Optical force acting on a molecule near a metal sphere: effects of decay rate change and resonance frequency shift

We present a theoretical analysis of optical force on a molecule near a metal sphere by taking into account the change of decay rate as well as the shift of resonance frequency in the response of the metal surface. As a result, the optical force is determined not only by the gradient of field but also the polarizability depending on the position and orientation of the molecule. In addition, we find the resulting reactive force cannot be expressed as a gradient of a scalar potential, in contrast to the usual situations.