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

Abstract

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 ${10}^{16}\text{ }\text{W}/{\text{cm}}^{2}$ 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 ${\ensuremath{\tau}}_{L}=(29\ifmmode\pm\else\textpm\fi{}5)\text{ }\text{fs}$ is derived for the soft x-ray pulses at FLASH.