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

Abstract

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 2 p 2A^{[script `]}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 the 1 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 repulsive 2 p 2E[script '] state which then surface ionizes to produce H⁺₃. 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 2 p 2A^{[script '][script ']}₂ metastable state of H3.