Session H03: Low-energy and novel-storage-ring collision physics

Progress in theoretical descriptions of dissociative recombination and other electron scattering processes in diatomic ions

The scattering process of an electron off of a molecular ion is a challenging yet important part of molecular chemistry. With newer, more accurate data from storage ring experiments along with ever more abundant computational resources, there arises a desire for more precise theoretical methods tackling this problem.

MQDT approaches such as the frame transformation method have been used to compute the dissociative recombination as well as rovibrational excitation processes for diatomic ions in multiple past studies. We describe and summarize our recent developments in the field progressing towards more accurate and less approximative descriptions. We study various cases of diatomics such as H₂⁺, HeH⁺ and CH⁺ and show good agreement of cross section data with the recent state-of-the-art storage ring experiments.   

Atomic and Molecular Physics at the Cryogenic Storage Ring

The Cryogenic Storage Ring (CSR) at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany is one of the most ambitious projects among the new electrostatic storage facilities. It features a nested vacuum structure with a circumference of 35 m, where the innermost vacuum chambers can be cooled to liquid helium temperature (~4K) by a closed cycle cooling unit. The cryogenic temperatures result in very low residual gas densities inside the experimental chambers (on the order of 10³ cm^-3), as well as a strong reduction of the blackbody radiation field. The electrostatic ion optics allow for storage of positive and negative ions with kinetic energies of up to 300 keV per unit charge, essentially independent of the particle mass.

   We will give a brief overview of the experimental capabilities of the CSR and report on recent photodetachment experiments with negative atomic and molecular ions. Furthermore, we will present the results of merged beams experiments of free electrons and neutral atoms with cold molecular ions of astrophysical interest.   

Probing charge-transfer neutralization reactions of atmospheric importance using the ion storage facility DESIREE

The Double ElectroStatic Ion Ring ExpEriment (DESIREE) facility located at Stockholm University, Sweden, uniquely allows for studies of mutual neutralization interactions between cations and anions at low and well-defined internal temperatures and centre-of-mass collision energies down to about 20 K and ~50 meV, respectively [1-3].

Using this facility, we aim for a better understanding of how small and large molecules are formed and processed in astrophysical, atmospheric, and combustion plasmas, where we combine several novel experimental methods to build a fundamental picture of the transfer of charge-, energy- and mass in collisional reactions. Control over the reaction environment [1-6] means that desired information, e.g., reaction products, can be obtained over many of the conditions needed to accurately model plasmas where these processes are important.

Here, I highlight how this facility can play a crucial role in studying in mutual neutralization reactions relevant to cool atmospheric plasmas: providing insights into ion balance processes, and, e.g., in the production of F-layer UV airglows. I focus on reactions involving the primary atomic and molecular oxygen and nitrogen ions [4,5], i.e., O^- + O⁺, O^- + N⁺, O^- + NO⁺, O^- + O₂⁺, and O^- + N₂⁺, where, in the processes with molecular cations, I can further determine the fractionation into two- and three-body product channels and study the effects of the rovibrational energy of the molecular ion on the reaction process.

 

References

[1] R. D. Thomas et al., Rev. Sci. Instrum. 82, 065112 (2011)

[2] H. T. Schmidt et al., Rev. Sci. Instrum. 84, 055115 (2013)

[3] H. T. Schmidt et al., Phys. Rev. Lett. 119, 073001 (2017)

[4] M. Poline et al. Phys. Chem. Chem. Phys. 23, 24607 (2021)

[5] M. Poline et al. Phys. Rev. A, 105, 062825 (2022)

[6] M. Poline et al. Phys. Rev. A, 106, 012812 (2022)   

Dissociation and dynamics of electron attachment to specific intramolecular structures.

Dissociative electron attachment (DEA) is one of the processes that involve low-energy electrons formed upon high-energy irradiation. Typical DEA studies focus on the detection and yield measurements of negative ions produced as a function of the incident electron energy using mass spectroscopic techniques. Such studies are useful in determining the reaction pathways and the resonant states, which are responsible for molecular decay. However, to provide more knowledge on this process other techniques are necessary to employ. Therefore, in the last two years, we have been developing a new experimental set-up, a velocity map imaging (VMI) spectrometer, which is capable to measure the kinetic energy and angular distributions of the negative ions formed in the DEA process. This type of study provides information about the DEA energy threshold and reveals the symmetry of the resonant states, which are created prior to the dissociation. The construction of the experimental setup has been completed and recently we have made significant progress on the spectrometer calibration, development of analysis codes, and obtained preliminary data for several molecular exemplars. For our preliminary studies, we investigated dissociation dynamics of low-energy electron attachment to simple molecules, containing specific chemical bonds (e.g., peptide bonds) and functional groups (e.g., hydroxyl group). Differential laboratory-frame momentum distribution of fragmented ions has been measured for different incident electron energies over the energy range of resonances, which we measured previously.