Selection rules for nuclear spin modifications in ion-neutral reactions involving H-3(+)

We present experimental evidence for nuclear spin selection rules in chemic al reactions that have been theoretically anticipated by Quack [M. Quack, M ol. Phys. 34, 477 (1997)]. The abundance ratio of ortho-H-3(+) (I = 3/2) an d para-H-3(+) (I = 1/2), R = [o-H-3(+)]/[p-H-3(+)], has been measured from relative intensities of their infrared spectral lines in hydrogen plasmas u sing para-H-2 and normal-H-2 (75% o-H-2 and 25% p-H-2). The observed clear differences in the value of R between the p-H-2 and n-H-2 plasmas demonstra te the spin memory of protons even after ion-neutral reactions, and thus th e existence of selection rules for spin modifications. Both positive column discharges and hollow cathode discharges have been used to demonstrate the effect. Experiments using pulsed plasmas have been conducted in the hollow cathode to minimize the uncertainty due to long-term conversion between p- H-2 and o-H-2 and to study the time dependence of the o-H-3(+) to p-H-3(+) ratio. The observed R(t) has been analyzed using simultaneous rate equation s assuming the nuclear spin branching ratios calculated from Quack's theory . In p-H-2 plasmas, the electron impact ionization followed by the ion-neut ral reaction H-2(+) + H-2--> H-3(+) + H produces pure p-H-3(+), but the sub sequent reaction between p-H-3(+) and p-H-2 scrambles protons. While the pr oton hop reaction (rate constant k(H)) maintains the purity of p-H-3(+), th e hydrogen exchange reaction (rate constant k(E)) produces o-H-3(+) and act s as the gateway for nuclear spin conversion. The value of R(t), therefore, depends critically on the ratio of their reaction rates alpha = k(H)/k(E). From observed values of R(t), the ratio has been determined to be alpha = 2.4. This is in approximate agreement with the value reported by Gerlich us ing isotopic species. (C) 2000 American Institute of Physics. [S0021-9606(0 0)01016-3].