THEORETICAL AND ELECTRON-SPIN-RESONANCE STUDIES OF THE H-CENTER-DOT-CENTER-DOT-CENTER-DOT-H, H-CENTER-DOT-CENTER-DOT-CENTER-DOT-D, AND D-CENTER-DOT-CENTER-DOT-CENTER-DOT-D SPIN-PAIR RADICALS IN RARE-GAS MATRICES - A CASE OF EXTREME SINGLET-TRIPLET MIXING

The H ... H, H ... D, and D ... D spin-pair radicals have been thoroug hly investigated in neon, argon, krypton, and xenon matrices near 4 K by electron spin resonance (ESR). A theoretical model has been develop ed that treats these spin-pairs as weakly interacting atoms. The model includes the effects of (3)Sigma/(1)Sigma mixing in the analysis of t he observed ESR spectral results and yields a consistent set of magnet ic parameters for these three isotopomers in all four rare gas hosts. The consideration of H atoms interacting with other H atoms over a dis tribution of internuclear distances in the rare gas lattice is include d in the theoretical and experimental analyses. Application of the mod el to earlier ESR results for H ... CH3 reveals a value for its Heisen berg exchange interaction (J) which is found to be considerably larger than that for the H ... H spin-pair. The effects of methane and neon on the J value are calculated for these spin-pairs. The H ... H case i s unusual in that the nuclear hyperfine interaction (A) is considerabl y larger than D (the anisotropic dipole-dipole magnetic interaction be tween electrons) which is much larger than J. The H ... H spin-pairs e xhibit internuclear distances greater than 7 Angstrom and have the fol lowing magnetic parameters (MHz) based upon this model of ''weakly int eracting atoms;'' g(iso) = 2.0016, A(iso) = 1426, D = - 200, and J = 6 . Since a distribution of distances is involved, other spin-pairs woul d be separated by even greater distances in the matrix and thus have s maller absolute values of D and J. (C) 1998 American Institute of Phys ics.