Two-colour time-resolved spectroscopy of helium using high-order harmonics

The radiative lifetime of the 1s2p 1 P state of helium is measured in a two-colour ionization experiment with a 5% accuracy. The state is excited by the 13th harmonic of a tunable 80 ps laser and ionized by a synchronous ultraviolet laser, with a variable time delay. This experiment demonstrates that the high harmonics generated in a jet of rare gas exposed to an intense laser field provide a tunable short-pulse XUV source ideally suited for pump/probe type of studies and, in particular, short lifetime measurements.