Coupling of an excited molecule to surface plasmons

The decay of an excited molecule deposited at distances less than its dominant fluorescence wavelength ${\lambda }_F$ from a metal surface is calculated including the interaction with surface charge-density oscillations. Both nonresonant and resonant cases, ${\omega }_0\ll {\omega }_p$ and ${\omega }_0\sim {\omega }_p$, are considered for ${\omega }_0$, the electronic transition frequency in the visible part of the electromagnetic spectrum. The plasma frequency ${\omega }_p$ determines the dielectric response of a free-electron-gas metal, $\epsilon \left(\omega \right)=1-\frac{{\omega }_p^2}{{\omega }^2}$, in the region $\omega <{\omega }_p$. We find considerable changes from the pure image-theory result given by us earlier, in particular for the molecular transition dipole moment oriented parallel to the metal surface. The coupling to the surface-plasmon excitation spectrum induces a rapidly increasing contribution to the linewidth of the decaying molecule with decreasing distance from the metal surface. As the image result for the case of a molecular transition dipole oriented parallel to the surface tends to zero with decreasing distance, the inclusion of the coupling to surface plasmons drastically affects the lifetime of this configuration. For both cases, i.e., transition dipole parallel and perpendicular to the metal surface, the contribution to the width due to coupling to surface excitations diverges with decreasing distance, leading to dramatic shortening of the lifetime of the decaying molecule due to conversion of the localized electronic energy into surface charge-density oscillations.