  Spin excitations in atomic-scale nanostructures have been investigated with
inelastic scanning tunneling spectroscopy, sometimes with conflicting results.
In this work we present a theoretical viewpoint on a recent experimental
controversy regarding the spin excitations of Co adatoms on Pt(111). While one
group [Balashov et al., Phys. Rev. Lett. \textbf{102}, 257203 (2009)] claims to
have detected them, another group reported their observation only after the
hydrogenation of the Co adatom [Dubout et al., Phys. Rev. Lett. \textbf{114},
106807 (2015)]. Utilizing time-dependent density functional theory in
combination with many-body perturbation theory we demonstrate that, although
inelastic spin excitations are possible for Cr, Mn, Fe, and Co adatoms, their
efficiency differs. While the excitation signature is less pronounced for Mn
and Co adatoms, it is larger for Cr and Fe adatoms. We find that the tunneling
matrix elements related to the nature and symmetry of the relevant electronic
states are more favorable for triggering the spin excitations in Fe than in Co.
An enhancement of the tunneling and of the inelastic spectra is possible by
attaching hydrogen to the adatom at the appropriate position.
