H-atom electron-spin polarization in irradiated water and ice confined in the nanopores of Vycor glass

Chemically Induced Dynamic Electron-spin Polarization (CIDEP) in radiolytically generated H atoms in water-saturated Vycor glass is observed at different temperatures (+29 to -94degreesC) by time-resolved pulsed EPR spectroscopy. The kinetics of polarization is investigated by means of the numerical solution of the stochastic Liouville equation (SLE) as applied to a two-site kinetic model. The experimental results are quantitatively simulated assuming that the H atom is one of the partners of the random (or triplet geminate) radical pairs and that the Heisenberg spin-spin exchange interaction is the only inter-radical interaction in such pairs. The recombination of H atoms with radical pair partners is a spin-selective reaction with activation parameters close to those of H-atom diffusion. It is shown that the rate of longitudinal spin relaxation of the partner, the rate of encounters, and the rate of the spin-selective recombination of the H atom with a partner are the most important kinetic parameters defining the observed time evolution of CIDEP. The kinetics of the CIDEP is defined mostly by the polarization generated in random pairs. The contribution of CIDEP from the geminate triplet radical pairs is important only at short times.