to interact with any other atoms. On the other hand, the halide gases are highly reactive. Still, they cannot bond with the inert gases to form a molecule. But when sufficient energy is provided to these atoms they bind together in a short-lived excited state that soon (few nanoseconds) decays back into the original two separate atoms (i.e. the molecule dissociates). Because of this rapid molecular dissociation, these lasers obtain population inversion just by excitation alone! In fact, the word excimer is short for "excited dimer," although most excimer lasers do not use two identical atoms as a strict dimer would.

The excimer molecules are created from a mixture of inert gases along with one of the halides. Typically a few percent of Ar, Kr and Xe are mixed with a few percent of a halide to form excimer molecules: ArF, KrF , and XeF. The other 90% of the gas mix consists of other inert gases such as He and Ne which act simply as a buffer and do not take part in the reaction. A large electric pulse is often used for the excitation and formation of the excimer molecule. The rapid decay of the short-lived molecule then leads to a very short laser pulse lasting 10 - 100 ns (10-8 - 10-7 s). So, another unusual feature of the excimers is that they do not require an optical amplifier. They are very efficiently formed in the reaction with an efficiency of around 30% so that the gain is extremely high. A high reflector and a glass (really quartz - why?) window is sufficient for laser light production. This means that only about 4% of the light is reflected back into the cavity at the front window, but the gain is so high that only a single pass through the cavity is needed to produce lots of uv light. Typically about 1 Joule of energy is in a 10 ns pulse, so that the pulse power is 1J/10ns = 100 MW. If this power were steadily produced it would be equivalent to powers generated by large power plants. However, only about 1 - 100 pulses are produced per second, so that the average power produced is about 1 - 100 W.

Because of the highly reactive nature of the halide gas used in these lasers, excimers are not very easy to operate. The halides tend to be very corrosive and therefore add a great deal to the operational cost (as well as the danger!) of these lasers. But still these lasers are very much in use because their output wavelength is in the UV, from 350 nm down to as low as 193 nm; all with a good deal of power.