Frequency shifts of molecules at rough metal surfaces

The frequency shifts for dipolar transitions of molecules at rough metallic surfaces are studied in a phenomenological model following the approach of our previous work [Phys. Rev. B 36, 4664 (1987)], with the surface roughness modeled by a shallow grating profile in most cases. The main findings here are as follows: (1) Such surface-induced shifts are indeed observable for molecular frequencies away from the surface-plasmon resonance frequency of the metal; (2) the presence of surface roughness can either enhance or suppress the flat-surface-induced shifts, leading to extra morphology-dependent resonances originated from the radiative coupling between the molecular emission and the substrate surface plasmon; and (3) the effects in the perfectly reflecting limit can be worked out analytically with numerical results showing interesting features that are unique for this case. Correlations are made with respect to previous experiments on vibrational shifts as well as to recent calculations from a microscopic approach.