2 0 0 8 J I N S T 3 P 0 2 0 0 3

Since the summer of 2005, the vacuum ultra-violet Free-ele ctron LASer in Hamburg (FLASH) has operated as a user facility at the Deutsches Elek tronen-Synchrotron (DESY), delivering ultra-short laser pulses of tens of femtosecond durat ion with a high peak brilliance of up to 1028photons/(smm2 mrad2 0.1%bandwidth). Due to the statistics of the Self-Amplified Spontaneous Emission (SASE) process, each photon pulse differs fr om the previous one in the number of modes per pulse, the wavelength (0 .5% fluctuations) and the intensity, making experiments more complicated. Thus, for certain experiments the detail ed knowledge of the beam properties on a shot-to-shot basis is mandatory. In this paper we descri be an online method to gain spectral information about the individual Free-Electron Laser (FEL) pulses that is based on rare-gas photoionization and photoelectron spectroscopy.