Charge exchange collisions of hydrogen atoms and ions with laboratory surfaces

Scattering and negative ion formation processes have been investigated for the atomic hydrogen projectiles (H, H+, H− ) incident on polycrystalline Cu and Cu(100) surfaces with kinetic energies in the range of 25 to 200 eV. Hydrogen beams were directed to the target surfaces at a fixed incident angle of 18° from the surface plane, and an electrostatic energy analyzer was used to record the energy distributions of backscattered ions within the plane of incidence. Experiments were performed in a high vacuum system with an ultimate pressure of approximately 10 −9 mbar, resulting in efficient adsorption of H₂O molecules on the target surfaces. The total H− ion yield per incident H atom was dependent on the surface conditions and was typically around 0.5% at room temperature for all energies investigated. Energy loss analysis of the resulting H− ions indicated that the electron capture process occurs directly through the valence electrons of adsorbed gases, rather than via resonant transitions of conduction electrons from the Cu surface. Measurements performed with incident H+ and H− ions demonstrated a suppressed neutralization as compared to clean metals, resulting in image potential effects amounting to approximately 3 to 4 eV. In contrast to the H+ experiments, incident H− ions yielded a second contribution of backscattered ions that were reflected directly from the adsorbed layer without losing memory of the initial charge state. These incident charge state effects are attributed to the dielectric and wide band gap insulating properties of an adsorbed H ₂O layer.