By virtue of the fact that its low-lying $^{2}\mathrm{\ensuremath{\Pi}}_{3∕2}$ state is paramagnetic, nitric oxide $(\mathrm{NO})$ can be an effective agent for inducing nuclear spin relaxation during isolated binary collisions with other species. At the same time, the strong coupling between the electronic angular momentum and the internuclear axis of this particular molecule leads to a situation in which its effective magnetic moment depends critically on molecular motions that take place on the time scale of the collision. Here we present the results of an investigation in which the NMR-detected longitudinal nuclear relaxation rate ${T}_{1}^{\ensuremath{-}1}$ of room temperature $^{3}\mathrm{He}$ gas adulterated with $\mathrm{NO}$ was measured as a function of $\mathrm{NO}$ density $([\mathrm{NO}])$ in two different magnetic fields. We find ${T}_{1}^{\ensuremath{-}1}=0.0502(3)[\mathrm{NO}]\phantom{\rule{0.3em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ at $1.50\phantom{\rule{0.3em}{0ex}}\mathrm{T}$, and ${T}_{1}^{\ensuremath{-}1}=0.0506(3)[\mathrm{NO}]\phantom{\rule{0.3em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ at $2.35\phantom{\rule{0.3em}{0ex}}\mathrm{T}$, where $[\mathrm{NO}]$ is expressed in amagat. Under these conditions approximately two-thirds of the $\mathrm{NO}$ molecules occupy the diamagnetic $^{2}\mathrm{\ensuremath{\Pi}}_{1∕2}$ ground state and one-third occupy the paramagnetic $^{2}\mathrm{\ensuremath{\Pi}}_{3∕2}$ state. Our data can be understood in terms of a semiclassical treatment of collision dynamics that involves the paramagnetic moment autocorrelation function.

We describe a neutron radiography technique that can be used to map the distribution of 3He impurities in liquid 4He, providing direct and quantitative access to underlying transport processes. Images reflecting finite normal- and superfluid-component 4He velocity fields are presented.

The time-dependent diffusion of 3Heatoms perpendicular to the axis of a single macroscopically large cylindrical pore is studied using a steady (or constant) gradient-recalled echo sequence. Measurements of the effective 3Hediffusion coefficient extending from the free-diffusion regime to the motionally averaged regime are presented, and are well-described by analytic solutions to the Bloch–Torrey equation based on the gaussian phase approximation. Our data yield the value 0.140(6)m2/s for the self diffusion coefficient of 3Heat a temperature of 296K and a pressure of 1.00Torr. Adaptations of these methods should enable the study of complex pore geometries as model systems.

We have used a neutron radiography technique to investigate the spatial distribution of 3He atoms in very dilute liquid 3He–4He mixtures at temperatures below the superfluid transition temperature. By imposing heat currents and monitoring the subsequent redistribution of 3He within the sample volume we obtain a direct measurement of the relevant mass diffusion coefficient. Data from these experiments provides a striking demonstration of the manner in which the distribution of impurity atoms within the liquid can be influenced by the presence of a free surface.

The role of 3He-3He collisions in our diffusion experiment is addressed and shown to not be relevant to the measurement of 3He diffusion against phonons in superfluid helium.

We have used a neutron radiography technique to investigate the spatial distribution of 3 He atoms in very dilute liquid 3 He- 4 He mixtures at temperatures below the superfluid transition temperature. By imposing heat currents and monitoring the subsequent redistribution of 3 He within the sample volume we obtain a direct measurement of the relevant mass diffusion coefficient. Data from these experiments provides a striking demonstration of the manner in which the distribution of impurity atoms within the liquid can be influenced by the presence of a free surface.