Question

Why is it impossible to measure an excimer absorption spectrum, even for highly concentrated solutions? Illustrate your answer with an energy diagram

Answer 1

An excimer (excited + dimer = excimer) is a short-lived dimeric or heterodimeric molecule formed from two species, at least one of which has completely filled valence shell by electrons (i.e. noble gases).

It is impossible to measure an excimer absorption spectrum, even for highly concentrated solutions, because:

• A significant intensity of excimer fluorescence can only be observed at high concentrations .
• The long‐wavelength band is attributed to an emission from an excimer (a transient dimer) formed intramolecularly by the association of excited and unexcited groups.
• formation of molecules is possible only if such atom is in an electronic excited state.
• The lifetime of an excimer is very short, on the order of nanoseconds.
• Excimers are only formed when one of the dimer components is in the excited state.
• The wavelength of an excimer's emission is longer (smaller energy) than that of the excited monomer's emission. An excimer can thus be measured by fluorescent emissions.
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