XUV+IR resonant two-photon ionization of helium atoms

Abstract We present a theoretical study on the two-photon resonant photoionization of helium. In our model, we consider a Ti:Sapphire laser pulse combined with its $15^{th}$ harmonic as representative driving fields. Our analysis focuses on the momentum and energy distributions of near-threshold photoelectrons when the ground state of helium is ionized through the $1snp$ excited states. We examine the influence of the IR driving laser and its \textcolor{blue}{$15^{th}$ harmonic pulse}. By tuning the pulse characteristics and considering dipole selection rules and Fano's propensity rule, we identify possible resonant pathways while the carrier frequencies of the infrared and harmonic fields remain fixed. A key parameter in our study is the temporal duration (FWHM) of the XUV pulse, which controls its spectral bandwidth and consequently the number of intermediate resonant states that can be accessed. These results provide insight into resonant ionization mechanisms relevant to attosecond orbital imaging and resonant RABBIT experiments.