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 1s5p P-1(1) states using the 15(th) 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 17(th) harmonic. We perform experiments with angular resolution using a velocity map imaging spectrometer and with high energy resolution using a magnetic bottle electron spectrometer. Our results are compared to calculations using the two-photon random phase approximation with exchange to account for electron correlation effects. We give an interpretation for the multiple pi-rad phase jumps observed, both at and away from resonance, as well as their dependence on the emission angle.

Automated summary

We studied the resonant two-photon ionization of helium atoms and measured the phase of the photoelectron wavepackets using an attosecond interferometric technique. Experiments were performed with angular and high energy resolution. The results were compared to calculations and an interpretation was given for the observed phase jumps at and away from resonance as well as their dependence on the emission angle.