2-COLOR PHOTOEMISSION PRODUCED BY FEMTOSECOND LASER-PULSES ON COPPER

Single-color illumination of a copper surface by a red or an ultraviol et femtosecond laser pulse yields a three-photon (red) or a two-photon (UV) photoemission process. A multicolor, multiphoton process is gene rated when the red and the UV pulses overlap both in space and in time on the photocathode. It is shown that this emission process results f rom the absorption by an electron of one red and one UV photon. It pro vides a means to correlate ultrashort laser pulses of different wavele ngths.