Theory of photoelectron spectroscopy for organic molecules and their crystals

In this short review we discuss recent progress in photoemission theory for organic molecules and their crystals. We discuss some important features in Keldysh Green's function theory for the photoemission. We briefly discuss many-body aspects in photoemission from core and extended levels. In particular phonon effects are investigated in more detail since organic solids are typically soft where electron-phonon interaction is important. Debye-Waller factor suppresses the interference effects of photoelectron waves which makes ARPES analyses useless, particularly in high energy region.