Critical role of electronic states above the vacuum level in photoelectron and secondary electron emission in few-monolayer pentacene films

Electron states above the vacuum level are known to play an important role in secondary electron processes, such as photoelectron emission and secondary electron emission where they act as "final" (or better "intermediate") states from which an electron is emitted to the vacuum. However, despite their relevance, these states are typically not well known, nor independently investigated, mostly due to a lack of proper spectroscopic techniques. Here, we present a spectroscopy study on crystalline pentacene, used as a model system to investigate the influence of these states on secondary electron processes. Using low-energy electron (LEE) spectroscopy, we first gauge the spectrum of such states in few-monolayer pentacene films. We, subsequently, relate these states to photoelectron and secondary electron emission. Specifically, photoemission experiments (Hg lamp) show a decrease in intensity with each additional pentacene layer grown. Given an absence of increase in the ionization energy or change in the crystal structure with increasing layer count, we relate the decrease in photoemission intensity to the emergence of a band gap just above the vacuum level as observed in LEE reflectivity spectra. Second, we study the energy distribution of secondary electrons. We use electron-beam damage to cause controlled changes in the band structure, and find a clear correlation between the evolution of the LEE spectra and the distribution of secondary electrons.