  The reversible control of a single spin of an atom or a molecule is of great
interest in Kondo physics and a potential application in spin based
electronics.Here we demonstrate that the Kondo resonance of manganese
phthalocyanine molecules on an Au(111) substrate have been reversibly switched
off and on via a robust route through attachment and detachment of single
hydrogen atom to the magnetic core of the molecule. As further revealed by
density functional theory calculations, even though the total number of
electrons of the Mn ion remains almost the same in the process, gaining one
single hydrogen atom leads to redistribution of charges within 3d orbitals with
a reduction of the molecular spin state from S = 3/2 to S = 1 that directly
contributes to the Kondo resonance disappearance. This process is reversed by a
local voltage pulse or thermal annealing to desorb the hydrogen atom.
