Phase dependence of resonant and antiresonant two-photon excitations

Measurements of the phase two-photon matrix elements are presented for resonant and antiresonant two-color ionization helium. A tunable, narrow-bandwidth, near-infrared (NIR) laser source is used extreme ultra-violet (XUV) high-harmonic generation (HHG). The 15th harmonic within (1+1') XUV+NIR ionization, tuned in out resonance with members 1s$n$p $^1$P$_1$ ($n=3,4,5$) Rydberg series, covering a broad spectral range high resolution. technique allows to observe characteristic rapid changes around resonances and, previously unobserved, at antiresonances between resonances. Similar effects observed (1+2') three-photon ionization. experimental results compared perturbative model numerical solutions time-dependent Schr\"odinger equation (TDSE) single active electron (SAE) approximation, elucidating origin dependences phenomena.