A theoretical description of the dissociative recombination process for the HCO+ ion suggests that the nonadiabatic Renner-Teller coupling between electronic and vibrational degrees of freedom plays an important role. This finding is consistent with a recent study of this process for another closed-shell molecule, the H-3(+) ion, where Jahn-Teller coupling was shown to generate a relatively high rate. The cross section obtained here for the dissociative recombination of HCO+ exhibits encouraging agreement with a merged-beam experiment.

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Renner-Teller effects in HCO$^{+}$ dissociative recombination

Cross sections and thermal rate coefficients are computed for electron-impact dissociative recombination and vibrational excitation/de-excitation of the N 2 + molecular ion in its lowest six vibrational levels, for collision energies/temperatures up to 2.3 eV/5000 K.

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Low-energy electron impact dissociative recombination and vibrational transitions of N 2 +

We review the study of dissociative recombination and rovibrational excitation of diatomic and small polyatomic molecular ions initiating complex organic molecules formation. In particular, we show how multichannel quantum defect theory (MQDT) and R-matrix methods are used to compute cross-sections and rate coefficients for cations in well-defined rovibrational levels of the ground electronic state, from sub-meV up to a few eV collision energies. The most recent MQDT results are compared either with other theoretical data or with measured data obtained in storage-ring experiments.

/pdf/electron-induced-excitation-recombination-and-dissociation-243hwlpemz.pdf

Electron-Induced Excitation, Recombination, and Dissociation of Molecular Ions Initiating the Formation of Complex Organic Molecules

A theoretical approach for the electron-impact vibronic excitation of molecular ions with low-lying excited electronic states is described. In this approach, the fixed-nuclear $R$-matrix method is employed to compute electron-ion scattering matrices in the Born-Oppenheimer approximation. A vibronic frame transformation and the closed-channel elimination procedure in a spirit of molecular quantum defect theory are employed to construct an energy-dependent scattering matrix describing interactions between vibronic channels of the target ion induced by the incident electron. The obtained scattering matrix accounts for Rydberg series of vibronic resonances in the collisional spectrum. The approach is applied to the ${\mathrm{CH}}^{+}$ ion of an astrophysical and technological interest. Cross sections for vibronic excitation for different combinations of initial and final vibronic states are computed. A good agreement between electronic-excitation cross sections, obtained using the quantum defect theory and in a direct $R$-matrix calculation, demonstrates that the present approach provides a reliable tool for determination of vibronic (de-)excitation cross sections for targets with low-energy electronic resonances. Such targets were difficult to treat theoretically using earlier methods.

Cross sections for vibronic excitation of CH + by low-energy electron impact

We present state-selected measurements of rotational cooling and excitation rates of CH^{+} molecular ions by inelastic electron collisions. The experiments are carried out at a cryogenic storage ring, making use of a monoenergetic electron beam at matched velocity in combination with state-sensitive laser dissociation of the CH^{+} ions for simultaneous monitoring of the rotational level populations. Employing storage times of up to 600 s, we create conditions where electron-induced cooling to the J=0 ground state dominates over radiative relaxation, allowing for the experimental determination of inelastic electron collision rates to benchmark state-of-the-art theoretical calculations. On a broader scale, our experiments pave the way to probe inelastic electron collisions for a variety of molecular ions relevant in various plasma environments. 

Laser Probing of the Rotational Cooling of Molecular Ions by Electron Collisions

We used the multichannel quantum defect theory to compute cross sections and rate coefficients for the dissociative recombination of CH + initially in its lowest vibrational level v i + = 0 with electrons of incident energy below 0.2 eV. We have focused on the contribution of the 2 2 Π state which is the main dissociative recombination route at low collision energies. The final cross section is obtained by averaging the relevant initial rotational states ( N i + = 0 , ⋯ , 10 ) with a 300 K Boltzmann distribution. The Maxwell isotropic rate coefficients for dissociative recombination are also calculated for different initial rotational states and for electronic temperatures up to a few hundred Kelvins. Our results are compared to storage-ring measurements.

https://www.mdpi.com/2218-2004/7/3/82/pdf

Dissociative Recombination of CH+ Molecular Ion Induced by Very Low Energy Electrons

The processes as: dissociative recombination, dissociative excitation and inelastic or super-elastic collision of molecular cations with electrons are major elementary process in the kinetics and in the energy balance of astrophysically-relevant ionized media (interstellar molecular clouds, planetary atmospheres, early Universe), in edge fusion and in many other cold media. In this work the main interactions and mechanisms governing these processes, successfully modelled by the Multichannel Quantum Defect Theory (MQDT) are presented. Using a stepwise method based on this theory, cross sections and rate coefficients have been obtained for reactions induced on H2+, BeH+ and BF+.

Michel D. Epée Epée

Papers

Electron-Induced Excitation, Recombination, and Dissociation of Molecular Ions Initiating the Formation of Complex Organic Molecules

Dissociative Recombination of CH+ Molecular Ion Induced by Very Low Energy Electrons

Reactive collisions of electrons with molecular cations: Application to H2+, BeH+, BF+