Spin correlation effects in spurs. Production of ortho- and para-hydrogen in the radiolysis of liquids

Abstract

A theoretical description is given of the evolution in time of the spin wavefunction (initially singlet overall) of a spur containing two free radicals and two hydrogen atoms. Transfer of excess singlet character from the radical plus hydrogen pairs to the two hydrogen atoms (initially uncorrelated) by a variety of electron-exchange processes is discussed. When this happens, the hydrogen molecules formed by geminate recombination in the spur have unequal nuclear state populations. This leads to departures from the normal ortho/para ratios, 3 for H₂, 2 for D₂, and to strong nuclear polarisation (detectable by n.m.r.) in the case of HD. Application of a magnetic field and scavenging of the atoms or the radicals can change the extent and direction of the effects. Spin relaxation in radicals plays an important role in water but not in most other liquids. Random reactions of two hydrogen atoms are also discussed; similar effects (but of opposite sign) are predicted when re-encounters are considered.