| We observe the capture and field ionization of individual atoms near the side-wall of a single, positively-charged, suspended carbon nanotube. The steep cylindrically-symmetric field gradient around the nanotube creates an attractive force that captures polarizable atoms, corresponding to a highly singular inverse-square potential. In this potential, atoms with angular momenta below a critical threshold value, determined by the charge on the nanotube, will be pulled towards the singularity at the origin of the potential-energy landscape. The strong fields near the nanotube will ionize the captured atoms, and these ions can be individually counted with a nearby ion detector operated in discrete pulse-counting mode. Extremely large cross-sections for ionization from a laser-cooled atomic beam are observed at modest voltages due to the nanotube's small radius and extended length. Efficient and sensitive neutral atom detectors can be based on the capture and field-ionization processes, as we have demonstrated.;The effects of the field strength on both the atomic capture and the ionization process are clearly distinguished in the data. When atoms are captured, we expect that the capture cross-section will increase as a linear function of the voltage applied to the nanotube. We observe this behavior for charging voltages of 150--300 V, which demonstrates the proportionality between the capture cross-section and the strength of the electric field. The field affects the ionization process as well, and we report two pertinent observations: (1) the creation of ions is suppressed below 150 V, corresponding to the conditions where the field strength at the surface of the nanotube is no longer sufficient to rapidly ionize captured atoms; and (2) we observe prompt and delayed ionization events related to the locations at which these events occur, revealing that the strength of the field determines whether an ion generated very close to the surface will quickly escape from the attraction of its own image charge. Investigation of these effects reveals a rich variety of physical behaviors in the nanoscale regime. |
