We employed photoelectron spectroscopy, namely ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS), to study (1) the structural phase transition from a crystalline to an amorphous phase initiated by low-energy ion irradiation, and (2) the bias-enhanced nucleation (BEN) and subsequent deposition of polycrystalline diamond films. Both experiments are performed in-situ, the samples are transferred from the preparation to the analysis chamber without breaking the vacuum, thus avoiding surface contamination. At least partial amorphisation of the surface of a polycrystalline CVD-diamond film and the destruction of graphitic inclusions in an amorphous carbon (a-C) film by ion irradiation (1 keV Ar+ ions) are examples for a loss of crystallinity. These structural changes are observed as a gradual function of ion dose and the results of the UPS analysis are discussed in some detail. The bias-enhanced nucleation, on the other hand, does not constitute a macroscopic structural phase transition, but the co-deposition of an amorphous carbon and a crystalline diamond phase as well as the formation of a thin SiC interface are observed. The temporal evolution of the Cls core level peak shows that the amorphous phase is mostly removed again by etching when switching to deposition parameters appropriate for the subsequent diamond growth. The experimental results are discussed with respect to the nucleation mechanism when bias-enhanced nucleation is employed.