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The possibility of modifying electron interactions
by external means provide an opportunity to test
theory and develop new insights to use electronic properties
for (nano)technological applications.
We present experimental evidence of the
variation of the lifetime and effective mass of electrons
in image potential states (IPS) on a Cu(111) surface
induced by a selective alteration of the electron density in
the bulk band states through laser pulse excitation.
The electron wavefunction in a IPS resides outside the
solid and is reflected at the surface into the vacuum by
the crystal potential with a Bragg scattering mechanism.
The penetration of the wave function into the bulk and
its phase at the surface self-consistently determine the
IPS binding energy, that collocates the energy position
of the IPS in a band gap of the surface projected bulk
bands. The (111) surfaces of metals are characterized by
the presence of both an occupied surface-state n=0 and
the unoccupied n=1 IPS, located close to the upper edge
of the bulk sp band gap. For Cu(111) this energy difference
is about 180 meV at zero parallel momentum,
allowing to control the penetration of the hybridized IPS
wavefunction into the bulk by exciting electrons into
the upper edge of the band gap to expel the IPS electron
into the vacuum through Coulomb repulsion and Pauli
exclusion principle.
In this way the interaction is turned on smoothly and the
non-interacting IPS states evolve on a one-to-one basis
into interacting states with modified lifetime and
mass.
Electrons in Cu(111) IPS are probed either by two-photon
photoemission at different wavelength either by a
two-color pump-probe photoemission technique, allowing
to measure their effective mass and intrinsic linewidth as
a function of the pump intensity, i.e. as a function of the
pump induced electron density into the quasi-isoenergetic
bulk bands. |