Photoelectron spectroscopy of two-photon ionisation of rare-gas atoms by multiple high order harmonics

Two-photon ionisation processes of rare-gas atoms produced by a superposition of multiple high order harmonics are studied by using a photoelectron spectroscopy technique. Second-order nonlinearity is confirmed by comparing the intensity-dependent photoelectron yields between the linear and nonlinear processes, and the cross sections of these processes are estimated. We also demonstrate the autocorrelation measurement of harmonic envelopes as a preliminary step toward characterisation of an attosecond pulse train.