Room temperature absorbance spectra of a thick crystal (a) and a thick crystal (b) of 4T as measured at normal incidence with different angles of polarization with respect to the axis. The experimental spectrum for in (b) is affected by saturation between 3.3 and , where the curve has been extrapolated from the spectrum in (a) scaled by the ratio between the thicknesses of the two samples. (c) Absorbance spectra of the previous two crystals taken at on an extended scale.
Absorbance calculated using Eq. (1) with and two possible values of ( representing a reasonable absorption coefficient in the low energy portion of the oligothiophene band and representing a reasonable absorption coefficient at the center of the main absorption peak) for samples of different thicknesses , namely, 50 and in (a) and (b), respectively.
Absorbance of 4T crystals of different thicknesses and of a thick film as measured at normal incidence at either (black diamonds) or (blank squares) as a function of the angle of polarization with respect to the axis, together with the curves calculated using Eq. (1) with and deduced from the measured absorbance of each sample at either 3.70 or . For the thick crystal in (a), the absorbance at has been taken from the spectrum artificially scaled in intensity reported in Fig. 1.
(a) Absorbance spectra of a thick 6T crystal measured at normal incidence with different angles of polarization with respect to the axis. (b) Absorbance spectrum of the same crystal taken at on an extended scale. Inset: Absorbance measured at normal incidence at either (black diamonds) or (blank squares) of the same 6T crystal as a function of , together with the curves calculated using Eq. (1) with and deduced from the measured absorbance for and , respectively, at 3.47 and .
Energy separation and Davydov splittings as defined in the text deduced from the absorbance spectra taken at room temperature and at normal incidence of different oligothiophene crystals ( is the number of thiophene units of the molecule).
Schematic illustration of the and absorption spectra of oligothiophenes at room temperature (not scaled), the latter for either transverse or mixed transverse/longitudinal excitation as in normal incidence measurements. The parameters , , and reported in Fig. 5 are also indicated.
Reflectance spectra of oligothiophene single crystals as measured at room temperature for different angles of incidence on the accessible ab face with ac as plane of incidence and ac polarization. Few interference fringes are observed in the 6T spectra below about . Reflectance is given in arbitrary units, and the spectra have been slightly shifted with respect to each other to better observed the main structure attributed to the transition under investigation (vertical line).
Positions of the peaks in the spectra at room temperature and energy separation , , and as also reported in Fig. 5.
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