Angle-resolved ultraviolet photoelectron spectroscopy and theoretical simulation of a well-ordered ultrathin film of tetratetracontane (n-C44H90) on Cu(100): Molecular orientation and intramolecular energy-band dispersion

The electronic structure and molecular orientation of a tetratetracontane ( n-C44H90; TTC) ultrathin film on a Cu(100) surface were studied by angle-re solved ultraviolet photoelectron spectroscopy (ARUPS using synchrotron radi ation. A well-oriented thin film of TTC was successfully prepared by vacuum evaporation in ultrahigh vacuum at room temperature. We observed a(2x1)-li ke low-energy electron-diffraction (LEED) pattern for the deposited TTC fil m. This result indicates that the TTC molecules lie on the Cu(100) surface in two types of domains, rectangular to each other, in which the alkyl-chai n axes are along the [110] and [1 (1) over bar 0] directions of the Cu(100) surface. The application of the dipole selection rules to the normal-emiss ion ARUPS spectrum revealed that the C-C-C plane of TTC is parallel to the Cu(100) surface plane (flat-on orientation). The intramolecular energy-band dispersion of TTC was examined by changing the take-off angle of emitted e lectron along the [110] direction of the Cu(TOO) surface. The observed resu lts support the conclusion about the direction of alkyl-chain axes by LEED observation. In order to analyze the molecular orientation more quantitativ ely, we also performed theoretical simulations of the angle-resolved photoe mission spectra using the independent-atomic-center (LAC approximation comb ined with nb initio molecular-orbital th (MO) calculations for various mole cular orientations. The simulated spectra for flat-on orientation are in ex cellent agreement with the observed spectra. These results once again verif y the deduced molecular orientation, and also demonstrate the reliability o f theoretical simulation with the LAC/MO approximation for compounds withou t a rr-electron system. Furthermore, we observed a work function change of about -0.3 eV by adsorption of TTC. Such a decrease of the work function in dicates the formation of a dipole layer at the interface, in contrast to th e traditional picture of energy-level alignment assuming a common vacuum le vel at the organic/metal interface.