Nonclassical behavior of energy transfer from molecules to metal surfaces: Biacetyl(3<i>n</i>π*)/Ag(111)

The distance dependent lifetime of biacetyl separated from a Ag(111) crystal by NH3 spacer layers ranging in thickness from 28 to 457 Å has been measured. We extended previous work, where the molecular emission was resonant with the silver interband/plasmon transition, to the case where the emission is below the interband transition. The modulation of the radiative rate is described inadequately by the classical theory for our experimental geometry. At short distances where nonradiative energy transfer to the metal is important, the classical prediction deviates from the data as well. These observations are consistent with a model in which energy is transferred to electrons localized at the metal surface but might also be explained by an inability of the classical theory to model the radiative rate properly.