Observation of quantum capture in an ion-molecule reaction

In 1954, Vogt and Wannier (Phys. Rev. 95 , 1190) predicted that the capture rate of a polarizable neutral atom or molecule by an ion should increase by a factor of two compared to the classical Langevin rate as the collision energy approaches zero. This prediction has not been verified experimentally. The H +2 + H 2 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 fol-lows the classical Langevin capture model down to cold-collision conditions (about k B · 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 H 2 ( J = 1 ) was observed. Here we present an experimental study of the reaction of H +2 and para H 2 ( J = 0 ), which has no dipole and no quadrupole moments, at collision energies below k B · 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.