OBSERVATION OF SOLITON-LIKE EXCITATIONS IN ALL-TRANS-BETA-CAROTENE SINGLE-CRYSTALS

The crystal structure of all-trans-beta-carotene was determined at 293 K. The space group determined in this investigation was consistent wi th that determined at 130 K by Senge et al. [Z. Naturforsch, C 47 (199 2) 474]. Anisotropic displacements of lattice constants and molecular structures were observed upon raising the temperature from 130 K to 29 3 K. Photoinduced and time-resolved absorption spectroscopies were app lied to all-trans-beta-carotene single crystals. By exciting visible a bsorption bands of the crystals, we observed, for the first time, the photoinduced bleaching of infrared absorption bands. The bleaching typ ically showed a single-exponential recovery with a lifetime of similar to 10 ms, which shows that some kind of metastable state is produced by exciting the visible absorption bands of the crystals. Depending on the crystals, however, we observed stretched-exponential and power-la w recoveries. By inference based on the results for trans-polyacetylen e, the cause of the latter two recoveries was attributed to the recomb ination of solitonlike excitations. Semi-empirical and ab initio molec ular orbital calculations indicated the presence of solitonlike excita tions in finite polyenes.