ESR Pumping of Atomic Hydrogen Gas at Low Temperature

We have used the method of the dynamic nuclear polarization for the proton polarization enhancement and nuclear relaxation rate determination in the atomic hydrogen gas at low temperatures T 0.5 K. Experiments were performed in the magnetic field 5 T using electron spin resonance at 140 GHz for density measurements and polarization enhancement. Nuclear polarization values exceeding 95% were achieved after 5 min pumping of the forbidden longitudinal transition a → c with 0.7 mW resonant microwave power. We have recognized the role of spin-exchange process c + a → d + b, and investigated process of the first order nuclear relaxation b → a, preventing the creation of larger polarization values.