Dynamical constraints and nuclear spin caused restrictions in collision systems

Abstract

This contribution summarizes a variety of results and ongoing activities, which contribute to our understanding of inelastic and reactive collisions involving hydrogen ions. In an overview of our present theoretical knowledge of various HmD+ collision systems (m + n < or = 5), it is emphasized that although the required potential energy surfaces are well characterized, no detailed treatments of the collision dynamics are available to date, especially at the low energies required for astrochemistry. Instead of treating state-to-state dynamics with state of the art methods, predictions are still based on: (i) simple thermodynamical arguments, (ii) crude reaction models such as H atom exchange or proton jump, or (iii) statistical considerations used for describing processes proceeding via long-lived or strongly interacting collision complexes. A central problem is to properly account for the consequences of the fact that H and D are fermions and bosons, respectively. In the experimental and results sections, it is emphasized that although a variety of innovative techniques are available and have been used for measuring rate coefficients, cross-sections or state-to-state transition probabilities, the definitive experiments are still pending. In the centre of this contribution are our activities on various m + n = 5 systems. We report a few selected additional results for collisions of hydrogen ions with p-H2, o-H2, HD, D2 or well-defined mixtures of these neutrals. Most of the recent experiments are based on temperature variable multipole ion traps and their combination with pulsed gas inlets, molecular beams, laser probing or electron beams. Based on the state-specific model calculations, it is concluded that for completely understanding the gas phase formation and destruction of HmDn+ in a trap, an in situ characterization of all the experimental parameters is required with unprecedented accuracy. Finally, the need to understand the hydrogen chemistry relevant for dense pre-stellar cores is discussed.

Controlled synthesis and analysis of He–H+3in a 3.7 K ion trap

“…It is not known how efficient the traces of H 2 in the present experiment are. To answer this question and for preparing dedicated experiments, a simple model is used, briefly mentioned in [70]. It is based on the assumptions that (1) H + 5 complexes are formed with the statistical weight 2I + 1 where I is the total nuclear spin, (2) I is conserved restricting the final states, and (3) the decay of the complex towards an allowed state is proportional to the square root of the translational energy after the collision (E t = E tot − E v,j ).…”

Nuclear spin dependence of the reaction of ${\rm H}_3^+$H3+ with H2. I. Kinetics and modeling

“…30 It is possible to account for the interplay of nuclear spin selection rules and energetic restrictions by employing a microcanonical statistical approach. Park and Light 23 have developed a microcanonical statistical model for the H + 3 + H 2 system that conserves energy, angular momentum (both motional and nuclear spin), and parity, and also allows for incomplete proton scrambling.…”

Simulation of the infrared predissociation spectra of H5+

“…Until now only statistical studies of these processes have been performed [11,12,20]. In these studies two limiting models are assumed: a complete scrambling in which an exchange of any proton is produced, or a restricted-protonhop mechanism, in which the direct proton transfer from the H 3 + to the H 2 subunit becomes more favorable .…”

“…In addition, the rates of isotopic exchange processes are important in low-pressure plasmas of H 2 -D 2 mixtures [8]. The spin statistics of these species determines selection rules for spectroscopy [9][10][11][12], and it is therefore also interesting to model the ortho-to-para conversion of H 3 + and isotopologues [11,13]. Particularly intriguing is the relative abundance of deuterated species, which is approximately 10 4 times higher than the D-to-H ratio of the galaxy, [14] attributed to zero-point energy differences important at the low temperatures of the interstellar medium [15,16].…”