Observation of a nonthermal ortho-to-para ratio of H3+ in hydrogen plasmas: Spin selection rules in plasma chemistry

Last year at this conference, the authors reported the observation of a nonthermal nuclear spin distribution of H3+ in hydrogen plasmas. Positive column and hollow cathode discharges with 100% parahydrogen as the source gas produced an H3+ ortho-to-para ratio of about 3:2 instead of the usual 1:1. Observing H3+ enriched in the para modification when para-H2 is used proves that the nuclear memory is kept even after a chain of ion-neutral reactions. The selection rules based on permutation symmetry are the most robust ones in spectroscopy and collisional interactions and are not broken except in near degenerate cases. The reaction H2 + H2+ → H3+ + H will produce only para H3+ if both reactants are para, but a combination of ortho and para reactants would produce ortho and para H3+. The authors have formulated a group theoretical method adapted from Quack's paper to determine the branching ratios of chemical reactions according to the nuclear spin species of the reactants. The method is also applicable to subsequent hydrogen exchange reactions involving H3+. Using steady-state rate equations in which the spin modification is explicity stated, an H3+ ortho-to-para ratio was calculated which approximately explains the experimental value. The group theoretical method for determining spin modification branching ratios as well as the ion-neutral chemical kinetics will be presented