Interface originated modification of electron-vibration coupling in resonant photoelectron spectroscopy

We present a comprehensive study of the photon energy ($h \nu$) dependent line-shape evolution of molecular orbital signals of large $\pi$-conjugated molecules by resonant photoelectron spectroscopy (RPES). A comparison to RPES data of small molecules suggests that the excitation into different vibrational levels on the intermediate state potential energy surface of the electronic excitation is responsible for the observed effect. In this simplified picture of electron-vibration couping the character of the potential energy surfaces involved in the RPES process determines the line-shape of the molecular orbital signal for a particular $h \nu$. We use the sensitivity of this effect to probe the influence of different interfaces on the electron-vibration coupling in the investigated systems. The magnitude of the variation in line-shape throughout the particular $h \nu$ region allows to reveal significant differences within the physisorptive regime.