The spectrum of hindered rotation of 14N2 and 15N2 molecules in an Ar matrix

A comparative analysis has been carried out between 2 and 10° K of excess linear expansion coefficients (LEC) determined experimentally for Ar−14N2 and Ar−15N2 solid solutions, due to rotational motion of the nitrogen molecules, and the corresponding values of the excess LEC calculated on the Manz model, 1 taking account of the relaxation of the lattice near the impurity molecules. A method is proposed for separating out the contributions of ortho and para nitrogen molecules to the impurity LEC of the solid solutions. It is shown that the difference between the behavior of the experimental and theoretical values of the excess LEC is determined by a feature of the lowest rotational energy level T1u namely, its splitting into a singlet and a doublet corresponding to a higher energy. The value of the energy gap between them that we determined is 4Beff ≈ 4.2° K on the Manz–Mirsky model, using values of the rotational barrier and of the renormalization coefficient of the rotational constant obtained from dilatometric studies.2

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