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Universität zu Köln
Mathematisch-Naturwissenschaftliche Fakultät
Fachgruppe Physik

I. Physikalisches Institut

Para-Hydrogen Converter

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Para Hydrogen
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 wavefunction. 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.
As hydrogen naturally comes with an ortho to para ratio of 3/1 ("normal" hydrogen) on earth, a reliable conversion method is needed. As part of the SFB 956 project B2 the cologne para hydrogen converter was modified to allow for the production of higher amounts of para hydrogen, which contain almost the natural abundance of deuterium. The higher amount of para hydrogen enables Raman spectroscopy as a test method for the purity, the higher amount of deuterium makes chemical tests of the purity in the ion trap more reliable.
Production of Para Hydrogen:
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.
1) Continous Flow:
The para hydrogen converter used with the cologne ion trap experiment was originally constructed to operate in continuos flow mode. This means that a very small amount of normal hydrogen is flowing through the cold converter box and is immediately used in the trap experiment.
This method is sometimes diffcult for experiments which investigate deuterium fractionation reactions. The problem in that case is, that normal hydrogen always contains a small fraction of deuterium as HD molecules. Since HD freezes out on cold surfaces prefential to H2, the para hydrogen leaves the converter box with a different HD to H2 ratio than the normal hydrogen had. This process reaches an equlibrium after several hours of stable operation conditions of the para hydrogen converter, but as soon as there are even slight changes to the operation conditions, such as temperature deviations of the coldhead or fluctuations in the hydrogen flow, the deuterium amount of the produced para hydrogen is changing again an is always much smaller than in the normal hydrogen.
2) Freeze Out:
In order to get para hydrogen of a defined and constant quality, the cologne para hydrogen converter was modified. SFB 956 provided the means (financial and technical support) to add a gas inlet system, which can be used for the production of a larger amount of para hydrogen. In this method, a storage bottle is filled with hydrogen, which is then slowly frozen out into the converter box. The box is at 12 K for this purpose. After evacuating all remaining normal hydrogen from the tubes and the bottle, the converter box is slowly heated up to 20 K. In this process the hydrogen evaporates from the catalyst as para hydrogen. This method has several advantages. The most important are: The produced para hydrogen contains an amount of HD that is nearly as high, as in the normal hydrogen. The HD amount of this gas stays constant over the whole experiment time. The amount of para hydrogen produced is sufficient to enable Raman Spectroscopy as test mechanism for the ortho to para ratio. The storage bottle is coated with teflon, preventing a back conversion into normal hydrogen (which would be the equilibrium at room temperature), so the quality of para hydrogen stays the same during the experiment time and is not subject to any fluctuations in the production conditions. This method has a higher risk of back conversion from para to ortho hydrogen.
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.
1) 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.
2) 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.