Observation of quantum capture in an ion-molecule reaction

Abstract

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 ${\mathrm{H}}_{2}^{+}$ $+$ ${\mathrm{H}}_{2}$ 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 follows the classical Langevin capture model down to cold-collision conditions (about ${k}_{\mathrm{B}}\ifmmode\times\else\texttimes\fi{}1$ 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 ${\mathrm{H}}_{2}^{+}$ and ground-state ortho ${\mathrm{H}}_{2}$ ($J=1$) was observed. Here we present an experimental study of the reaction of ${\mathrm{H}}_{2}^{+}$ and para ${\mathrm{H}}_{2}$ ($J=0$), which has no dipole and no quadrupole moments, at collision energies below ${k}_{\mathrm{B}}\ifmmode\times\else\texttimes\fi{}1$ 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 ${\mathrm{HD}}^{+}$ with HD support this conclusion.