Cold reactions of He with OCS and CO: competitive kinetics and the effects of the molecular multipole moments.

The reactions of He with OCS and CO have been studied at collision energies between ∼ ⋅ 200 mK and ∼ ⋅ 30 K by merging a beam of Rydberg He atoms with rotationally cold (∼3.5 K) seeded supersonic expansions containing either OCS or CO or a mixture of OCS (mole fraction 23.2%) and CO (76.

Cold Ion-Molecule Chemistry: The Very Different Reactions of He⁺ with CO and NO.

The ion-molecule reactions He+ + CO → He + C+ + O and He+ + NO → He + N+ + O have been measured at collision energies between 0 and kB · 10 K. Strong variations of the rate coefficients are observed below kB · 5 K. The rate of the He+ + CO reaction decreases by ~30% whereas that of the He+ + NO reaction increases by a factor of ~1.

Reaction of an Ion and a Free Radical near 0 K: He + NO → He + N + O.

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 ∼ ·10 K.

Multipole-moment effects in ion-molecule reactions at low temperatures: part III - the He + CH and He + CD reactions at low collision energies and the effect of the charge-octupole interaction.

We present experimental and theoretical studies of the He + CH and He + CD reactions at collision energies in the ·(0-10) K range. Helium atoms in a supersonic beam are excited to a low-field-seeking Rydberg-Stark state and merged with a supersonic beam of CH or CD using a curved surface-electrode deflector. The ion-molecule reactions are studied within the orbit of the helium Rydberg [He()] electron, which suppresses stray-electric-fields-induced heating and makes it possible to reach very low collision energies.

Multipole-moment effects in ion-molecule reactions at low temperatures: part II - charge-quadrupole-interaction-induced suppression of the He + N reaction at collision energies below ·10 K.

We report on an experimental and theoretical investigation of the He + N reaction at collision energies in the range between 0 and ·10 K. The reaction is studied within the orbit of a highly excited Rydberg electron after merging a beam of He Rydberg atoms (He(), is the principal quantum number), with a supersonic beam of ground-state N molecules using a surface-electrode Rydberg-Stark decelerator and deflector. The collision energy is varied by changing the velocity of the He() atoms for a fixed velocity of the N beam and the relative yields of the ionic reaction products N and N are monitored in a time-of-flight mass spectrometer.

Multipole-moment effects in ion-molecule reactions at low temperatures: part I - ion-dipole enhancement of the rate coefficients of the He + NH and He + ND reactions at collisional energies / near 0 K.

The energy dependence of the rates of the reactions between He and ammonia (NY, Y = {H,D}), forming NY, Y and He as well as NY, Y and He, and the corresponding product branching ratios have been measured at low collision energies between 0 and ·40 K using a recently developed merged-beam technique [Allmendinger , , 2016, , 3596]. To avoid heating of the ions by stray electric fields, the reactions are observed within the large orbit of a highly excited Rydberg electron. A beam of He Rydberg atoms was merged with a supersonic beam of ammonia using a curved surface-electrode Rydberg-Stark deflector, which is also used for adjusting the final velocity of the He Rydberg atoms, and thus the collision energy.

Ion-Molecule Reactions below 1 K: Strong Enhancement of the Reaction Rate of the Ion-Dipole Reaction He^{+}+CH_{3}F.

The reaction between He^{+} and CH_{3}F forming predominantly CH_{2}^{+} and CHF^{+} has been studied at collision energies E_{coll} between 0 and k_{B}·10  K in a merged-beam apparatus. To avoid heating of the ions by stray electric fields, the reaction was observed within the orbit of a highly excited Rydberg electron. Supersonic beams of CH_{3}F and He(n) Rydberg atoms with principal quantum number n=30 and 35 were merged and their relative velocity tuned using a Rydberg-Stark decelerator and deflector, allowing an energy resolution of 150 mK.