SolidH2andD2: Remarkable differences in some NMR …

The differences in the observed properties of solid ${\mathrm{H}}_{2}$ and ${\mathrm{D}}_{2}$ are reviewed, and in particular those encountered in NMR experiments. The failure to detect a sharp NMR (I=1) impurity ``isolated pair'' spectrum in p-${\mathrm{D}}_{2}$ is discussed in terms of a larger crystalline field than in ${\mathrm{H}}_{2}$, where an intense and sharp pair spectrum has been observed. Furthermore, we discuss the dramatic (I=1) solid echo signal loss with decreasing temperature which is observed in solid ${\mathrm{D}}_{2}$, but not for solid ${\mathrm{H}}_{2}$. A theory of the solid echo damping through orientational fluctuations is developed. This theory accounts for the observed solid echo decay in ${\mathrm{D}}_{2}$ as a function of the pulse spacing time \ensuremath{\tau} and leads to an estimation of the order parameter fluctuation amplitude and the correlation time ${\mathrm{\ensuremath{\tau}}}_{\mathit{c}}$. However, the theory cannot account for the loss of spin (as determined from Curie's law), which must occur for very small values of \ensuremath{\tau} that are not covered by the theory.