Hindered rotation of oxygen molecules in solid argon

The heat capacity of the solid solution Ar–O2 is measured in the temperature interval 1.8−8 K with an oxygen concentration 1 and 2 mol.%. The anomaly observed in the heat capacity is due to coupled spin-rotational degrees of freedom of impurity molecules of oxygen. The spin-rotational spectrum and the impurity contribution to the thermodynamics of O2 molecules in a crystal field are calculated. It is shown that the crystal field acting on O2 molecules in an Ar matrix is a superposition of an octahedral and a weak tetragonal field.

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