Electric field effects on the Davydov components of a strong intramolecular transition:: α-sexithiophene single crystals

Low-temperature absorption and electroabsorption spectra of a thin single crystal of alpha-sexithiophene reveal a distinctly different line shape and sensitivity to electric fields of the Daydov components of the first intramolecular pi-pi* transition. The weak component of very small linewidth responds by a quadratic Stark shift which is attributed to a molecular polarizability of 900 Angstrom(3). The strong Davydov component of much larger linewidth responds by an additional effect which dominates its spectrum and results in an electroabsorption spectrum of second derivative line shape. This signal is attributed to coupling of the field to the large transition dipole of the four molecules of the unit cell. The spectra show no response of a charge-transfer state and no Franz-Keldysh effect of a band continuum. The data indicate that Davydov components of the same molecular state are not mixed by the field.