The relative kinetic energy distribution of the hydrogen atoms formed by the dissociation of the electronically excited H2 molecule Experimental observations of excited dissociative and metastable states of H3 in neutralized ion beams
Results are presented of an experimental study of the relative concentration of ions in the positive column of small-diameter noble-gas discharges. The ratio of the molecular ion concentration to the atomic ion concentration has been measured as a function of discharge pressure and current for dc discharges in argon, krypton, and xenon. Discharge pressures and currents were varied from 0.1 to 10 Torr and 15 to 40 mA. A simple theoretical model of the positive column was developed and the pressure dependence of the ion concentration ratio was calculated. By parametrically fitting the predicted ion ratio equation to the experimental data, estimates of several reaction rate coefficients could be derived. Where possible, the ion ratio data and the derived rate coefficients have been compared to previously published data.
A study has been made of the angular distribution of scattered particles induced by proton impact on a target of helium. Cross sections are presented for scattering of protons, of neutral hydrogen atoms, and of metastable hydrogen atoms. Impact energies range from 4 to 20 keV; scattering angles are from 0' to 2. 0'. Measurements of the fractional metastable content of the scattered neutral flux rise from 1% or less at 0' to as much as 8% at angles of 1'.Theoretical predictions of the cross sections for scattering of all particles, neutrals and protons, show reasonable agreement with experiments. Theoretical predictions, by Colegrave and Stevens, of the probability for forming a neutral atom disagree with the experimental values by a constant phase factor. Measurements of the probability for forming a metastable atom do not agree with available theoretical predictions.
ing of the H(2s) atom on the basis of these results. Electron capture into autoionizing states is a likely explanation for the small H measurements. Electron capture into autoionizing states could not be detected in either our apparatus or the apparatus of Gilbody et al. The results of Donnally eg g). 0 support the concept of electron capture into autoionizing states. If electron capture is as important as we suggest, then approximately 90%%uo of the capture events result in autoionizing states of H . The discrepancy between the experimental quenching cross sections of Krotkov et gl. ' and their calculation can at least be partially explained by the exclusion of the electron-capture process in the calculation. The discrepancy between their experimental quenching measurements and the quenching measurements of Gilbody et al. 3 precludes further quantitative comparison at this time. ACKNOWLEDGMENT The authors wish to thank H. D. Kisner for the use of the computer program used in the Bates-Walker calculation.A study has been made of H2' dissociation by impact on targets of He, Ar, H2, and N2. Cross sections were measured for the angular distributions of metastable hydrogen, protons, and neutral hydrogen atoms resulting from dissociation. Projectile energies are in the range 4-12 keV; the angular range of the measurements extended from 0. 5' to 5. 0 . The angular distributions of metastables are consistent with their formation through excitation of the H2' projectile into the 2s0~and 3p0"repulsive states of H2'. In all cases the dissociation cross section appears to be strongly peaked towards small angles between the internuclear axis of the molecule and the direction of the H2' projectile; in the case of H(2s) formed by dissociation on He, the cross section appears to vary as approximately the square of the cosine of that angle. Angular distributions vary quite markedly from one target to another.
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