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

A 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. [Russian Text Ignored].