We present experimental evidence for nuclear spin selection rules in chemical reactions that have been theoretically anticipated by Quack ͓M. Quack, Mol. Phys. 34, 477 ͑1997͔͒. The abundance ratio of ortho-H 3 ϩ (Iϭ3/2) and para-H 3 ϩ (Iϭ1/2), Rϭ͓o-H 3 ϩ ͔/͓ p-H 3 ϩ ͔, has been measured from relative intensities of their infrared spectral lines in hydrogen plasmas using 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 demonstrate the spin memory of protons even after ion-neutral reactions, and thus the 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 ϩ to p-H 3 ϩ ratio. The observed R(t) has been analyzed using simultaneous rate equations assuming the nuclear spin branching ratios calculated from Quack's theory. In p-H 2 plasmas, the electron impact ionization followed by the ion-neutral reaction H 2 ϩ ϩH 2 →H 3 ϩ ϩH produces pure p-H 3 ϩ , but the subsequent reaction between p-H 3 ϩ and p-H 2 scrambles protons. While the proton hop reaction ͑rate constant k H ͒ maintains the purity of p-H 3 ϩ , the hydrogen exchange reaction ͑rate constant k E ͒ produces o-H 3 ϩ and acts as the gateway for nuclear spin conversion. The value of R(t), therefore, depends critically on the ratio of their reaction rates ␣ϭk H /k E. From observed values of R(t), the ratio has been determined to be ␣ϭ2.4. This is in approximate agreement with the value reported by Gerlich using isotopic species.

A monobridged isomer of the disilyne Si2H2, the planar bridged-2 form Si(H)SiH, has been detected through its millimeter- and submillimeter-wave rotational spectrum. The molecules are produced in a low pressure, low power plasma in a mixture of SiH4 and Ar cooled at liquid nitrogen temperature. From the analysis of a total of 148 lines, a set of accurate rotational constants have been derived. Using the geometry predicted by ab initio calculations (see the preceding paper), the spectrum of Si(D)SiD has been predicted, observed, and analyzed. From the two sets of rotational constants, a preliminary molecular structure has been derived by fixing the Si1–Si2–bridged H angle to the ab initio value of 52.5°: Si1–Si2=2.119 Å; Si2–bridged H=1.629 Å; Si2–terminal H=1.474 Å; Si1–Si2–terminal H=157.5°.

Intensity ratios of infrared spectral lines of ortho-and para-H 1 3 have been measured in hydrogen plasmas using normal and para-H 2 . Large dependences of the intensity ratios on the parent gases have been observed, indicating the spin memory of protons even after chemical reactions. The results clearly demonstrate the existence of selection rules on nuclear spin modifications in chemical reactions theoretically anticipated by M. Quack [Mol. Phys. 34, 477 (1977)]. [S0031-9007 (97)03170-0] PACS numbers: 33.15.-e, 33.20.Ea, 34.90.+qSelection rules that relate quantum states before and after various physical and chemical processes are fascinating subjects which reflect both the symmetry and the orders of magnitude of the physicochemical interactions. It is well known that the ordinary radiative processes of atoms and molecules, spectroscopy, obey the most rigorous selection rules [1] while collisional processes obey less rigorous selection rules [2]. This is because (a) molecules retain higher symmetry in a more homogeneous field in the former processes than in the latter, and (b) the magnitude of interaction is much smaller in the former, thus its higher order effect is negligible. In both processes, however, the selection rules related to the nuclear spin quantum numbers, which result from the invariance of both interactions for permutations of identical nuclei, hold most rigorously.