Site-selective control of electron-induced Fe(CO)5 dissociation by hydration: a mechanistic pathway to high-purity nanofabrication.

Through first-principles simulations, this study elucidates the mechanism by which water molecules steer the electron-induced dissociation of iron pentacarbonyl (Fe(CO)5), a key process in focused electron beam-induced deposition (FEBID). We demonstrate that the adsorption site of a single water molecule is a critical determinant of the subsequent reaction dynamics. Adsorption at an equatorial site alters the frontier orbital landscape, specifically localizing the LUMO+2 wavefunction along a single axial Fe-C bond. Consequently, non-adiabatic molecular dynamics simulations show that equatorial hydration accelerates the selective cleavage of this targeted axial ligand, reducing the dissociation time by 25 femtoseconds compared to the axial hydration configuration. Furthermore, electron wave packet scattering simulations identify 7.5 eV as a key energy for process control. At 7.5 eV, the wave-packet calculation shows preferential depletion of the water-localized HOMO-2 orbital and population of the LUMO+5 to LUMO+8 target orbitals. Because electron capture and nuclear dissociation are not resolved in the present sub-femtosecond, frozen-nuclei simulation, this response is interpreted as an inelastic electronic-excitation channel in the same energy range in which experiments report water-initiated DEA and OH- formation. Collectively, these findings establish hydration site engineering as a precise strategy for controlling precursor fragmentation, providing a first-principles-based pathway to enhance the purity of metallic nanostructures fabricated via FEBID.