ESR observation of the H⋅⋅⋅H, H⋅⋅⋅D, and D⋅⋅⋅D spin-pair radicals in rare gas matrices

The first experimental observation of the fundamentally important spin-pair radicals, H...H, H...D, and D...D is reported. ESR (electron spin resonance) studies of rare gas matrices near 4 K containing extremely high concentrations of H and D atoms revealed the presence of these spin exchange-coupled molecules whose ESR spectra are analyzed in terms of a 3Σ electronic ground state. The observed D tensor (zero-field splitting) for a specific isotopic spin pair was surprisingly constant in all four rare gases but showed large changes among the three isotopic species. As expected, the hyperfine splitting (Aiso) for the spin-pair was observed to be one-half that of the free H or D atom.