Purely Spatial Quantum Diffusion of H Atoms in Solid ${\mathrm{H}}_2$ at Temperatures below 1 K

We report on a direct measurement of the quantum diffusion of H atoms in solid molecular hydrogen films at $T=0.7\mathrm{K}$. We obtained a rate of pure spatial diffusion of H atoms in the ${\mathrm{H}}_2$ films, $D^d=5(2)\times {10}^{-17}{\mathrm{cm}}^2{\mathrm{s}}^{-1}$, which was 2 orders of magnitude faster than that obtained from H atom recombination, the quantity used in all previous work to characterize the mobility of H atoms in solid ${\mathrm{H}}_2$. We also observed that the H-atom diffusion was significantly enhanced by injection of phonons. Our results provide the first measurement of the pure spatial diffusion rate for H atoms in solid ${\mathrm{H}}_2$, the only solid state system beside ${}^3\mathrm{He}-{}^4\mathrm{He}$ mixtures, where atomic diffusion does not vanish even at temperatures below 1 K.

Figure 1

ESR spectra of H atoms in the sample 1. (a) At the beginning of sample accumulation (day 2). (b) At the end of sample accumulation (day 16). See the text for further details. The relative scales are shown on the vertical axes.

Figure 2

(a) Time evolution of the absolute number of H atoms in samples 1, 2, and 3 (filled red circles, green diamonds, and blue squares, respectively). (b) Time evolution of the average local concentrations of H atoms in samples 1, 2, and 3 (open red circles, green diamonds, and blue squares, respectively).

Figure 3

Time evolution of the absolute number of H atoms (a) and concentrations (b) corresponding to $C1$ and $C2$ components in the $2.5\mu \mathrm{m}$ thick sample 1 shown by red and blue circles, respectively.

Figure 4

Evolution of the H-atom rich layer in samples 1, 2, and 3 (open red circles, filled green diamonds, and open blue squares) while running the discharge in the sample cell. The solid black line shows a fit to the experimental data using Eq. (2) at the second stage of H atom accumulation. The inset shows the propagation of the layer with a high concentration of H atoms deeper into the bulk due to spatial diffusion.

Figure 5

(a) Time evolution of the local concentration of H atoms corresponding to the broad component $C2$ in sample 3 after stopping the discharge. Inset: inverse concentration of H atoms corresponding to component $C2$ as a function of time. (b) Time evolution of the concentration of H atoms corresponding to the narrow component $C1$. Inset: inverse concentration of H atoms corresponding to component $C1$ as a function of time.