Prof. Dr. Reinhard Kienberger
Technische Universität München

Wolfram Helml
Max Planck Institute of Quantum Optics
International Max Planck Research School of Advanced Photon Science

Prof. Dr. Jerome Hastings
Stanford University
Stanford Linear Accelerator Center (SLAC)

The measurements planned aim at demonstrating an experimental technique, which will be capable to measure the X-ray pulse duration of the LINAC Coherent Light Source (LCLS) at Stanford, California, the world’s largest and brightest coherent X-ray facility at present, with sub-femtosecond precision utilizing photoelectron spectroscopy. One of the essential characteristics of LCLS and other Free Electron Lasers (FELs) which are being built up nowadays is their ultrashort pulse duration. Notwithstanding its importance for the characterization of LCLS itself, the precise measurement of such short FEL pulse durations is a prerequisite for the design of many future experiments with the goal to obtain detailed information on physical, chemical and biological dynamics, therefore paving the way to novel and attractive applications in medicine, especially in the fields of soft tissue imaging and cancer diagnostics.

Final report:

The goal of this project was the temporal characterization of ultrashort and ultrabright X-ray pulses at the Linac Coherent Light Source (LCLS) operated by the Stanford University in California. The ultrashort pulse duration available at these novel free-electron lasers (FEL) is a fundamental and crucial parameter for a host of high-intensity dynamical experiments aiming to obtain detailed structural and functional information on physical, chemical and biological systems The precise measurement of the temporal pulse structure might even lead to novel insights concerning the development of next-generation X-ray facilities, like the European X-Ray Free-Electron Laser (XFEL) in Hamburg.

In the frame of this project the method of streaking spectroscopy was transferred from the optical to the X-ray regime. This method uses the electromagnetic field of a co-timed conventional laser to 'streak' (PEs) photoelectrons excited by the X-ray pulses from a noble gas. The temporal distribution of these photoelectrons mimics the X-ray pulse shape. Via the interaction of these PEs with the optical laser field this distribution in time is mapped to a corresponding one of the PEs' kinetic energy, which can then be measured with a magnetic bottle electron time-of-flight spectrometer.

We demonstrated that this experimental technique is capable of measuring X-ray pulse durations for every single-shot with a precision of a few 100 attoseconds In addition this method also permits us to directly investigate the substructure of the FEL pulses on an attosecond time-scale. The results have been submitted to a major peer reviewed journal and are currently under consideration for publication.