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

Abstract

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 excitona 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 (An/n) depending upon the strength of the magnetic field ( H ) , the lifetimes of para-(tl) and ortho-(tS), excitons, and the rate constant of the para-ortho-transition (A). These results can be used for the interpretation of the effert 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 ( A i / i ) in a magnetic field is investigated. The relationships Ai/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 ( t S / t l ) of the Wannier excitons. The magnetic field strength ( H i p ) , consistent with A i / i ( H l p ) = 1/2 ( A i / i ) (H + a), is found t o decrease with the sample thickness. The obtained results agree with the predictions of the developed theory.