  Scattering of electrons by localized spins is the ultimate process enabling
electrical detection and control of the magnetic state of a spin-doped
material. At the molecular scale, this scattering is mediated by the electronic
orbitals hosting the spin. Here we report the selective excitation of a
molecular spin by electrons tunneling through different molecular orbitals.
Spatially-resolved tunneling spectra on iron porphyrins on Au(111) reveal that
the inelastic spin excitation extends beyond the iron site. The inelastic
features also change shape and symmetry along the molecule. Combining DFT
simulations with a phenomenological scattering model, we show that the
extension and lineshape variations of the inelastic signal are due to
excitation pathways assisted by different frontier orbitals, each of them with
a different degree of hybridization with the surface. By selecting the
intramolecular site for electron injection, the relative weight of iron and
pyrrole orbitals in the tunneling process is modified. In this way, the spin
excitation mechanism, reflected by its spectral lineshape, changes depending on
the degree of localization and energy alignment of the chosen molecular
orbital.
