OBSERVATION OF ORTHO-PARA $H_{3}{^{+}}$ SELECTION RULES IN PLASMA CHEMISTRY. A TIME DEPENDENT STUDY IN A HOLLOW CATHODE DISCHARGE.

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1997

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Ohio State University

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During the past two Columbus meetings, we reported our observation of a nonthermal ortho-to-para distribution of $H_{3}{^{+}}$ in hydrogen plasmas, where we found selection rules on nuclear spin modifications to hold in chain of plasma chemical reactions. The solution of rate equations incorporating nuclear spin branching ratios gave an ortho-to-para ratio of $H_{3}{^{+}}$ in good agreement with experiments. However, our observations were limited to steady-state plasma conditions, and no direct information relative to the kinetics of the plasma could be derived. For this reason, a new time dependent experiment has been carried out in a hydrogen plasma. Our study consisted of monitoring the evolution of ortho- and para-$H_{3}^{+}$ absorption during and after a short pulse discharging normal $H_{2}$ or para-$H_{2}$ confined in a hollow cathode. Diviations from thermal conditions were found stronger than those observed in the steady-state work, and the study of the $H^{+}_{3}$ absorption signals allowed a more detailed analysis of the chemical processes occurring during the thermalization of the o-$H^{+}_{3}$ and p-$H^{+}_{3}$ in the cell. An experimental value for the ratio of rate constants of the proton hop ($\widetilde{H}_{2} + H^{+}_{3} \rightarrow H_{2}\widetilde{H}^{+} + H_{2}$) and the hydrogen exchange ($\widetilde{H}_{2} + H^{+}_{3} \rightarrow H_{2}\tilde{H}^{+} + H\tilde{H}$) reactions has been obtained to explain the ortho-to-para $H^{+}_{3}$ ratios observed in the pulsed plasma.

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Author Institution: Laboratoire de Spectroscopie Hertzienne Associ\'e au C.N.R.S, U.F.R. de Physique; JILA; Department of Chemistry and Department of Astronomy and Astrophysics

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