Solid Nitrogen: A Nuclear Quadrupole Resonance Study

Nuclear quadrupole resonance of ¹⁴N(I=1) has been studied for the two diatomic species ¹⁴N-¹⁴N and ¹⁴N-¹⁵N in solid α-nitrogen. Spectra from carefully prepared and annealed samples display well-resolved intramolecular dipolar splittings consisting of an asymmetric triplet for ¹⁴N-¹⁴N and a doublet for ¹⁴N-¹⁵N. From the order of the components of the triplet, the sign of the nuclear quadrupole coupling constant is deduced to be negative. Measurements of the resonance frequencies νQ(14-14) and νQ(14-15) have been made as a function of temperature from 4.2 K to the phase transformation at 35.6 K, and as a function of isotopic concentration for five concentrations ranging from 0.37 to 99% ¹⁵N. The hydrostatic pressure dependence of νQ(14-14) in a natural isotopic abundance sample has been observed at seven fixed temperatures. These results were combined with detailed thermal expansion and compressibility data to obtain the resonance frequency νQ(14-14) as a function of temperature at constant volume and as a function of volume at constant temperature. The observed strong temperature dependence of the resonance frequency is ascribed to excitation of large-amplitude molecular librations having substantial anharmonic character. A quasiharmonic theory of motional averaging incorporating published Raman spectroscopy data provides a satisfactory fit to the resonance frequency at zero pressure. An independent determination of the motional averaging obtained from the dipolar splitting is in excellent agreement with the averaging of the quadrupole resonance frequency. A fit of the theoretical equation for motional averaging with the experimental volume dependence of νQ(14-14) yields the volume dependence of the libration frequencies. Three isotopic mass effects were observed and studied at 4.2 K: (i) The frequencies νQ(14-14) and νQ(14-15) increase approximately linearly with increasing concentration of ¹⁵N (isotope shift); (ii) νQ(14-15)-νQ(14-14)=7.95±0. 30 kHz independent of concentration (isotope splitting); and (iii) the individual lines of the spectra broaden in mixed samples due to dynamic disorder. The total isotope shift is 2.6 ±{} 0.25 kHz for both molecules; a qualitative explanation of the effect is given based on the volume dependence of νQ(14-14), the known difference in lattice constants of pure ¹⁴N₂ and ¹⁵N₂,and the assumption of a defect volume for a ¹⁴N-¹⁴N impurity molecule in a ¹⁵N₂ lattice. The isotope splitting is a consequence of unequal masses and force constants for the two molecules.