In
commercial excimer lasers, preionization is normally by a pulse of
ultraviolet light, but an electric discharge pulse, a pulse of x rays, or a
shot from an electron beam can also be used.
The electrons in the
excitation pulse then produce the excited rare gas atoms and halogens
that react to form excited diatomic molecules or excimers, such as
xenon fluoride. (The excited state is often indicated by following the
chemical formula with an asterisk, for example, XeF*.)