CHARACTERISTICS OF A POSITRONIUM BOUND-STATE WITH NITROBENZENE IN METHANOL AS DEDUCED FROM MAGNETIC-FIELD EFFECTS AT VARIOUS TEMPERATURES

The characteristics of a positronium (Ps) bound state with nitrobenzen e in methanol are determined in positron lifetime spectroscopy (LS) ex periments at various temperatures (T= 263, 294 and 323 K) by applying an extemal magnetic field (B). At zero field, measurements of the LS p arameters of the pure solvent and of a 0.01 M nitrobenzene solution at various T show a non-Arrhenius behaviour of the reaction, denoting it s reversibility. The total apparent reaction rate constant (k') shows a smooth maximum at about 308 K and its variation with T is well descr ibed by supposing that the forward reaction rate constant (k1) is pure ly diffusion-controlled. By contrast with what was obtained previously in n-hexane, the variations with B of parameter R and of the o-Ps(m = 0) lifetime are well fined without supposing any significant effect o f B on the lifetimes of the spin states of the bound state. They are t herefore described by a single parameter, the contact density of Ps in the pure solvent (eta), which is temperature independent, at 0.68+/-0 .03. The absence of field effect on the bound-state lifetimes arises f rom a low contact density parameter for the latter, which is estimated at about 0.12 at 294 K. The data do not allow to distinguish whether thermal decomposition of the bound state occurs with or without Ps spi n reorientation. Although the bound state formed is weak as regards th e kinetics, because of the reversibility of the reaction, it may be te rmed ''strong'' in the sense that the positron wavefunction overlaps o nly weakly with the odd electron in this complex.