Experimental study on multiphoton ionization of Ar with high intensity short las

We present high resolution photoelectron energy spectra from multiphoton ionization (MPI) of Ar subject to laser pulses with wavelength of 400 nm, pulse duration of 35 fs, and maximum intensity of 5×1013 W/cm2. Ionizations into Ar+ 2P3/2 and 2P1/2 channels are observed and distinct resonance structures are found in both ionization channels. The intensity dependence of the resonance structures is explained in terms of th