Direct autocorrelation of soft-x-ray free-electron-laser pulses by time-resolved two-photon double ionization of He

The pulse duration of soft x-ray free-electron laser (FEL) radiation is directly measured by time-resolved observation of doubly charged helium ions at 51.8 eV. A wave front splitting autocorrelator produces two correlated FEL pulses with a resolution of better than a femtosecond. In the interesting intensity range from 10(13) to 10(16) W/cm(2) direct and sequential double ionization contribute to the ion yield which has significant influence on the correlation width, being a general feature at high photon energies. Here, a duration of tau(L) = (29 +/- 5) fs is derived for the soft x-ray pulses at FLASH.