Excimer

An excimer, short for excited dimer, is a short-lived molecule that is bound only when in an electronic excited state. If returned to the ground state, the components of the molecule are strongly disassociative and repulsive. The lifetime of an excimer is typically of the order of nanoseconds. An example is xenon chloride (XeCl), where the xenon atom will only form a molecule with the chorine atom while in an excited state (Xe^*), and on reverting to the ground state returns to an unreactive noble gas electronic structure.

Excimers can be used as the gain medium of a type of powerful ultraviolet laser known as an excimer laser.