The interaction of a beam of Rydberg molecules with a metal surface is investigated for the first time. Hydrogen molecules in a supersonic expansion are excited to Rydberg states with principal quantum number n, in the range 17–22 and are directed at a small angle onto a flat surface of either aluminum or gold. Detection of ions produced when Rydberg electrons tunnel into the metal surface provides information on the interaction between the Rydberg molecules and the surface potential. The experimental results suggest that, when close to the metal surface, the Rydberg molecules undergo a process of surface induced rotational autoionization. It is found that the surface-ionization cross section shows strong resonances as a function of the applied electric field, which are independent of the metal studied.
The interaction of Rydberg atoms with metallic surfaces and thin metallic films has been the subject of several previous experimental and theoretical studies. As the atom approaches the surface the Rydberg electron is subject to fields caused by the presence of image charges in the metal, and thus the physics is closely related to the Stark effect behavior of the atom. It has been shown that the predominant process at sufficiently close distance of approach is the tunneling ionization of the Rydberg electron into the conduction band of the metal. The ability to manipulate the Rydberg electron spatial distribution is an important aspect of these studies. Not only can the mean radius of the Rydberg orbit be adjusted by selection of principal quantum number (r ~ n2a0) but the angular distribution of the electron density can also be controlled by selection of specific Stark states in the presence of a field; thus, it is possible to polarize the atom such that the Rydberg electron distribution lies either in front of the ionic core, or trails behind the ionic core, as the atom approaches the surface. Measurements of the field required to pull the resultant ion away from collision with the surface have previously shown that the distance at which ionization occurs is strongly dependent on the principal quantum number, but surprisingly shows little dependence on the angular distribution . The tunneling ionization probability should also be dependent on the long-range surface potential and the degree of surface roughness, as well as being sensitive to the presence of adsorbates.
