Effects of the charge–dipole and charge–quadrupole interactions on the He⁺ + CO reaction rate coefficients at low collision energies

Abstract

The reaction between He$^+$ and CO forming $\textrm{He} + \textrm{C}^+ + \textrm{O}$ has been studied at collision energies in the range between $0$ and $k_\textrm{B} \cdot 25 \, \textrm{K}$. These low collision energies are reached by measuring the reaction within the orbit of a Rydberg electron after merging a beam of He$(n)$ Rydberg atoms and a supersonic beam of CO molecules with a rotational temperature of $6.5 \, \textrm{K}$. The capture rate of the reaction drops by about $30\%$ at collision energies below $k_\textrm{B} \cdot 5~\textrm{K}$. This behavior is analyzed in terms of the long-range charge-dipole and charge-quadrupole interactions using an adiabatic-channel capture model. Although the charge-dipole interaction has an effect on the magnitude of the rate coefficients, the effects of the charge-quadrupole interaction determine the main trend of the collision-energy dependence of the rate coefficients at low collision energies. The drop of the capture rate coefficient at low collision energies is attributed to the negative sign of the quadrupole moment of CO ($Q_{zz} = -2.839 \, \textrm{D}$\AA) and is caused by the $|J|M|\rangle = |00\rangle$ and $|11\rangle$ rotational states of CO, which represent about $70\%$ of the CO molecules at the rotational temperature of $6.5 \, \textrm{K}$.

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

“…The averaged data show no significant dependence on E coll / k B between ∼10 and ∼1.5 K. However, below ∼1 K, a striking increase of the reaction yield can be observed, with a pronounced maximum at E coll = 0. This sharp enhancement near zero collision energy represents the most remarkable result of the present investigation and differs markedly from the behavior observed under similar experimental conditions for the He + + CO → He + C + + O reaction 50 , 59 although NO and CO have similar molecular structures and electric dipole moments (−0.1574(14) D 60 , 61 vs 0.112 D, 62 respectively). In the case of the He + + CO reaction, the rate coefficient was found to decrease by about 30% at collision energies below ∼ k B ·5 K, which was interpreted as arising from the negative value of the quadrupole moment of CO ( Q zz = −2.839 D Å) 63 on the basis of rotationally adiabatic channel calculations.…”

“…The presence of the Λ-doubling in NO effectively enhances the effect of the molecular electric dipole moment and makes the ion-dipole interaction the dominant long-range interaction, in contrast to the case of CO where the ion-quadrupole was found to be the dominant long-range interaction. 59 Our experimental results have verified theoretical predictions made by Clary and his co-workers more than 30 years ago and responded, with some delay, to their encouragement and recommendation to experimentalists that “particular emphasis should be placed on the variation of the rate coefficient with temperature close to 0 K”. 21 …”

“…The collision-energy dependence of the reaction yield of the He + + NO reaction (sharp increase near 0 K) markedly differs from the behavior observed in the He + + CO reaction (a decrease of ∼30% near 0 K), , although NO and CO have similar masses, polarizabilities, rotational constants, and electric dipole and quadrupole moments. The presence of the Λ-doubling in NO effectively enhances the effect of the molecular electric dipole moment and makes the ion-dipole interaction the dominant long-range interaction, in contrast to the case of CO where the ion-quadrupole was found to be the dominant long-range interaction .…”

“…In the case of the He + + CO reaction, the rate coefficient was found to decrease by about 30% at collision energies below ∼ k B •5 K, which was interpreted as arising from the negative value of the quadrupole moment of CO (Q zz = −2.839 D Å) 62 on the basis of rotationally adiabatic channel calculations. 50,58…”

“…The overall behavior of the rate coefficients differs from what was observed for the He + + CO → He + C + + O reaction. In that reaction, the effects of the charge-quadrupole interaction were found to be dominant over those of the charge-dipole interaction (compare Figures and with Figure 4 of ref ). Moreover, the negative sign of the CO quadrupole moment ( Q zz = −2.839 D Å) was found to be crucial to explain the ∼30% reduction of the rate coefficients at collision energies below ∼ k B ·5 K. In the case of the He + + NO → He + N + + O reaction, changing the sign of the quadrupole moment does not significantly affect the behavior of the rate coefficients below 1.5 K, where the behavior is dominated by the effects of the charge-dipole interaction.…”