Measurements of the phase of two-photon matrix elements are presented for resonant and antiresonant two-color ionization of helium. A tunable, narrow-bandwidth, near-infrared (NIR) laser source is used for extreme ultraviolet (XUV) high-harmonic generation (HHG). The 15th harmonic of the laser is used within ($1+1^{\prime }$) $\mathrm{XUV}+\mathrm{NIR}$ two-photon ionization and tuned in and out of resonance with members of the $1snp{}^{1}P_1$ ($n=3,4,5$) Rydberg series, covering a broad spectral range with high spectral resolution. The technique allows us to observe characteristic rapid changes in the phase of the two-photon matrix elements around the resonances and at the antiresonances between the resonances. Similar effects are observed for ($1+2^{\prime }$) $\mathrm{XUV}+\mathrm{NIR}$ three-photon ionization. The experimental results are compared to a perturbative model and to numerical solutions of the time-dependent Schrödinger equation in the single-active-electron approximation, elucidating the origin and dependences of the observed phenomena.

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• Electronic excitation & ionization
• Electronic transitions
• High-order harmonic generation
• Multiphoton or tunneling ionization & excitation
• Photoemission
• Ultrafast optics
• Ultrafast phenomena