A METASTABLE HYDROGEN ATOMIC-BEAM - CONSTRUCTION AND CHARACTERISTICS

A detailed report on a thermal beam of 2S metastable hydrogen (deuterium) atoms is given. This beam is produced in two steps: molecular dissociation in a radiofrequency discharge and electronic bombardment. We have obtained a good collimation (beam spread ≃0.8°) and a very accurate shielding against electric parasitic fields so that quenching losses from the source region to the detection region are reduced to ≃2%. Velocity distribution in the beam has been thoroughly studied. We have recently used this beam to remeasure the fundamental physical constant R∞ (Rydberg constant) with a very high accuracy, limited only by the wavelength and frequency standard in the optical region. The characteristics of such a beam, although specifically designed for this measurement, can also be interesting for different types of experiments.