In principle, the temporal beating of superposed high harmonics obtained by focusing a femtosecond laser pulse in a gas jet can produce a train of very short intensity spikes, depending on the relative phases of the harmonics. We present a method to measure such phases through two-photon, two-color photoionization. We found that the harmonics are locked in phase and form a train of 250-attosecond pulses in the time domain. Harmonic generation may be a promising source for attosecond time-resolved measurements.

We present calculations of two-photon matrix elements for two-photon two-colour ionization. We focus on ionization processes that use one infrared and one extreme-ultraviolet photon. The paper describes a novel algorithm used for the calculation of radial matrix elements, and also of radial phase shifts of continuum states.

In this paper we present some results of a high-harmonic generation experiment in argon, performed with a Ti:sapphire laser (787 nm, 140 fs) and a spatially shaped focus (flat top). The range of intensities is 3-100 TW cm −2 . The intensity dependence of the harmonic signal shows an interesting behaviour of the 13th harmonic, namely an initial suppression followed by a strong enhancement at 85 TW cm −2 , which exceeds the magnitude of the 11th harmonic. This behaviour is a consequence of a resonant atomic process. Numerical calculations were also performed. They partly reproduced the experiment.

We present an alternative method for measuring ultrashort extreme-ultraviolet pulses that can be synchronized with an intense infrared pulse. The method, based on photoionization of a target atom by the extremeultraviolet pulse in the presence of the infrared pulse, has a potential accuracy of close to 1 fs and is susceptible to single-shot operation. It is demonstrated on harmonic 15 of a titanium:sapphire laser. The minimum pulse duration that can be measured is limited only by the frequency of the radiation used for the ponderomotive shift of the ionization potential ͑3 fs in the case of the titanium:sapphire fundamental͒.

Generally, high optical intensity is required for effective multiphoton excitation of quantum systems to highly excited states. In certain situations, however, lower-intensity, chirped pulses can provide more efficient transfer of population to the upper states by the process of adiabatic passage. We have studied the relative importance of these two mechanisms in the anharmonic 5s-5p-5d quantum ladder system of rubidium using frequency chirped laser pulses from an amplified Ti:sapphire laser ͑ϭ780 and ⌬ϭ10 nm͒. We measure simultaneously the three-photon ionization signal due to the Ti:sapphire and the population that remains in the 5d state with a postionizing 532-nm Nd:YAG pulse ͑where YAG denotes yttrium aluminum garnet͒. At low infrared fluences ͑80 J/cm 2 ͒, the transfer to the 5d state is significantly enhanced when the pulse frequency is swept from the red to the blue, such that it follows the frequency spacing of the rubidium ladder. Counterintuitively, population is also transferred efficiently for the blue-to-red chirp at high fluences ͑Ͼ5 mJ/cm 2 ͒. We attribute both of these effects to adiabatic passage from the 5s state to the 5d state. Even at the highest fluences, more efficient transfer occurs for either direction of chirp than occurs at zero chirp, where the intensity is maximal. A comparison to theoretical predictions reveals striking agreement in both absolute magnitude and functional form. These results have important implications for the understanding of population transfer in complex ladder systems, such as molecular anharmonic vibrational ladders.