Ionization of Rydberg H2 molecules at doped silicon surfaces.

The present study focuses on the interaction of H2 Rydberg molecules with doped silicon semiconductor surfaces. Para-H2 Rydberg states with principal quantum numbers n = 17-21 and core rotational quantum number N(+) = 2 are populated via resonant two-colour two-photon (vacuum ultraviolet-ultraviolet) excitation and collide at grazing incidence with a surface. For small Rydberg-surface separation, the Rydberg states are ionized due to the attractive surface potential experienced by the Rydberg electron and the remaining ion-core is detectable by applying a sufficiently strong external electric field. It is found that the surface ionization profiles (ion signal vs applied field) of H2 on p-type doped Si surfaces show a higher detected ion signal than for n-type Si surfaces, while an Au surface shows lower detected ion signal than either type of Si surface. It is shown that ion detectability decreases with increasing dopant density for both types of Si surfaces. Higher-n Rydberg states show higher ion detectability than lower-n Rydberg states but this variation becomes smaller when increasing the dopant density for both p- and n-type surfaces. Theoretical trajectory simulations were developed with a 2D surface potential model and using the over-the-barrier model for the ionization distance; the results help to explain the observed variations of the experimental surface ionization profiles with dopant density and type.