Geminate ion recombination in anisotropic media: Naphthalene

Electron-hole recombination in naphthalene is studied using our extension of the conventional Onsager treatment to anisotropic media. While the geminate escape probability is determined by the magnitude and the direction of the initial inter-ion vector, our calculations indicate that anisotropic effects are more significant in naphthalene than is the case for anthracene. In the presence of an external electric field the escape probability is affected by both the absolute crystallographic direction of field and by the direction of the field relative to the inter-ion vector. For small fields, the escape probability is proportional to the field strength, however, the slope-to-intercept ratio depends upon the crystallographic direction of the field. The calculated dependence of the escape probability on field strength agrees with experiment for fields up to 15 kV cm^-1. The width parameter of the Gaussian distribution of separations used in the calculations is 2.1 nm and is significantly larger than the mean separation (0.5 nm) originally estimated by fitting the normalized high-field experimental values using the conventional isotropic treatment.