Phase Measurement of Resonant Two-Photon Ionization in Helium
We study resonant two-color two-photon ionization of helium via the 1s3p P-1(1) state. The first color is the 15th harmonic of a tunable Ti:sapphire laser, while the second color is the fundamental laser radiation. Our method uses phase-locked high-order harmonics to determine the phase of the two-photon process by interferometry. The measurement of the two-photon ionization phase variation as a f