Spin Exchange and Surface Relaxation in the Atomic Hydrogen Maser

An experiment using the atomic-hydrogen maser is described which confirms several predictions of the theory of spin exchange and which provides new information on the spin relaxation of hydrogen at solid surfaces. Atoms in the ($F=1\mathrm{}$, $m_F=0\mathrm{}$) state, which are confined to a storage bulb, are put into a coherent radiating state by a microwave pulse at the $\Delta m_F=0\mathrm{}$ hyperfine frequency. The initial amplitude and the decay rate of the induced signal are measured. Atoms or molecules containing unpaired electrons are added to the storage bulb at various constant rates, and the pulse measurements are repeated. In this manner, the rate at which spin-exchange collisions decrease the oscillating dipole movement of the radiating gas, a $T_2$ process, is compared with the rate at which the same collisions decrease the population difference $(F=1, m_F=0)-(F=0, m_F=0)$, a $T_1$ process. It is shown for a spin-exchange theory which neglects spin-orbit coupling and all direct magnetic interactions that the ratio of the relaxation times, $\frac{T_2}{T_1}$, is independent of all collision parameters. For collisions between atomic hydrogen and atomic deuterium, nitric oxide, and molecular oxygen, the predicted $\frac{T_2}{T_1}$ ratios are $\frac{4}{3}$. For hydrogen-hydrogen collisions, the predicted $\frac{T_2}{T_1}$ ratio is 2. The deuterium, nitric oxide, and oxygen results agree with the theory, but the hydrogen results are at least 5% low. The magnitude of the discrepancy depends upon the composition of the storage-bulb wall, and the error is explained by a wall relaxation process which has a rate proportional to the hydrogen density. A second wall relaxation process is observed with a rate independent of the hydrogen density, but which increases rapidly with temperature. Both wall relaxation processes appear to be related to the chemical reactivity of atomic hydrogen. These phenomena are examined in detail for a series of hydrocarbon, fluorocarbon, and mixed hydro-fluorocarbon surfaces. Approximate experimental hydrogen-radiation decay-rate cross sections also are given for a series of molecular gases.