Resonant two-photon ionization of helium atoms studied by attosecond interferometry
We study resonant two-photon ionization of helium atoms via the 1s3p, 1s4p and 1s5p1P1 states using the 15th harmonic of a titanium-sapphire laser for the excitation and a weak fraction of the laser field for the ionization. The phase of the photoelectron wavepackets is measured by an attosecond interferometric technique, using the 17th harmonic. We perform experiments with angular resolution usin