Summary

Stars are formed from the collapsing dense and cold clumps contained in in interstellar clouds. In these cold environments, astronomers observe a wealth of molecules, in which a hydrogen atom (H) has been replaced by its heavier sibling deuterium (D). Astonishingly, the abundance of deuterated molecules in these cold clouds is by far more than one would expect by the environmental D/H ratio. This phenomenon is called deuterium enrichment (or deuterium fractionation).  Deuterium fractionation is a central tool in understanding the different phases of star formation. Deuterated molecules are sometimes the only tracers for the coldest environments in space and thus are key to characterize star forming regions, especially prestellar cores and young protostellar envelopes.

 

Among the chemical processes occurring in cold clouds, the family of  ion-molecule reactions H3+ +H2 (in all its  isotopic variants) is of utmost importance, in particular for the deuteration processes.  The sequential deuteration reactions

H3+     + HD↔ H 2 D+ + H 2 
H2D+ + HD ↔ D2H+ + H 2
D2H+ + HD ↔ D 3 +    + H2

are the main initial drivers of deuteration in cold interstellar clouds and  determine the chemistry in the last moments before star formation. Additionally, H 2 D+ and D2H+  are  one of the few remaining molecular tracers, when all other heavier elements have been frozen out onto dust grains (H3+ does not have a dipole moment an d cannot be detected by its rotational motion).

It is the aim of this project to investigate these ion-molecule-reactions in quantum-mechanical detail, and supply the astrochemical community and observers with accurate rate coefficients as well as transition frequencies for deuterated species. Besides the important reactions system H3+ +  H2, similar  reactions involving CH  3  + and C 2 H2+ will b e examined.


Laboratory tools:

Ion-molecule reaction systems are best investigated in low temperature ion traps (22-pole traps). For the reaction system H 3 + + H2 the chemistry can be examined by changing the conditions, as the ambient temperature, the ortho/para-ratio of the H2  molec ules  as well as the HD/H2-ratio of the reaction gas. For spectroscopic investigations,  the method of laser induced reactions (LIR) can be used. It allows to obtain state-specific reaction rates and accurate ro-vibrational and rotational frequencies of the investigated deuterated ions. In detail, following laboratory tools are available for the investigation:

  • 10K 22-pole ion trap machine. Allows to do spectroscopy and investigate ion-molecule reactions from 300K to the lowest temperature of 10K
  • 4K 22-pole ion trap machine. As all gases are frozen out at 4K (except Helium), this machine is used for high-resolution spectroscopy in combination wit a  frequency comb (see below)
  • para-H2 generator.  Using a paramagnetic catalyst at low temperature, commercial hydrogen gas (normal-hydrogen) can be converted into high-purity para-hydrogen samples. A Raman-spectrometer is available to control the purity.
  • frequency comb.  For high-resolution spectroscopy of cold (4K) ions, the calibration of the frequency axis is very important. This is achieved with a frequency comb locked to a rubidium atomic clock. For mid-IR spectroscopy, this system enables relative  accuracies better than 10-9.  

Goals and workplan:

  • Develop reliable methods to produce and characterize high-purity p-H2 samples.
  • For a H3+ like sample stored in H2 gas (containing impurities of HD), determine the H2D+/H3+ and the D 2 H+/H3+ ratio as  a function of the trap temperature, the HD/H2 ratio and the o/p-ratio of H2.
  • Use a microcanonical model to compare the above experimental results. Further develop the model to include different reaction mechanisms.
  • Determine the o/p-ratio of H3+ embedded in a cold cloud of o/p-H2.
  • The ultimate goal for the H3+ system (and its deuterated siblings) is to obtain state-to-state rate coefficients which are finally needed for a proper comparison of theory and experiment as well as for the proper modelling of the deuterium chemistry.
  • Determine accurate transition frequencies for important deuterated ions, in particular CH 2 D+ and C2HD+.