The reactions between
ions and free radicals are among the fastest
chemical reactions. They are predicted to proceed with large rates,
even near 0 K, but so far, this prediction has not been verified experimentally.
We report on measurements of the rate coefficient of the reaction
between the ion He+ and the free radical NO at collision
energies in the range between 0 and ∼ k
B·10 K. To avoid heating of the ions by stray electric
fields, the reaction is observed within the large orbit of a Rydberg
electron of principal quantum number n ≥ 30,
which shields the ion from external electric fields without affecting
the reaction. Low collision energies are reached by merging a supersonic
beam of He Rydberg atoms with a supersonic beam of NO molecules and
adjusting their relative velocity using a chip-based Rydberg–Stark
decelerator and deflector. We observe a strong enhancement of the
reaction rate at collision energies below ∼k
B·2 K. This enhancement is interpreted on the basis
of adiabatic-channel capture-rate calculations as arising from the
near-degenerate rotational levels of opposite parity resulting from
the Λ-doubling in the X 2Π1/2 ground
state of NO. With these new results, we examine the reliability of
broadly used approximate analytic expressions for the thermal rate
constants of ion–molecule reactions at low temperatures.