Session S6: Molecular Dissociation Induced by Electron Collisions

Molecular dissociation induced by electron collisions

A remarkably large number of molecules have been observed in molecular clouds, and the question is of course how the molecules can be formed in the cold and dilute interstellar medium. Ion-molecule reactions are believed to be on route of formation, and the final step in these reaction networks is electron-molecular ion dissociative recombination. I will describe recent experiments at the ion storage ring CRYRING in Stockholm and in particular discuss results on H$_{3}^{+}$.   

Molecular Dissociation Induced by Electron Collisions

Free electrons can efficiently break molecules or molecular ions in low-energy collisions by the processes of dissociative recombination or attachment. These processes make slow electrons efficient chemical agents in many environments. For dissociative recombination, in particular, studies of the underlying reaction paths and mechanisms have become possible on a uniquely elementary level in recent years both for theory and experiment. On the experimental side, collisions can be prepared at resolved collision energies down to the meV (10 Kelvin) level, increasingly gaining control also over the initial molecular quantum level, and individual events are detected and kinematically analyzed by fast-beam coincidence fragment imaging. Experiments are reported from the ion cooler ring TSR in Heidelberg. Stored beams of molecular ions cooled in their external and internal degrees of freedom are collinearly merged with intense and cold electron beams from cryogenic GaAs photocathodes, recently shown to yield fast cooling of the center-of-mass motion also for heavy and correspondingly slow molecular ion beams. To reconstruct the molecular fragmentation events multiparticle imaging can now be used systematically with collision energies set a wide range, especially aiming at specific electron capture resonances. Thus, for CF$^+$ it is found that the electronic state of the C fragment ($^3P$ or $^1D$) switches resonantly when the collision energy is changed by only a small fraction. As a new powerful tool, an energy-sensitive multi-strip surface-barrier detector (EMU) has been set up to measure with near-unity efficiency the masses of all fragments together with their hit positions in high-multiplicity events. Among many uses, this device allows internal molecular excitations to be derived for individual chemical channels in polyatomic fragmentation. New results will be presented in particular on the breakup of the hydronium ion (D$_3$O$^+$).   

Theory of dissociative recombination of triatomic molecular ions

In this talk I will discuss several techniques recently developed for the theoretical description of dissociative recombination of molecular ions and related processes such as molecular photoionization and rovibrationally-inelastic electron-collisions with the ions. The techniques are based on quantum defect theory, an efficient tool for characterizing electron-ion collision processes. In order to apply the same theoretical techniques to both diatomic and triatomic ions, we utilize an appropriate adiabatic dissociative coordinate: for diatomic ions it is the internuclear distance, while for triatomic ions it is the hyperradius or some other suitable coordinate representing the dissociative degree of freedom. A unique adiabatic dissociative coordinate is needed in order to treat the dissociative flux escaping into different rovibrational and fragmentation channels in a uniform way. If needed, the approximation of adiabaticity of such dissociative coordinate can be lifted by taking non-adiabatic couplings explicitly into account. I will concentrate on a time-independent theoretical framework that has been applied to describe dissociative recombination in such ions as H$_3^+$ and its isotopologues (H$_2$D$^+$, D$_2$H$^+$ and D$_3^+$), HCO$^+$ (DCO$^+$). I will also discuss the time-dependent framework of the theory to represent dissociation recombination. For the linear triatomic ions with large dipole moment we have modified the approach to include the effect of the dipole moment of the ion on the motion of the incident electron. For this purpose we have applied the multichannel quantum defect theory generalized to deal with the Coulomb field with non-integer complex partial waves.   

Ab initio calculations of dissociative attachment and dissociative recombination of electrons and polyatomic species

Interactions of free electrons with neutral and positively charged molecular species play a role in various physical systems. In interstellar space, reactions such as dissociative recombination determine the balance of various charged and neutral species. In a laboratory equipped with an apparatus like a COLTRIMS device, the dissociative attachment process can be used as a microscope to study polyatomic molecular dynamics. We discuss the theoretical and numerical methods used to calculate dissociative attachment and dissociative recombination of electrons with larger molecules from first principles. Studies using these methods are complimentary to other methods that yield more approximate reaction rates at greatly lesser numerical cost; they may yield precise information about the dissociation dynamics, product distribution, and differential cross section that approximate methods cannot. We discuss calculations performed to date on the target species H$_2$O, NO$_2$, and LiH$_2^+$. We discuss the scaling of our numerical methods with the number of atoms, and the prospects of applying them to tetra-atomics.