S.V.Zavatsky, N.I.Kislukha,
Yu.P.Piryatinsky*
Spectra of stationary absorption, fluorescence excitation and
of time-resolved fluorescence and phosphorescence for crystals of polar
molecular compounds -2,2'-diamino-1,1'-dinaphtyl and 2,2',4,4'-tetraaminodiphenyl
have been obtained at various temperatures (300-4,2K). The doublet structure
of the absorption spectra and considerable Stokes' shifts (from 2000 up
to 6000cm-1) in the fluorescence ones have been revealed. Comparison
of the Stokes' shift magnitude with calculated values of the exciton zone
width in the crystals (~100-200cm-1) brings to the conclusion
that there is strong autolocalization of excitons in these crystals. Using
Hitler-London and adiabatic approximation, the energy and structure of
the excited states of polar crystals have been calculated. The
suggested model of the strong autolocalization of excitons supposes
the possibility of the emergence of excited states with a broken symmetry
and a long lifetime.