  The adsorption/desorption of ethene (C2H4), also commonly known as ethylene,
on Fe3O4(001) was studied under ultrahigh vacuum conditions using temperature
programmed desorption (TPD), scanning tunneling microscopy, x-ray photoelectron
spectroscopy, and density functional theory (DFT) based computations. To
interpret the TPD data, we have employed a new analysis method based on
equilibrium thermodynamics. C2H4 adsorbs intact at all coverages and interacts
most strongly with surface defects such as antiphase domain boundaries and Fe
adatoms. On the regular surface, C2H4 binds atop surface Fe sites up to a
coverage of 2 molecules per (rt2xrt2)R45{\deg} unit cell, with every second Fe
occupied. A desorption energy of 0.36 eV is determined by analysis of the TPD
spectra at this coverage, which is approximately 0.1-0.2 eV lower than the
value calculated by DFT + U with van der Waals corrections. Additional
molecules are accommodated in between the Fe rows. These are stabilized by
attractive interactions with the molecules adsorbed at Fe sites. The total
capacity of the surface for C2H4 adsorption is found to be close to 4 molecules
per (rt2xrt2)R45{\deg} unit cell.
