Ion-dipole and ion-quadrupole interaction effects in ion-molecule reactions at collisional energies Ecoll between 0 and 40⋅kB K.
Many ion-molecule reactions are barrierless, exothermic reactions which proceed with high rate coefficients even at zero temperatures and are important for the synthesis of molecules in the interstellar medium [1]. These reactions are usually modelled by the classical Langevin model which predicts a temperature- and collisional-energy-independent reaction capture rate coefficients. However, at low collision energies, significant deviations from the Langevin rate coefficients arise resulting from the electrostatic interaction between the charge of the ion and the electric dipole [2-3] and quadrupole [4-5] moments of the neutral molecule.
We present experimental and theoretical studies of ion-molecule reactions involving He+ and simple neutral molecules. These reactions are studied within the orbit of a Rydberg electron in a merged Rydberg-neutral beams set-up as described in Refs.[2,4,6]. The helium atoms are excited to a low-field-seeking Rydberg-Stark state, and deflected and merged with a supersonic beam of the neutral molecule using a curved surface-electrode Rydberg-Stark decelerator [2]. We monitor the product-ion yield in a time-of-flight mass spectrometer as a function of the Rydberg helium velocity.
The measured total product yields, I, display a significant dependence on the collisional energy, Ecoll, when the molecule has a permanent dipole or quadrupole moments. With decreasing Ecoll below 10⋅kB K , we observe (i) a significant increase of I in the case of a molecule with a permanent dipole moment (e.g., NH3, ND3 and CH3F), and (ii) a pronounced suppression of I in the case of a molecule with a negative Qzz component of the quadrupole moment (e.g., N2 and CO). We calculate the reaction rate coefficients using a capture model that includes the rotational-state-dependent energy shift of the molecule in the electric field of the He+ ion and average over the rotational state population distribution in the supersonic source, including nuclear-spin statistics effects. The agreement between the experimental data and the model is very good for a number of molecules: CH3F [2], NH3, ND3, N2 and CO. The observed significant deviation from the Langevin model is attributed to the locking of the molecular angular momentum at low collisional energies [3].
[1] D. Smith, Chem. Rev., 1992, 92, 1473.
[2] V. Zhelyazkova, F. B. V. Martins, J. A. Agner, H. Schmutz and F. Merkt, Phys. Rev. Lett., 2020, 125, 263401.
[3] J. Troe, Chem. Phys., 1987, 87, 2773.
[4] P. Allmendinger, J. Deiglmayr, K. Höveler, O. Schullian and F. Merkt, J. Chem. Phys., 2016, 145, 244316.
[5] E. I. Dashevskaya, I. Litvin, E. E. Nikitin and J. Troe, J. Chem.Phys., 2005, 122, 184311.
[6] K. Höveler, J. Deiglmayr, J. A. Agner, H. Schmutz and F. Merkt, Phys. Chem. Chem. Phys., 2021, 23, 2676–2683.