Phase measurement of resonant two-photon ionization in helium

Phys Rev Lett. 2010 Mar 12;104(10):103003. doi: 10.1103/PhysRevLett.104.103003. Epub 2010 Mar 12.


We study resonant two-color two-photon ionization of helium via the 1s3p (1)P(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 function of detuning from the resonance and intensity of the dressing field allows us to determine the intensity dependence of the transition energy.