No abstract

The H2(v″) vibrational distribution observed in a medium-density hydrogen discharge is analyzed in terms of standard collisional processes. All processes in the model are specified independently of adjustable parameters. The calculated distribution is found to be a sensitive function of the wall relaxation process. The warm-gas-temperature discharge analyzed here leads to a substantially more rapid H2(v″) wall relaxation than is inferred from cold-gas systems.

An optimized tandem two-chamber negative-ion source system is discussed. In the first chamber high-energy (E>20 eV) electron collisions provide for H2 vibrational excitation, while in the second chamber negative ions are formed by dissociative attachment. The gas density, electron density, and system scale length are varied as independent parameters. The extracted negative ion current density passes through a maximum as electron and gas densities are varied. This maximum scales inversely with system scale length R. The optimum extracted current densities occur for electron densities nR=1013 electrons cm−2 and gas densities N2R in the range 1014–1015 molecules cm−2. The extracted current densities are sensitive to the atomic concentration in the discharge. The atomic concentration is parametrized by the wall recombination coefficient γ and scale length R. As γ ranges from 0.1 to 1.0 and for system scale lengths of 1 cm, extracted current densities range from 8.0 to 80 mA cm−2. The relative negative-ion yields from single-chamber and tandem two-chamber systems are compared. Estimates are made for the rates of polar dissociation of H2 molecules and H+3 ions, and these rates are compared with the dissociative attachment rates.

The recombination and dissociation of H+2 and H+3 ions incident upon metal surfaces leads to H, H2(v″), and H− products rebounding from the surface. A four-step model for H+2 -ion recombination generates H2(v″) via resonant electron capture through the b 3Σ+u and X 1Σ+g states. A molecular trajectory analysis provides final-state H2(v″) distributions for incident energies of 1, 4, 10, and 20 eV. The calculated H2:H+2 yields compare favorably with the observed yields. A similar four-step model for incident H+3 proceeds via resonant capture to form the H3(2p 2E′→2p 2A1) ground state, in turn dissociating into H+H2(v_″), with the fragment molecule rebounding to give the final H2(v″) distribution. Comparing the final populations v″≥5 for incident H+2 or H+3 shows that the H+3 ion will be more useful than H+2 for H− generation via dissociative attachment. Molecular ions incident upon low-work-function surfaces generate additional H2(v″) via resonant electron capture through excited electronic states and provide two additional sources of H− production: Direct H− production by H dissociation products rebounding from the surface and H− production through the formation of H−2 in the surface selvage that in turn dissociates into H+H−. The H−2 in the selvage is formed by resonant capture to the low-lying vibrational levels of H2(v″), and complements dissociative attachment to high-lying levels in the discharge. The H, H2(v″), and H− yields are inventoried for H+3 incident upon barium surfaces.

Vibrational distribution functions are derived for a number of rocksalt-structure alkali halides using a more refined treatment of the interionic forces than that provided by regarding them as rigid point charges.The dipole moment at any given ion site is calculated taking into account the contribution from the deformation of the electron distribution resulting from both polarization and overlap repulsion between nearest neighbors. In this way the dipole-dipole part of the Coulomb interaction is treated self-consistently.Both room temperature and 0°K input parameters are used, and the derived specific-heat data are compared with experimental results. The over-all agreement with experiment is significantly better than that obtained by treating the ions as rigid point charges.Sets of phonon dispersion curves are also given. For NaI they are in much better agreement with those determined experimentally by inelastic neutron scattering than are the rigid ion curves. There appears to be close agreement with the results of the "shell-model" calculations.