激发二聚体
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In most molecules, electrons occupy the lower energy levels (molecular orbitals). Two electrons can be in one energy level, but three electrons cannot; this is Hund's rule. Molecules can absorb light, and if a molecule absorbs light whose energy corresponds to the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), the electron in the HOMO is excited to the LUMO. This situation is called the excited state.
An excimer forms a dimer only in the excited state; excimers do not form dimers in the ground state. When an excimer returns to the ground state, its components often strongly disassociate and repel each other. The lifetime of an excimer is typically on the order of nanoseconds. For example, pyrene forms an excimer.
The wavelength of an excimer's emission is longer than that of the excited monomer's emission, because the excimer is stabilized compared to the excited monomer. An excimer can thus be measured by fluorescent emissions.
Excimers form only in dense situations; in thin situations, excited monomers form. If two molecules are near each other, strong fluorescence can be detected. This is useful for the measurement of the distance between two molecules.
While an excimer is a molecule that forms a dimer from the same molecule in the excited state, an exciplex is a molecule that forms a dimer from different molecules in the excited state. Exciplexes are very commonly miscalled excimers.
Exciplexes can be used as the gain medium of a type of powerful ultraviolet laser known as an excimer laser. Despite its name, strictly speaking it is an exciplex laser.
An example is a xenon chloride (XeCl) laser. Its gain comes from an electric discharge of xenon (Xe) and chlorine (Cl2). The xenon chloride (XeCl) only forms in the excited state, and it returns to an unreactive noble gas electronic structure in the ground state.
Excimers are not only an interesting phenomenon themselves but also a phenomenon that can be applied to research.