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Timing of Femtosecond Pump-and-Probe Pulses

Pump-and-probe experiments using X-ray FELs may uncover the dynamics of ultrafast processes at the atomic level. The key to success is the ability to synchronize the femtosecond pump and probe pulses. An elegant experiment at FLASH proves it to be possible.

Many physical, chemical and biochemical processes happen so fast that it is only possible to uncover the intermediate steps using femtosecond light sources. At the moment FLASH produces femtosecond pulses in the extreme-ultraviolet and soft X-ray regime. When the XFEL will start operating, it will deliver femtosecond pulses of hard X-rays and enable scientists to record the evolution of extremely fast processes at atomic resolution; provided the necessary experimental techniques have been developed.

The method of choice to study femtosecond dynamics are pump-and-probe experiments. At FLASH the FEL beam serves as pump source to start a reaction, while the optical laser pulses may be used to probe the state of the system at different time intervals after the reaction has been initiated. The key to success for future time-resolved pump-and-probe experiments at FLASH and XFEL is the ability to synchronize the two femtosecond pulses. Now a series of experiments at FLASH performed by researchers from LIXAM/CNRS and Université Pierre et Marie Curie in France, Dublin City University in Ireland, Queen’s University Belfast in the United Kingdom, and DESY has proven this is indeed possible. In the fastest of these experiments optical pulses lasting 120 femtoseconds were crossed in space and time by 10- to 50-femtosecond FEL pulses. For this type of experiments, users can apply the laser pulses of the optical laser system available at the FLASH facility. (See also “THE USER FACILITY”)

Above-Threshold Ionization

A powerful tool to obtain information on the temporal overlap of two femtosecond pulses is given by the Above-Threshold Ionization (ATI) process. This technique utilizes electron spectroscopy of rare gas atoms to analyze the photoionization signal produced by FEL photons in the presence of the optical laser beam.


As the FEL pulse hits the rare gas, the energetic photons kick out electrons from the atoms. These electrons have a characteristic energy and give rise to the main line detected in the photoelectron spectrum. However, when the FEL pulse and the optical pulse overlap, the emitted photoelectrons are born into the strong optical field of the laser. Close to the atom these electrons can absorb or emit one or more photons from the optical laser field and thus change their energy by multiples of the photon energy of the optical laser.


This two-photon ionization process reveals itself as SideBands (SB) observed on both sides of the main atomic line in the photoelectron spectrum. These sidebands occur only when the two femtosecond pulses cross each other in space and time, and hence prove the overlap of the extremely fast pump and probe pulses. Since the effect depends strongly on the strength of the optical field, the intensity of the sidebands is a direct measure of the time interval between both laser pulses.


The experimental results from FLASH illustrate the successful combination of two very different laser types and clearly show the potential of short-wavelength FELs and optical laser pump-and-probe experiments. Apart from proving that synchronization is possible in the femtosecond regime, the experiments also open new and exciting opportunities for investigating fundamental photoionization and photodissociation processes.

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Time Resolution

At present the temporal resolution of the pump-and-probe experiments at FLASH depends on the length of the optical laser pulses, which is 120 femtoseconds at 800 nanometers in the infrared and 12 picoseconds for visible light at 523 nanometers. The width of the ATI cross-correlation signal is mainly determined by the temporal jitter of the FEL pulses with respect to the optical laser and represents the overall temporal resolution of around 250 femtoseconds (rms).


The temporal resolution will be considerably improved by using optical laser pulses of shorter duration and by eliminating the time jitter between FEL pulse and optical laser pulse, either with the help of single-shot techniques or by measuring the arrival times of the individual FEL pulses and correcting for the fluctuations.


A comparison of the measured intensities of single-shot sidebands and a theoretical analysis based on numerically solving the Time-Dependent Schrödinger Equation (TDSE) makes it possible to determine the relative temporal delay with an accuracy of better than 50 femtoseconds.

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