Nuclear relaxation in solid and liquid nitrogen

Abstract

A detailed spin-echo study combining NQR and NMR has been made of the solid phases α and β and the liquid phase of high-purity nitrogen. In f.c.c. α - N 2 the quadrupole coupling constant e 2 qQ, the spin-lattice relaxation time T 1, the spin-spin relaxation time T 2, and the inverse linewidth parameter T 2 ∗ have been measured as a function of temperature from 4.2°K to the α-β phase transition at 35.6°K. The temperature dependence of e 2 qQ is adequately explained by Bayer theory for T < 20°K, but at higher temperature it is shown that e 2 qQ accurately obeys an order parameter relation of the form A( T c - T) β with T c = 37.70±0.07°K and β = 0.0770±0.0008. This result is discussed in terms of the librational modes of the lattice and the orientational ordering of the molecules due to quadrupole-quadrupole interactions. The spin-lattice relaxation time follows a T −3.5 relation. Applying the theory of Bayer-Woessner-Gutowsky for relaxation via molecular librations, we have calculated the mean lifetime τ a , over librational states and find τ a ∞ T −3.2±0.1 with τa(4.2°K) ≅ 10 −10sec. In the temperature interval 20–30°K τ a becomes comparable with the period of libration implying that the librational excitations are not well defined at higher temperatures. Anomalies are also noted in T 2 and T 2 ∗ above about 20°K. In h.c.p. β - N 2 hindered rotations of the molecules about the c-axis of the crystal at an angle very near 54°44' reduces the average e 2 gQ by three orders of magnitude. Single crystals of β - N 2 were studied by means of NMR and the temperature dependence of the residual quadrupole coupling deduced from rotation patterns of the NMR line splitting. The coupling constant decreases with decreasing temperature, having the value 2.85±0.30 kHz at 63°K and being unobservably small (<0.5 kHz) below about 40°K. Spin-lattice relaxation in β - N 2 is well described by short correlation time theory and apart from a small discontinuity at the triple point has the same temperature dependence as in the liquid. T 1 was measured in the liquid in equilibrium with its vapor from 63 to 91°K and also as a function of pressure up to the fusion curve at 77 and 91°K. The activation energy for molecular reorientation was computed to be 180±15 cal/mole in both β - N 2 and liquid N 2. From the pressure measurements a correction was made for thermal expansion of the liquid and the activation energy at constant volume was calculated to be 100±30 cal/mole.