Shlomo Efrima

Classical theory of light scattering by an adsorbed molecule. I. Theory

We develop a classical theory for the intensity and the depolarization ratio of the light scattered (Raman or Rayleigh) by an absorbed molecule. It is assumed that the optical properties of the system can be described by the polarizability of the molecule and the dielectric constant of the two media. The presence of the surface modifies the field incident upon the molecule as well as the field emitted by the induced dipole. We compute these effects, exactly, by using a dyadic Green’s function method, and approximately, by using a perfect mirror model. The theory provides the angular distribution of the scattered radiation, and its polarization as a function of the polarization, the frequency and the direction of incidence of the incoming radiation, as well as of the dielectric properties of the metal and the position of the molecule with respect to the surface. We use these equations to analyze the possible sources for the experimentally observed enhancement of the scattering caused by the presence of the...

A one-dimensional microscopic quantum mechanical theory of light enhanced desorption

Abstract A one-dimensional microscopic quantum mechanical model is used to inquire whether it is possible to enhance the desorption rate by employing a laser to induce vibrational excitation of the chemisorptive bond. For model parameters simulating CO/Cu it is found that the required laser intensity is very large, roughly 10 4 times that of a conventional high power CO 2 laser (whose intensity we take to be 5 × 10 10 W cm 2 ). We suggest that surface roughness can be used to enhance substantially the effect of the laser and possibly to enable the observation of laser enhanced desorption.

Light scattering by a molecule near a solid surface. II. Model calculations

We use a theory, previously developed by us, to investigate numerically the properties of the light scattered by a molecule adsorbed near a solid surface. We illustrate how the angular dependence and the polarization of the scattered light can be used to infer, in certain limiting cases, the position of the molecule with respect to the surface. The frequency dependence of the scattered radiation is investigated and possible causes for the observed enhancement of scattering by a metallic surface are discussed.

Resonant Raman scattering by adsorbed molecules

We extend the classical model used in our previous work, to discuss the resonant Raman scattering by a molecule located near a metallic surface. We use the Drude model, modified to incorporate the electrodynamic effects of the surface. The frequency dependence of the polarizability is substantially modified and this lowers the frequency at which the resonant Raman effect appears. We derive the intensity of the fundamental and that of the overtones, for resonant scattering, and exemplify numerically their behavior.

A one‐dimensional model for phonon‐induced desorption

We derive the kinetic equations for a one‐dimensional model of phonon‐induced desorption. We start with a generalized master equation and discuss approximations that reduce it to a master equation of the Pauli type. This describes desorption as a stochastic process in which the energy of the chemisorptive bond has random variations caused by the thermal fluctuations of the lattice atoms. Desorption occurs when the bond energy exceeds a certain value, placing the adsorbed particle in a continuum state of the surface‐particle potential. The calculation of the rates of energy transfer includes multiphonon processes and allows transitions between all the bound levels of the adsorbed particle. Numerical calculations are presented in a subsequent paper.

Classical theory of light scattering by a molecule located near a solid surface

Abstract We develop a classical theory of Rayleigh and Raman scattering by a molecule located near a solid surface. The main physical factors determining the intensity enhancement, angular dependence, depolarization ratio and frequency dependence, are discussed.

Vibrational frequencies of a chemisorbed molecule: The role of the electrodynamic interactions

Abstract We use a simple model to discuss the role of the electrodynamic interactions between a chemisorbed molecule and the surface in shifting the vibrational frequencies of the molecule.

Optical response of concentrated colloids of coinage metals in the near-ultraviolet, visible, and infrared regions

The optical response of two‐phase composite materials (cermet topography with coinage metals) at high filling factors under a wide range of particles sizes, frequencies, and experimental conditions is calculated using a model that combines multipolar interactions and particle size effects. The validity of the model is established for a real system of a silver colloid and the simulations are discussed in terms of the microscopic structural parameters and the interaction between the particles. The limitations of the Maxwell–Garnett model are explored and the transition between a dense system which primarily reflects light and a dilute composite which mostly transmits light is followed with the same model.

The diffusion of an atom on a solid surface

A microscopic model for the diffusional motion of an atom chemisorbed on a solid surface is presented. The atom located at a given site is described by a set of states which can be divided into two classes. One class is composed of states whose energy is located below the intersite barrier. A particle described by one of these states can go to a neighboring site only through tunneling. The second class of states have higher energy than the intersite barrier. They are delocalized and an atom occupying such a state can change its site with relative ease. The vibrational motion of the lattice can transfer energy to or take it from the particle and it can also change the intersite transfer probability. As a result the particle may undergo three types of processes: 1) change state but not site (vertical transitions); 2) change site and state (oblique transitions); or 3) change site but not energy (horizontal transitions). The theory derives a kinetic equation for the probability that at a given time the partic...

A one-dimensional microscopic model for thermal desorption of an atom. Applications to the case of weak binding

Abstract A one-dimensional model is used to compute the rate of thermal desorption. The oscillation of the adsorbed atom with respect to the surface is described by a Morse potential, driven by the plionon field Multiphonon processes are used to compute the rate of energy transfer to the oscillator. The activation energy is found to be smaller than the desorption barrier.