  We have performed inelastic scanning tunneling spectroscopy (ISTS) on
individual Fe atoms adsorbed on a Ag(111) surface. ISTS reveals a magnetization
excitation with a lifetime of about 400 fsec which decreases linearly upon
application of a magnetic field. Astoundingly, we find that the g-factor, which
characterizes the shift in energy of the excitation in a magnetic field, is g =
3.1 instead of the regular value of 2. This enhancement can be understood when
considering the complete electronic structure of both the Ag(111) surface state
and the Fe atom, as shown by ab initio calculations of the magnetic
susceptibility.
