Inversed linear dichroism in F $K$-edge NEXAFS spectra of fluorinated planar aromatic molecules

The symmetry and energy distribution of unoccupied molecular orbitals is addressed in this work by means of NEXAFS and density functional theory calculations for planar, fluorinated organic semiconductors (perfluorinated copper phthalocyanines and perfluoropentacene). We demonstrate how molecular orbitals with significant density of states on the fluorine atoms show different symmetry from those mainly located on C and N atoms. As a result, the angle-dependent linear dichroism in NEXAFS F $K$-edge spectra is inversed with respect to that in the C and N $K$-edges. In addition, the significant overlap in energy of ${\pi }^{*}$ and ${\sigma }^{*}$ orbitals throughout the F $K$-edge spectrum hampers its use for analysis of molecular orientations from angle-dependent NEXAFS measurements.

Figure 1

(a) Molecular structures of perfluorinated copper phthalocyanine (F${}_{16}$CuPc) and PFP. (b) Schematic representation of electronic levels in a molecule, including localized core levels, as well as delocalized molecular orbitals. The experimental techniques used in this work include XPS and NEXAFS. In the former, a core level is excited by fixed energy photons above the vacuum level $E_{\mathrm{vac}}$ (excitation 2), and the distribution of kinetic energies are measured. In the latter, the photon energy is scanned in the range of transitions from core levels to empty molecular orbitals (excitation 1). The decay of such excited states causes the emission of secondary electrons that are then measured as a function of the photon energy.

Figure 2

Characteristic XPS spectra for F${}_{16}$CuPc and PFP monolayers on Au(111). The observed core levels correspond to the initial states for the electron transitions of our subsequently measured NEXAFS spectra. The specific atoms giving rise to the various components in the C 1$s$ spectra are labeled following the nomenclature of Fig. 1. The SU satellites are color coded according to the main peaks.

Figure 3

LDOS from unoccupied $p_z$ or $p_x$-$p_y$ orbitals projected onto the F${}_{16}$CuPc and PFP C, N, and F atoms, respectively. These correspond to the final states for the electron transitions of our subsequently measured NEXAFS spectra.

Figure 4

NEXAFS spectra from the $K$-edges of C, N, and F in F${}_{16}$CuPc and PFP monolayers on Au(111) for $p$- and $s$-polarizations. Solid lines show the experimental spectra, while the dotted lines depict the spectra calculated for gas-phase molecules. The latter are shifted to lower energies by the following amounts: C${}_{\mathrm{CuPc}}$ $=$ $-$1.5 eV, C${}_{\mathrm{PFP}}$ $=$ $-$1.3 eV, F${}_{\mathrm{CuPc}}$ $=$ $-$2.2 eV, F${}_{\mathrm{PFP}}$ $=$ $-$2.0 eV, and N${}_{\mathrm{CuPc}}$ $=$ $-$1.1 eV.