From the combination of OES, QMS, and DFT we propose a plasma decomposition mechanism of FeCp2 in the hollow cathode Ar plasma used in e-CVD. We propose that ferrocene is likely to decompose according to Fig. and Fig. . Initially, the neutral ferrocene molecule decomposes according to reactions 4-5 and Fig. ), since energies of 3.8 eV and 2.8 eV, respectively, are required, which are in line with our earlier measured electron temperatures. The decomposition of the Cp ligands from the iron center is also supported by the Fe I emission lines using OES. Other decomposition pathways were also investigated, such as decomposition initiated by ionization (reactions 6-9, Fig. )) and decomposition initiated by EA reactions (reactions 10-15, Fig. )). Our in-silico modeling showed that the initial first-and second ionization required 7.0 and 11.6 eV while the initial EA reactions required 0.3 and 2.6 eV, respectively. The ionization energies are roughly a factor 2 higher than the decomposition through the neutral ferrocene while the EA reactions have not been experimentally observed yet, indicating that EA is not a dominant decomposition pathway. 46 We suggest that the Cp rings are then further decomposed to C2H2 and C3H3. The energetically preferred decomposition path is through the negative C2H2 -after an EA reaction, eventually forming CH • , C2 and H • in several steps (see reactions 22-28) and Fig. . Energies required for the Cp decomposition ranges from 0.8 and 8.9 eV, as suggested by our DFT calculations and supported through observations of C2, C3 and CH using OES and C2H2 and C3H3 using MS. We do however note that the decomposition can as well proceed through neutral decomposition, although to our knowledge, no cross sections for such reactions have been displayed. H2 and H • . The required energies for these reactions range from 0.8 to 8.9 eV, which are energies that are accessible in the plasma.