AUTHOR=Neoričić L. , Busto D. , Laurell H. , Weissenbilder R. , Ammitzböll M. , Luo S. , Peschel J. , Wikmark H. , Lahl J. , Maclot S. , Squibb R. J. , Zhong S. , Eng-Johnsson P. , Arnold C. L. , Feifel R. , Gisselbrecht M. , Lindroth E. , L’Huillier A. 

TITLE=Resonant two-photon ionization of helium atoms studied by attosecond interferometry

JOURNAL=Frontiers in Physics

VOLUME=10

YEAR=2022

URL=https://www.frontiersin.org/journals/physics/articles/10.3389/fphy.2022.964586

DOI=10.3389/fphy.2022.964586

ISSN=2296-424X

ABSTRACT=<p>We study resonant two-photon ionization of helium atoms via the 1<italic>s</italic>3<italic>p</italic>, 1<italic>s</italic>4<italic>p</italic> and 1<italic>s</italic>5<italic>p</italic><sup>1</sup>P<sub>1</sub> states using the 15<sup>th</sup> 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<sup>th</sup> 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 <italic>π</italic>-rad phase jumps observed, both at and away from resonance, as well as their dependence on the emission angle.</p>