Effect of molecular binding on the resonance scattering of photons from the 6.324 MeV level in ¹⁵N

The temperature effect of nuclear resonance scattering from the 6.324 MeV level in ¹⁵N was studied as a function of temperature using a gaseous ¹⁵N₂ target and a solid Li¹⁵NO₃ target. The γ-source was produced by the Cr(n, γ) reaction using thermal neutrons. In order to reproduce the experimental variation of the scattering cross section versus temperature, the Lamb treatment for metallic elements was generalised to the case of diatomic and more complicated molecules. In ¹⁵N₂ an effective temperature was defined in which the zero-point energy of vibration of the diatomic molecule was included. In LiNO₃, an effective temperature was also defined by introducing a modified Debye temperature and accounting for the normal modes of vibration of ¹⁵N in the ¹⁵NO₃^- molecule. An excellent agreement between measured and calculated values was obtained. In a way, the present measurement may be viewed as detecting the zero-point vibrations in molecules.