  We report absolute values for the radiative relaxation quantum yield of
synthetic eumelanin as a function of excitation energy. These values were
determined by correcting for pump beam attenuation and emission re-absorption
in both eumelanin samples and fluorescein standards over a large range of
concentrations. Our results confirm that eumelanins are capable of dissipating
>99.9% of absorbed UV and visible radiation through non-radiative means.
Furthermore, we have found that the radiative quantum yield of synthetic
eumelanin is excitation energy dependent. This observation is supported by
corrected emission spectra, which also show a clear dependence of both peak
position and peak width upon excitation energy. Our findings indicate that
photoluminescence emission in eumelanins is derived from ensembles of small
chemically distinct oligomeric units which can be selectively pumped. This
hypothesis lends support to the theory that the basic structural unit of
eumelanin is oligomeric rather than hetero-polymeric.

Soft Condensed Matter;Materials Science