Interaction of Rydberg hydrogen atoms with metal surfaces

This thesis presents a theoretical and experimental investigation of the interaction of electronically excited Rydberg hydrogen atoms with metal surfaces and the associated charge-transfer process. As a Rydberg atom approaches a metal surface, the energies of the Rydberg states are perturbed by the surface potential generated by the image charges of the Rydberg electron and core. At small atom-surface separations, the Rydberg atom may be ionised by resonant charge transfer of the Rydberg electron to the continuum of delocalised unoccupied metal states, with which the Rydberg electron is degenerate in energy. Typically, this ‘surface ionisation’ can be measured by extracting the remaining positively charged ion-cores with externally applied electric fields. By applying various levels of theory, from classical to fully time-dependent quantum calculations, this thesis explores various experimentally relevant effects on the charge-transfer process, such as the magnitude and direction of the externally applied electric field, the atom collisional velocity, the presence of local surface stray fields and electronically structured surfaces. The theoretical results give insight into the previous experimental work carried out for the xenon atom and hydrogen molecule, and point out some of the fundamental differences from the hydrogen atom system.
Experiments involving Rydberg hydrogen atoms incident on an atomically flat gold surface, a rough machined aluminium surface and a single crystal copper (100) surface are presented, providing for the first time the opportunity to make a quantitative comparison of theory and experiments. The ability to control the critical distance at which charge-transfer occurs is demonstrated by using Rydberg states of varying dimensions and collisional velocities. By changing the collisional angle of the incident Rydberg beam, the effect of Rydberg trajectory is also investigated. By manipulating the polarisation of the Rydberg electron with electric fields, genuine control over the orientation of the electron density distribution in the charge-transfer process is demonstrated. This property was predicted by the theory and should be unique to the hydrogen atom due to its intrinsic symmetry. By reversing the direction of the electric field with respect to the metal surface, electrons rather than positive ions are detected, with ionisation dynamics that appear to be very different, as predicted by quantum calculations. Experiments involving the single crystal Cu(100) surface also suggests possible resonance effects from image states embedded in the projected bandgap which are shown from quantum calculations to play an important role in the surface charge transfer of electronically structured metal substrates.
The experimental technique developed in this work provides some exciting future applications to study quantum confinement effects with thin films, nanoparticles and other bandgap surfaces. The ability to control the Rydberg orbital size, electronic energy, collisional velocity and orientation in the charge-transfer process will provide novel ways of probing the surface’s electronic and physical structure, as well as being a valuable feature in offering new opportunities for controlling reactive processes at metallic surfaces.

Keywords: charge transfer
image charge
metal surfaces
Rydberg interaction
surface ionisation
Rydberg
surface interaction
hydrogen atoms
Advisor: Softley, T