The durations of extreme-ultraviolet femtosecond pulses (harmonics 21 and 27 of a Ti:sapphire femtosecond laser) have been measured by a cross-correlation technique. Photoelectron energy spectra resulting from ionization of helium atoms by one harmonic photon in the presence of the infrared laser field show sidebands that provide a cross-correlation signal from which the durations of the harmonic pulses can be obtained. The measurements yield a mean value of 130 fs for both harmonics. The accuracy of the method depends on the actual intensity dependence of the sidebands.
J. Opt. Soc. Am. B

Bouhal, A., Evans, R., Grillon, G., Mysyrowicz, A., Breger, P., Agostini, P., … von der Linde, D. (1997). Cross-correlation measurement of femtosecond noncollinear high-order harmonics. J. Opt. Soc. Am. B, 14, 950–956. doi:10.1364/JOSAB.14.000950