Application of photoelectron spectroscopy to the measurement of the flux of x-ray free-electron lasers irradiating clusters or biomolecules

This Brief Report shows the behavior of photoelectrons produced by the irradiation of x-ray free-electron lasers (XFELs) onto a cluster or a biomolecule and the measurement of the x-ray fluxes by using the photoelectrons. It is found that plasmas, which are produced through the interaction of XFELs with the target, give little contribution to photoelectrons. The minimum energies of the photoelectrons inform us of the x-ray flux for each target size. Further, these energies approximated by a simple equation agree well with the calculation results for relatively small x-ray fluxes.