Parallel-Mode EPR of Atomic Hydrogen Encapsulated in POSS Cages
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Applied Magnetic Resonance
ORIGINAL PAPER
Parallel‑Mode EPR of Atomic Hydrogen Encapsulated in POSS Cages George Mitrikas1 · Yiannis Sanakis1 · Nikolaos Ioannidis1 Received: 30 June 2020 / Revised: 23 August 2020 © Springer-Verlag GmbH Austria, part of Springer Nature 2020
Abstract In a typical EPR experiment, the transitions require that the static magnetic field B0 is oriented perpendicular to the microwave field B1 (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 B0 is oriented parallel to B1 (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 A0 = 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, gn , and the existence of a clock transition with f = 307 MHz.
This work is dedicated to Prof. Dante Gatteschi on the occasion of his 75th birthday in recognition of his outstanding contribution in Electron Paramagnetic Resonance Spectroscopy. * George Mitrikas [email protected] 1
Institute of Nanoscience and Nanotechnology, NCSR Demokritos, Athens, Greece
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1 Introduction Parallel-mode EPR spectroscopy, where the oscillating microwave magnetic field B1 is parallel to the static field B0 , gives access to ΔMS ≠ ±1 transitions in systems with S > 1∕2 where the zero-field splitting (ZFS) term determines the spin Hamiltonian. The conditions for such transitions to be observed can be fulfilled in either integer or non-integer spin systems and are governed by the relative magnitude of the ZFS terms with respect to the Zeeman term and the microwave energy. Typical (but not exclusive) examples where such conditions can be met are integer spin systems with non Kramers’ doublets where the axial ZFS term is large in comparison to the microwave energy and in the presence of rhombic second-order or higher order terms with appropriate values [1, 2]. In half integer spin systems, the conditions are met in cases with small ZFS in
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