Single ionization of helium by a superposition of selected XUV high harmonics and infrared radiation has been studied by a momentum imaging technique. The measured angular distributions of photoelectrons are compared to numerical time-dependent calculations, showing very good agreement after average. The calculated angular distributions appear to depend critically on the delay between harmonic and infrared pulses on the attosecond scale, and on the relative phases and intensities of the harmonics.

Energy- and angle-resolved photoionization spectra of He irradiated by linearly polarized intense 810 nm laser radiation and several of its XUV odd harmonics are investigated. The angular distribution of the odd-order peaks, produced by single-photon ionization by one harmonic, is, surprisingly, broadened by the IR field. The even-order ones, due to two-colour, two-photon ionization, show at 90° lobes which depend on the relative IR–XUV phase. Application to the characterization of attosecond pulses is suggested.