Is the ESR spectrum attributable to H2– or H2+(H2)2? Precise measurement of the g-value and anisotropic hyperfine structure in γ-irradiated solid parahydrogen

Abstract

Four ESR lines, found by Miyazaki et al. in 1995 in irradiated solid parahydrogen (ca. 92% para-purity) at 4.2 K, assigned as “H₂⁻,” were precisely re-measured with highly pure solid parahydrogen (>99.9%) and compared with calculated g-value and hyperfine coupling constants of H₂⁺(H₂)₂. The g-value of I,M_I = 0,0 feature, reported previously as 2.0033 or 2.0023, was found to be incorrect and has now been corrected to 2.002120 ± 0.000012. This corrected value is clearly smaller than that of free electron. We also found that the remaining three lines corresponding to I,M_I = 1,0; 1, +1; and 1, –1 features have axially symmetric hyperfine anisotropic structure i.e., A_∥ = 20.318 and A_⊥ = 20.502 mT. This anisotropy corresponds to the rotation of the species in solid parahydrogen. Although the experimental g-value and isotropic hyperfine coupling constants are in good agreement with those of H₂⁺(H₂)₂ estimated by ab initio calculation, it is still difficult to explain why H₂⁺ (H₂)₂ clusters turned into ESR inactive species by addition of 0.1∼1% D₂ in the present system. In this regard, we still maintain that the species in solid parahydrogen are H₂⁻.