Pulsed EPR characterization of encapsulated atomic hydrogen in octasilsesquioxane cages

Abstract

Hydrogen atoms encapsulated in molecular cages are potential candidates for quantum computing applications. They provide the simplest two-spin system where the 1s electron spin, S = 1/2, is hyperfine-coupled to the proton nuclear spin, I = 1/2, with a large isotropic hyperfine coupling (A = 1420.40575 MHz for a free atom). While hydrogen atoms can be trapped in many matrices at cryogenic temperatures, it has been found that they are exceptionally stable in octasilsesquioxane cages even at room temperature [Sasamori et al., Science, 1994, 256, 1691]. Here we present a detailed spin–lattice and spin–spin relaxation study of atomic hydrogen encapsulated in Si₈O₁₂(OSiMe₂H)₈ using X-band pulsed EPR spectroscopy. The spin–lattice relaxation times T₁ range between 1.2 s at 20 K and 41.8 μs at room temperature. The temperature dependence of the relaxation rate shows that for T < 60 K the spin–lattice relaxation is best described by a Raman process with a Debye temperature of θ_D = 135 K, whereas for T > 100 K a thermally activated process with activation energy E_a = 753 K (523 cm⁻¹) prevails. The phase memory time T_M = 13.9 μs remains practically constant between 200 and 300 K and is determined by nuclear spin diffusion. At lower temperatures T_M decreases by an order of magnitude and exhibits two minima at T = 140 K and T = 60 K. The temperature dependence of T_M between 20 and 200 K is attributed to dynamic processes that average inequivalent hyperfine couplings, e.g. rotation of the methyl groups of the cage organic substituents. The hyperfine couplings of the encapsulated proton and the cage ²⁹Si nuclei are obtained through numerical simulations of field-swept FID-detected EPR spectra and HYSCORE experiments, respectively. The results are discussed in terms of existing phenomenological models based on the spherical harmonic oscillator and compared to those of endohedral fullerenes.