  Historically, electron spin resonance (ESR) has provided excellent insight
into the electronic, magnetic, and chemical structure of samples hosting spin
centers. In particular, the hyperfine interaction between the electron and the
nuclear spins yields valuable structural information of these centers. In
recent years, the combination of ESR and scanning tunneling microscopy
(ESR-STM) has allowed to acquire such information on individual spin centers of
magnetic atoms bound atop a surface, while additionally providing spatial
information about the binding site. Here, we conduct a full angle-dependent
investigation of the hyperfine splitting for individual titanium atoms on
MgO/Ag(001) by measurements in a vector magnetic field. We observe strong
anisotropy in both the g-factor and the hyperfine tensor. Combining the results
of the hyperfine splitting with the symmetry properties of the binding site
obtained from STM images and a basic point charge model allows us to predict
the shape of the electronic ground state configuration of the titanium atom.
Relying on experimental values only, this method paves the way for a new
protocol for electronic structure analysis for spin centers on surfaces.
