I know what an exited atom is : any of it's electron is elevated from the ground state. But I can't really figure out what is an excited molecule (e.g diene of polyene)...... I have read molecules like diene or polyene can be excited and they absorb light wave for this as atoms do to send some of their electrons to higher energy level. Is it something like , hmm....the molecular orbital is enlarged so that the shared electrons can enjoy extra freedom (higher energy) or something else? I think you have understood the problem.