Measurements, using 55-MHz deuteron magnetic resonance (DMR), are reported of deuteron spin-lattice relaxation times for HD in solid argon at concentrations of 300–1100 ppm over the temperature range of 10–70 K. The relaxation times increase rapidly, from 10 to 4000 sec, as the temperature is reduced and are independent of the sample’s para-${\mathrm{D}}_2$ concentration. Comparisons of deuteron spin-lattice relaxation times for HD in solid argon are made with previously reported relaxation times for solid HD–n-${\mathrm{D}}_2$ mixtures and for ortho-${\mathrm{H}}_2$ and para-${\mathrm{D}}_2$ in solid argon. The very different relaxation behavior for HD can be understood because it is an asymmetric molecule. The lack of exchange symmetry results in an increasing probability of the molecule being in a J=0 rotational state as the temperature is reduced. Nuclear spin-lattice relaxation in HD arises from phonon-induced Δ${\mathit{m}}_{\mathit{J}}$ transitions for those molecules in the J=1 states. A theory is presented to calculate the nuclear spin-lattice relaxation rate (1/${\mathit{T}}_1$) in terms of a molecular decay rate (Γ) that arises from Δ${\mathit{m}}_{\mathit{J}}$ or ΔJ transitions. The decay rate Γ as a function of temperature is determined from the relaxation data. It is found that the asymmetric rotor HD molecules have a coupling to the lattice phonons that is much stronger than for ortho-${\mathrm{H}}_2$ and para-${\mathrm{D}}_2$.

• Received 27 August 1992

DOI:https://doi.org/10.1103/PhysRevB.47.2581