  The basis of low-temperature superconductivity has been set to be the pair
formation of electrons, due to their effective attraction. The appearance of an
effective attraction potential has also been predicted for electron-electron
scattering in the presence of a strong, inhomogeneous radiation field. In the
present work the strong electromagnetic fields were created by femtosecond
Ti:Sa lasers, used to excite surface plasmons in gold films at room
temperature, in the Kretschmann geometry. Experimental investigations were
carried out using a surface plasmon near-field scanning tunneling microscope,
by measuring its response to the excitation at hot spots on the gold surface.
Furthermore, the spectra of photoelectrons, liberated by multi-plasmon
absorption, have also been measured by a time-of-flight spectrometer. In both
cases new type of anomalies in the electon signal have been measured in the
same intensity range, whose existence may be qualitatively understood, by using
the intensity-dependent expression for the effective electon-electron
scattering potential, derived earlier in a different context.
