Dr. Aniruddha Chakraborty

An electron near a metal surface feels the charge of its image in the metal. Consequently near a metal surface it moves under the influence of this coulombic attractive potential, and there can be quantized energy levels which may lie in the band gap of the metal and whose wave function lie outside the surface. These image potential states can be studied using two photon photoemission spectroscopy, which is a valuable tool for the study of relaxation process at the interfaces.
Harris et al.[1] have carried out interesting studies of image potential states using this technique. We suggest a theoretical description of the states of the electron interacting with two dimensional layer of the polar adsorbate [2]. We model the adsorbed layer as a polarizable continuum. Due to the image effects, the dominant interaction is between the electron and the perpendicular component of induced polarization of the adsorbed layer. We also include the dipole-dipole interaction within the adlayer. With this simple model it is easy to make an analysis of the self trapping of electron. Depending upon the values of the parameters, the self trapped state can have any arbitrary size. Also, there are regimes in which (1) there is no localized state, (2) a localized state and a delocalized state coexists, with the delocalized state being a saddle point on the potential energy surface, and the localized state a minimum and (3) both the state exists as stable minima, and there is a barrier between the two. In the second case, self-trapping would be a barrierless process while for the third, it would be an activated process.
We find that our model can explain the salient features of the experimental results of Harris et al. [Science 297 1163 (2002)]. At the parameter values required to fit the experimental data, self trapping is barrierless. We study the dynamics of electron solvation by assuming a trial wave function for the electron and the solvent polarization, and then using the Dirac-Frenkel variational method [3]. The electron is initially photoexcited to a delocalized state, which has a finite but large size and causes the polar molecules to reorient. This reorientation acts back on the electron and causes its wave function to shrink, which will cause further reorientation of the polar molecules and the process continues until the electron would get self trapped.

References

1. C. B. Harris et. al. Science, 297, 1163 (2002).
2. K. L. Sebastian, A. Chakraborty and M. Tachiya, J. Chem. Phys. 119, 10350 (2003).
3. K. L. Sebastian, A. Chakraborty & M. Tachiya, J. Chem. Phys. 123, 214704 (2005).