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

Low-temperature absorption and electroabsorption spectra of a thin single c rystal of alpha-sexithiophene reveal a distinctly different line shape and sensitivity to electric fields of the Daydov components of the first intram olecular pi-pi* transition. The weak component of very small linewidth resp onds by a quadratic Stark shift which is attributed to a molecular polariza bility of 900 Angstrom(3). The strong Davydov component of much larger line width responds by an additional effect which dominates its spectrum and res ults in an electroabsorption spectrum of second derivative line shape. This signal is attributed to coupling of the field to the large transition dipo le 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. Th e data indicate that Davydov components of the same molecular state are not mixed by the field.