Electronic band structure of pentacene: An experimental and theoretical study

The intermolecular and intramolecular dispersions of pentacene are measured by angle resolved photoemission spectroscopy using a uniaxially aligned crystalline thin film. The band structure perpendicular to the molecules displays a small dispersion in agreement with density functional theory (DFT). Parallel to the molecules, two $\pi$ bands consisting of five and six orbitals are clearly observed. In these intramolecular bands, the orbital emissions are shown to be in agreement with calculations of the photoemission intensities based on DFT both in terms of position and width in momentum space.

Figure 1

A model of 5A(022) bulk termination (left) and the top view (right). Interring spacing $a=2.35\mathrm{\AA }$, interplane spacing $d=14.77\mathrm{\AA }$, and lateral spacing $b=6.41\mathrm{\AA }$ are indicated.

Figure 2

(Color online) ARUPS measurements along the 5A molecular axes ($x$ axis in Fig. 1). In (a), spectra from $\theta =-44°$ to $+70°$ in 2° steps are shown. The inset indicates the experimental geometry. Spectra in 1° steps are displayed as an $E\left(k\right)$ map in (b) after conversion from $\theta$ to $k_x$ by Eq. (1) and after the quadratic curve at the bottom of (a) was subtracted from the data. The Brillouin zone boundaries of a hypothetical infinitely long acene $(\pi ∕a=2.35{\mathrm{\AA }}^{-1})$ are indicated. The color scale (gray scale) represents an arbitrary linear intensity scale. Calculations for an isolated molecule are represented by symbols which have been rigidly shifted to agree with the measured HOMO energy: upper (×) and lower (+) $\pi$ bands and first $\sigma$ band (○).

Figure 3

(Color online) Constant energy line scans of the photoemission intensity map of Fig. 2b at 5A $\pi$ orbital energies. Next to each line scan, the Kohn-Sham orbitals are shown. The Fourier transform of each orbital is displayed as a solid red line (solid gray line) with the experimental data (solid black circles). The binding energies of the line scans are $-1.53$, $-2.78$, $-3.88$, $-4.68$, $-5.13$, $-3.13$, $-4.18$, $-5.48$, $-6.38$, $-6.88$, and $-7.63\mathrm{eV}$ from top to bottom. All experimental intensities have the same scale, and the calculations have been scaled so that the HOMO intensities match.

Figure 4

(Color online) ARUPS measurements perpendicular to the 5A molecular axes ($y$ axis in Fig. 1). Spectra from $\theta =-10°$ to $+40°$ in 1° steps are displayed as an $E\left(k\right)$ map after conversion from $\theta$ to $k_y$ by Eq. (1) and subtraction of a quadratic curve to simulate the secondary electron background. The Brillouin zone boundaries of the 5A crystal $(\pi ∕b=0.49{\mathrm{\AA }}^{-1})$ are indicated. The color scale (gray scale) represents an arbitrary intensity scale. The top panel shows the center of a Gaussian peak fit of the HOMO emissions. Theoretical calculations in the corresponding crystal direction are shown by purple (gray) dashed lines.