Pulsed NMR of dilute ortho-H2in solid Ne, Ar, Kr, and para-H2

Abstract

Pulsed NMR was used to study ortho-${\mathrm{H}}_2$ as a dilute impurity in solid neon, argon, krypton, and para-${\mathrm{H}}_2$. The nuclear relaxation arises principally from intramolecular spin-spin and spinrotation interactions. Many features of the observed relaxation are in excellent agreement with Fedders' predictions of the effects of local crystal fields. The ${\mathrm{H}}_2$ sites in solid neon and argon are found to have crystal fields with very low symmetry and the sites in $p$-${\mathrm{H}}_2$ have fields with axial symmetry. The temperature variation of the rotational-quantum-state correlation frequencies of the $o$-${\mathrm{H}}_2$ molecules is discussed in terms of a two-phonon Raman process. The data fit well the model of Van Kranendonk, but with an unusually low characteristic temperature near 40 K in all four host solids. In the rare-gas solids motional narrowings of intermolecular broadenings indicate that the coefficients of ${\mathrm{H}}_2$ impurity diffusion are similar to the coefficients of host self-diffusion.