Change of the Concentration of Wannier Excitons in Molecular Crystals Caused by a Magnetic Field

Abstract

AbstractA calculation is made of wave functions of para‐ and ortho‐states of excitons of the Wannier type in a magnetic field. These functions are used for the calculation of the changes in the population rates and lifetimes of para‐ and ortho‐states and also of the rate constant for a para‐ortho‐transition in a magnetic field. The results obtained are used for the calculation of the changes in the total concentration of Wannier excitons, which are generated by excitation of molecular crystals in a magnetic field. It is found that in a magnetic field the total concentration of Wannier excitons can either be increased by excitation of only Wannier para‐excitons, or decreased when both para‐ and ortho‐excitons are generated. Expressions are obtained for the relative change in the total concentration of excitons (Δn/n) depending upon the strength of the magnetic field (H), the lifetimes of para‐ (τ1) and ortho‐ (τ3), excitons, and the rate constant of the para‐ortho‐transition (λ). These results can be used for the interpretation of the effect of a magnetic field on photocurrent and luminescence of organic molecular crystals, which was discovered earlier. A relative change of the photocurrent in tetracene thin films (Δi/i) in a magnetic field is investigated. The relationships Δi/i (H) are measured for different thicknesses of tetracene polycrystalline films. The increase of the sample thickness is shown to be equivalent to that of the lifetime ratio of ortho‐ and para‐states (τ3/τ1) of the Wannier excitons. The magnetic field strength (H1/2), consistent with Δi/i(H1/2) = 1/2 (Δi/i) (H → ∞), is found to decrease with the sample thickness. The obtained results agree with the predictions of the developed theory.