Photodetachment of Negative Ions Near a Surface

Abstract

In this dissertation, theoretical study of photo-detachment of hydrogen negative ion near spher ical surface is presented and it is investigated that how spherical shape of a surface can generate additional oscillatory structure in the photo-detachment cross section. Keeping in view the cor relation effects, the photo-detachment cross sections are important to study the structure of negative ions and trajectories of the detached-electrons in the presence of external environ ments. The differential and total photo-detachment cross sections are derived using the theo retical imaging method and quantum approach for system composed of hydrogen negative ion (H−) placed near (5-10 bohr radii) a spherical surface. When a polarized laser-light interacts with the fragile quantum particle H−, which acts as a coherent source of the detached-electron waves. The detached-electron waves propagate away from the ion core in all possible directions. Far from the ion core, the waves propagate according to the semi-classical mechanics, and are correlated with the classical trajectories. The detached-electron waves initially propagating away from the ion core are reflected from the spherical surface, which then interfere with the waves originating in that direction, provides quantum interference patterns on a screen. These patterns reveal structural analysis of the negative ion and spherical surface induced effects in the photo-detachment cross section. To calculate the detached-electron flux on a screen placed at a large distance from the system, two detached-electron waves; one direct and the other reflected from the spherical surface are considered. These two coherent waves superimpose with each other at a large distance giving rise a resultant wave at a screen. This wave carries spherical effects in addition to the structural information about the negative ion. This wave is used to derive an expression for the detached-electron flux which displays strong oscillating structure on the observation screen. The idea of quantum interference is employed to explain this structure. The total photo-detachment cross section is derived by integrating the detached-electron flux in space around the system. It is observed that both, differential and total photo-detachment cross sections display oscillatory structure similar to the case of hydrogen negative ion placed near a plane surface, where the difference is caused by curvature in the spherical surface. The oscillatory structure in the cross section strongly depends upon the radius of curvature of the surface. Further, the laser polarization direction affects oscillations in the detached electron spectra. Maximum amplitude is obtained when laser is polarized parallel to the z-axis and it gradually diminishes with increasing angle and eventually goes to zero for laser polarized perpendicular to the z-axis. We also discussed the absorbing nature of the surface using the idea of tunnelling of waves. It is hoped that this theoretical study will guide future photo detachment microscopy experiments for anions near spherical surfac