Interference of two-photon transitions induced by XUV light

The relative phase of first (?) and third harmonics (?) extreme ultraviolet light pulses was varied to control the population of the 2s state in helium through the interference of ? + ? and ? - ? two-photon excitation paths. The population was monitored by observing the total electron yield due to the 2s autoionization decay. Maximum yield occurs when the relative phase of the two harmonics matches the phase difference of complex atomic amplitudes governing the two excitation paths. The calculated trend of atomic phase differences agrees well with the measured data in the spectral region of the resonance, provided that time-reversed -? + ? path is also taken into account. These results open the way to accessing phase differences of two-photon ionization paths involving energetically distant intermediate states and to perform interferometry in the extreme ultraviolet range by monitoring final state populations.