The superposition of harmonics of a femtosecond laser beam may form a
train of pulses with duration in the attosecond regime  or even
isolated as pulses. Since this extreme temporal localization of light
has been demonstrated in the laboratory, its rigorous characterization
became a challenging problem that has set off intense experimental and
theoretical efforts. Among them, the one targeting the extension of well
established methods of optical fsec metrology to the XUV asec regime
resulted to the demonstration of a second order autocorrelation (AC)
yields measurement of an asec pulse train, formed by the superposition
of higher-order harmonics of a Ti:Sapp laser, the theoretical analysis
of which verified the experimental findings .
In a subsequent work, the temporal width of an attosecond (asec) XUV radiation pulse train, has been determined, utilizing a 2nd order autocorrelation measurement of the XUV radiation field . Theoretical ab-initio two-photon autocorrelated ionization yields have been calculated in order to explain the deviation of the measured width from the fourier-transform limited value . Effects of the phase-dependence of the harmonics on the measured width are explored on the basis of the classical three-step model .
As a continuation of this approach, measured and calculated energy-resolved photoelectron spectra, by the two-photon ionization of Helium subject to an XUV attosecond pulse, is also presented. The theoretical results are in reasonable agreement with the experimental data, thus verifying the feasibility of performing AC-frequency resolved optical gating measurements (AC-FROG) in the XUV-attosecond regime .
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