Lifetimes of electronic excited states of a molecule close to a metal surface

We present a model for the evaluation of the metal-induced deexcitation rate for molecules close to a metal surface. In this model, the molecule is treated quantum mechanically at the density-functional theory level and its excitations with a time-dependent density-functional theory procedure. The metal is described through its dielectric response properties. The nonlocal features of such a response are taken into account by using a modified Lindhard-Mermin dielectric constant. The presence of small random roughness on the metal surface is described with the model of Rahman and Maradudin [Phys. Rev. B 21, 504 (1980)]. Systems in which the molecule close to the metal is immersed in an homogeneous matrix or deposited on a spacer are considered. The matrix and the spacer are treated with the polarizable continuum model. The molecule-metal and the molecule-matrix (or molecule-spacer) electrostatic interactions are described by using the integral equation formalism, numerically treated with a boundary element method. As a test case, the method has been applied to the study of phosphorescence from biacetyl close to a silver surface and immersed in or deposited on solid ammonia. Our results show that nonlocal metal response and surface roughness have an important role in explaining measured lifetimes. (C) 2003 American Institute of Physics.