Early excimer lasers incorporated lasing species such as Kr2, which is a dimer molecule that exists only in the excited state. Excimers have evolved to include lasing species of excited rare gas halides such as KrF. This species, as well as others (XeF, ArF, XeCl and KrCl), is bound only in the excited state, while the ground state is weakly bound. The KrF molecule is created by a chemical reaction of its constituents after they have been ionized by a high-speed electrical discharge. From the excited state, the excimer molecule transitions quickly to the ground state and spontaneously disassociates back in to its constituents. During this transition, a UV photon is emitted.
The actual kinetics of KrF formation are quite complex and involve the formation of other intermediary excited states than those depicted here. For the purposes of this discussion, the simplified diagram below conveys the basic concept of excimer formation.
Essentially, however, an electrical discharge creates Kr+ and F- ions, which combine to form the excited KrF molecule. This combination involves the interaction of neon as a third body collision partner that lends or absorbs energy in the reaction. When the KrF molecule transitions to the unbound lower state, a UV photon is emitted. After KrF molecule dissociation, a short relaxation time elapses as the Kr and F2 molecules settle back down to their normal energy states.