Observation of quantum capture in an ion-molecule reaction

Vogt and Wannier [Phys. Rev. 95, 1190 (1954)] predicted that the capture rate of a polarizable neutral atom or molecule by an ion should increase by a factor of 2 compared to the classical Langevin rate as the collision energy approaches zero. This prediction has not been verified experimentally. The H+2 + H2 reaction is ideally suited to observe this effect because the small reduced mass makes quantum effects related to s-wave scattering observable at higher collision energies than in other systems. Moreover, the reaction rate for this barrierless, strongly exothermic reaction follows the classical Langevin capture model down to cold-collision conditions (about kB x 1 K) and is not affected by short-range interactions. Below this temperature, a strong enhancement of the reaction rate resulting from charge-quadrupole interaction between H+2 and ground-state ortho H2 (J = 1) was observed. Here we present an experimental study of the reaction of H+ 2 and para H2 (J = 0), which has no dipole and no quadrupole moments, at collision energies below kB x 1 K. We observe an enhancement at the lowest collision energies which is attributed to the quantum enhancement predicted by Vogt and Wannier. Measurements of the reaction of HD+ with HD support this conclusion.