Parallel-Mode EPR of Atomic Hydrogen Encapsulated in POSS Cages

In a typical EPR experiment, the transitions require that the static magnetic field $$B_0$$ is oriented perpendicular to the microwave field $$B_1$$ (perpendicular mode). This is determined by the transition rules either in the classical or in the quantum mechanical description. However, there are cases where EPR transitions are observed when $$B_0$$ is oriented parallel to $$B_1$$ (parallel mode). Quite numerous studies can be found in the literature where EPR transitions in both modes (dual-mode EPR) are feasible. In the majority of cases, dual-mode EPR studies are typically applied in $$S>1/2$$ systems where non-zero transition probabilities for the parallel mode are the result of the state mixing provided by the zero-field splitting interaction. On the other hand, the observation of parallel-mode EPR signals in $$S=1/2$$ systems becomes feasible when strong hyperfine interaction between the electronic and nuclear spin is present, as has been theoretically predicted for the hydrogen atom having a hyperfine coupling constant of $$A_0=1420$$  MHz (Weil in Concepts Magn Reson Part A 28:331, 2006). Herein, we report the first dual-mode X-band EPR experiments of hydrogen atom (both isotopes $$^1$$ H and $$^2$$ H) encapsulated in polyhedral oligomeric silsesquioxane cages. We extend the theory to the case of deuterium and we extract analytical formulas for transition probabilities. For the forbidden transitions, this study revealed a first-order dependence of resonance fields on the nuclear g-factor, $$g_{\mathrm{n}}$$ , and the existence of a clock transition with $$f=307$$  MHz.