Hyperfine Resonance of Gaseous Atomic Hydrogen at 4.2 K

We have measured the hyperfine frequency and wall shift of hydrogen atoms at 4.2 K stored in a bulb coated with solid ${\mathrm{H}}_2$. The phase shift per wall collision is - 0.29(1) rad. The adsorption energy of H on ${\mathrm{H}}_2$ is 9(2) K, and the adsorption time at 4.2 K is approximately 30 nsec. Transverse and longitudinal relaxation times have been measured, and atomic densities greater than ${10}^{14}$/${\mathrm{cm}}^3$ have been observed.