The total cross section for thermal neutrons of liquid and solid normal-hydrogen, para-hydrogen, deuterium and deuterium hydride have been measured at several temperatures. The cross section of the liquid and solid phase of para-hydrogen shows that the para-hydrogen molecule scatters coherently. The coherence effects in the cross sections of liquid para-hydrogen and deuterium indicate a short range order of the molecules in the liquid.The cross sections of para-hydrogen and deuterium hydride show clearly the excitation of the first rotational level of the molecule. The threshold indicates that the molecules of the liquid phase are not completely free. The cross section of liquid and solid para-hydrogen for neutrons with energies larger than the threshold energy show a different behaviour that can be explained by the different dynamics of liquid and solid.

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