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para-H2  –  Para Hydrogen Experiments


  • asvany
  • schlemmer


Molecular hydrogen can exist in two different nuclear spin configurations, named "ortho" and "para". The difference between these two configurations is the symmetry of the nuclear wave function. The effect of this difference can be seen in the rotational energy of the molecule. Due to symmetry reasons the rotational ground state of hydrogen can only be occupied by para hydrogen molecules, while the lowest possible state for ortho hydrogen is the first excited rotational state. For the investigation of reactions relevant to astrochemistry, it is important to perform experiments at low temperatures. At these temperatures the rotational energy from the first excited rotational state often is sufficient to change the equilibrium of the observed reaction. Therefore, astrochemical experiments often need high quality para hydrogen. ...more




Para-Hydrogen Converter
For the conversion of ortho hydrogen into para hydrogen two environmetal conditions are necessary: The environment has to be cooled down below 20 K and a paramagnetic catalyst material has to present. The low temperature shifts the equilibrium of energy level occupancy to almost 100 % of para hydrogen (rotational ground state). The paramagnetic catalyst enables the spin flip of the molecules from ortho to para hydrogen. There are several methods how to bring the hydrogen into and out of this conversion environment.
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Recent Results

Characterization of Para Hydrogen:

There are several methods to determine the ortho to para ratio of hydrogen. Direct methods like spectroscopy or nuclear magnetic resonance observe the population of rotaional levels or the nuclear spin configuration. Indirect methods use the influence of the rotational energy on chemical equlibria.

Ion Trap:

In the ion trap apparatus the investigation of the ortho to para ratio of H2 is only possible indirectly. There are several ions whose reactions with H2 are only slightly endothermic. In such a case the rotational energy of ortho H2 is sufficient to react, while para H2 does not contain enough energy for a reaction at cryogenic temperatures. Therefore, the observation of such chemical equlibria allows for the determination of the ortho to para ratio of the reactant hydrogen.

Suitable reactions are for example N+ + H2 → NH+ + H, H2D+ + H2 → H3+ + HD. The second reaction is also dependent on the HD to H2 ratio of the reactant gas, so the first reaction would be preferable.

During Experiments on the reactions of H3+ isotopologues, unfortunately only the second reaction can be performed, as no N+ ions are available at that time. Therefore, ist is mandatory to have a second test mechanism in order to derive the ortho to para and the HD to H2 ratio. It is also of great advantage to know, that these two ratios will not change suddenly during the experiments.

Raman Spectrometer:

Raman Spectroscopy is a direct test mechanism for the ortho to para ratio of hydrogen and it is independent of the HD to H2 ratio. By comparing the intensites of transitions starting from either ortho or para levels, the ortho to para ratio can be derived. This method became available only recently for two reasons. The amounts of para hydrogen produced by the continous flow method were far to small for spectroscopic investigations, which changed by using the freeze out method. Also, there was no Raman Spectrometer for gas probes available in the colonge labs. This could be changed by modifing a lab course Raman Spectrometer for solid state probes, using "Qualitäts-Verbesserungs-Mittel" from North Rhine-Westphalia.
Figure 1: Schematic of the conversion chamber of para hydrogen generator and the freeze out production of p-H2.
Figure 2: The para hydrogen generator setup.
Figure 3: Raman spectrum of normal and para hydrogen. For the para hydrogen sample, the ortho-line in the center of the spectrum is below the detection limit.


  • Funded by SFB 956.