Time-Resolved (Pump-Probe) Experiments

Figure 3: CAD drawing of the experimental setup for time-resolved XAFS experiments with liquid jet system and on-axis microscope.

Figure 4: XAFS transient signal of an OLED metal complex as a function of energy (top) and of time delay (bottom) measured at beamline P11. The time-delay scan was performed at the energy where the maximum transient signal was obtained.

For the investigation of dynamical processes with pico- to milli-second time resolution beamline P11 offers two laser systems for optical pumping:
An actively Q-switched Nd:YAG laser provides laser pulses with a duration of 1.2 ns and repetition rates between 1 kHz and 20 kHz. It provides wavelengths of 1064 nm, 532 nm, and 355 nm with pulse energies of about 10 µJ and higher.
A femtosecond PHAROS laser (Figure 1) built by Light Conversion provides laser pulses with a duration of 180 fs with repetition rates of up to 300 kHz. The PHAROS system is synchronized to the PETRA III sychrontron with a timing jitter of around 1 ps rms. The repetition frequency can freely be chosen between 1 Hz and 300 kHz. The fundamental wavelength of the laser system is at 1030 nm. Additionally, a harmonics generator (“HIRO”) allows for the use of higher harmonics at wavelengths of 515 nm (SHG), 343 nm (THG), and 257 nm (FHG). The high repetition rates are well suited for experiments with fast sample exchange as is the case with a liquid jet system.

Figure 1: PHAROS laser system installed in the experimental hutch of beamline P11 (courtesy of Light Conversion).

Furthermore, an optical parametric amplifier (OPA) allows to freely tune the wavelength between 260 and 2600 nm. The maximum operation frequency when using the OPA is limited to 10 kHz. The PHAROS system in combination with the OPA is ideally suited for experiments with solid state samples where the sample is not permanently replaced such as liquid samples in a jet.
The PHAROS laser system is fully integrated into the P11 beamline control software and parameters such as laser power, repetition frequency and delay time between laser pump and X-ray probe pulse can be controlled by the software. A screen shot of the software GUI for time-resolved experiments is shown in Figure 2.

Figure 2: P11 control software for time-resolved XAFS experiments at beamline P11.

A time-resolved XAFS experiment has been developed at beamline P11. A schematic drawing is shown in Figure 3. The setup consists of an on-axis microscope for visualization of the liquid jet and, by inserting a scintillator screen at the same position, for visualization of the X-ray and laser beam. By this means, spatial overlap of the laser, X-ray and the jet can easily be achieved and permanently checked without entering the hutch. The X-ray fluorescence photons are detected with an avalanche photodiode with gated readout.
Figure 4 shows the result of a pump-probe XAFS experiment delivering the clear evidence of a Metal-to-Ligand-Charge-Transfer (MLCT) in a metal-organic compound commonly used for OLED devices. In addition, the rise of the transient signal as function of temporal delay between laser and X-ray pulse at a fixed energy is presented. By analyzing the first derivative of the rising signal the bunch length of PETRA III has been determined to be 75 ps (FWHM) in 40 bunch mode.
For time-resolved crystallography experiments the laser beam can also be delivered to the crystallography endstation. An X-ray chopper allowing for single bunch extraction is currently being built and will be available in summer 2014.