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A diatomic molecule that exists only when one of the atoms is in an excited energy state, especially one whose atoms are the same chemical element.
(General Physics) physics an excited dimer formed by the association of excited and unexcited molecules, which would remain dissociated in the ground state
ex•ci•mer(ˈɛk sə mər)
a molecular complex of two, usu. identical, molecules that is stable only when one of them is in an excited state.