INVESTIGATION OF ABOVE-THRESHOLD IONIZATION WITH FEMTOSECOND PULSES -CONNECTION BETWEEN PLATEAU AND ANGULAR-DISTRIBUTION OF THE PHOTOELECTRONS

We present high-resolution photoelectron spectra and angular distribut ions produced by multiphoton ionization of Ar and Xe atoms by intense ultrashort laser pulses. The measurements have been performed in order to investigate the connection between two recently discovered effects in above-threshold ionization, namely a plateau in the photoelectron spectra and side lobes in the corresponding angular distributions. The side lobes are found to be restricted to the region where the plateau begins. This affords a new explanation of the origin of the side lobe s.