Multicolour above-threshold ionization of helium:: quantum interference effects in angular distributions

Energy- and angle-resolved photoionization spectra of He irradiated by linearly polarized intense 810 run 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 degrees lobes which depend on the relative IR-XUV phase. Application to the characterization of attosecond pulses is suggested.