Total scattering, surface ionization, and photoionization of a beam of H₃ metastable molecules

In a previous paper we reported a technique for generating an intense hyperthermal beam of hydrogen atoms and metastable H₃ molecules. From the flight time of the H₃ species between the source and detector we estimated that its lifetime exceeds 40 μs and that it must therefore be in the 2p ²A″₂ excited Rydberg state. In this paper we report experiments utilizing this novel source of H₃ molecules. Beam-gas attenuation measurements indicate that the H₃-Ar cross section is roughly ten times larger than the H-Ar cross section for translational energies in the1 to 10 eV range. This observation is consistent with the assignment of the H₃ to that excited state, which has a much larger effective radius than a ground state hydrogen atom. The temperature dependence of the surface ionization of H₃ by heated tungsten and platinum filaments is used to obtain effective ionizational potentials of this species. These potentials suggest that upon interaction with a metal surface, the metastable state decays to the repulsive2p ²E′ state which then surface ionizes to produce H₃⁺. The production H ₃⁺ and H⁺ when the H₃ beam is irradiated with UV light from a high pressure mercury lamp was also observed and is attributed to the relatively low ionization potential (∼3.7 eV) of the 2p ²A″₂ metastable state of H₃.