Pump-Probe X-Ray Absorption Spectroscopy at the Synchrotron with the UHF-BOX Boxcar Averager

January 9, 2025 by Heidi Potts

Synchrotron facilities enable scientists to study the interaction of materials with X-rays and thereby gain an understanding of the fundamental properties of materials. A synchrotron generates short X-ray pulses which are repeated periodically, since they arise from Bremsstrahlung of particle bunches circulating in a ring. These X-ray pulses are then used for various experiments in beamlines (Figure 1).

Figure 1: Schematic illustration of a synchrotron and a beamline with the experimental hutch.

A common experiment with synchrotron radiation is pump-probe X-ray absorption spectroscopy, in which a sample is excited with a so-called pump-pulse followed by a probe-pulse. The effect of the pump-pulses excitation is imprinted onto the probe-pulses, changing, for example, the intensity of the transmitted probe-pulses. By varying the pump-probe time delay, it is possible to reconstruct the dynamics of the excitation effects in the sample. Figure 2 shows a schematic representation of such an experiment.

Figure 2: Schematic illustration of a pump-probe experiment.

The Challenge

The challenge is to measure these effects in a fast and precise manner. The repetition rate of the pulses is typically in the regime of several MHz, requiring fast data analysis, and the signals are often very weak, requiring the averaging of many pump-probe sequences in order to see the dynamics of the sample.

The Solution

The UHFLI 600 MHz Lock-in Amplifier with the UHF-BOX Boxcar Averager Option provides an excellent solution for such experiments: It enables to capture the data from the pulses, subtract any background signal, and then display and save the relevant information in real time.

Measurements at BESSY II

Recently, we had the opportunity to perform measurements with our UHF-BOX at the KMC-3 XPP beamline at BESSY II. Here, the repetition rate of the synchrotron pulses is 1.25 MHz. For the pump-probe experiment, a pulsed laser is synchronized with the beamline repetition rate, and the frequency of the excitation pulses is chosen to be f_{_rep}=1.25 MHz / 12 = 104.2 kHz. After setting up the experiment, the electrical signal from the X-ray detector is connected to one of the Signal Input channels of the UHFLI and the repetition rate is provided as a reference.

To start the experiment, the detector signal is visualized using the periodic waveform analyzer (PWA) function of the Boxcar Averager tool. The PWA allows to display a full period - or a fraction – of the pulses’ repetition period and to average it over many subsequent periods. The averaging is performed on the FPGA of the instrument, allowing to achieve averaging of millions of periods in real-time, thereby enabling the observation of features in the signal that would otherwise be buried in noise. Figure 3(a) shows the signal from the X-ray detector. The probe pulse can be identified as a sharp pulse (marked with a green rectangle) in between lots of other features and noise. Here, we already zoomed in to analyze only 1/12^th of the period because the pulse duration is much shorter than the repetition period. Since the rest of the period does not contain meaningful information for the experiment, boxcar averaging is the ideal analysis method (more information in our whitepaper), as it can effectively single out the fraction of the period where the information is concentrated.

Figure 3: (a) Measurement of the signal from the X-ray detector showing a short pulse in between many other features. (b) Measurement of the probe X-ray pulse where the boxcar windows are defined.

The next step is to define the region of interest during which the signal is analyzed. This can be done by zooming in on the pulse even more and then defining the so-called gate window using the mouse cursors. A second region can be marked as a baseline, as shown in Figure 3(b). Activating the boxcar measurement, the signal is integrated during the defined windows, with the baseline region subtracted from the gate window.

Now the measurement can be run, and the data analyzed in real-time using the LabOne GUI or saved to the PC using the APIs. Depending on the requirements of the measurement, it is possible to measure the information from individual pulses or perform real-time averaging directly with the UHF-BOX.

For more a detailed tutorial on how to set up such a boxcar experiment, please refer to this blog post or this video.

Outlook

In many experiments, the dynamics of the sample are much faster compared to the time between two consecutive X-ray pulses. A typical experiment therefore consists of measuring one X-ray pulse at different time-delays after excitation and comparing it with one X-ray pulse before excitation. However, if the dynamics of the sample are slow compared to the repetition rate of the X-ray pulses, the effect of the excitation can be measured over multiple X-ray pulses after the excitation.

With the UHF-BOX, this can be done with real-time averaging using the OutPWA functionality. The OutPWA takes the results of the boxcar averager and displays them over one period of the pump excitation. The results can then be averaged over many periods. Figure 4 shows the OutPWA with a demo where 12 individual pulses are visible, showing a decay related to the sample dynamics after excitation.

Figure 4: Measurement with the OutPWA showing 12 consecutive probe pulses, averaged over 10 000 periods.

Pump-probe experiments at synchrotrons require to capture the information from short pulses in a noisy environment. The UHF-BOX offers a solution to set up the experiment and optimize the detection windows in a graphical user interface. The results can be captured in real-time using the GUI and the APIs. With this, the experiment can be optimized on the fly and only the relevant data is saved - reducing the complexity of the experiment.

We thank Thomas Rossi for the possibility to perform the measurements together at BESSY II.

Related Publication

R. F. Wallick et. al. Excited-State Identification of a Nickel-Bipyridine Photocatalyst by Time-Resolved X-ray Absorption Spectroscopy. J. Phys. Chem. Lett. 15, 4976–4982 (2024)