Abstract
The technical implementation of a multi-MHz data acquisition scheme for laser-X-ray pump-probe experiments with pulse limited temporal resolution (100 ps) is presented. Such techniques are very attractive to benefit from the high-repetition rates of X-ray pulses delivered from advanced synchrotron radiation sources. Exploiting a synchronized 3.9 MHz laser excitation source, experiments in 60-bunch mode (7.8 MHz) at beamline P01 of the PETRA III storage ring are performed. Hereby molecular systems in liquid solutions are excited by the pulsed laser source and the total X-ray fluorescence yield (TFY) from the sample is recorded using silicon avalanche photodiode detectors (APDs). The subsequent digitizer card samples the APD signal traces in 0.5 ns steps with 12-bit resolution. These traces are then processed to deliver an integrated value for each recorded single X-ray pulse intensity and sorted into bins according to whether the laser excited the sample or not. For each subgroup the recorded single-shot values are averaged over ∼107 pulses to deliver a mean TFY value with its standard error for each data point, e.g. at a given X-ray probe energy. The sensitivity reaches down to the shot-noise limit, and signal-to-noise ratios approaching 1000 are achievable in only a few seconds collection time per data point. The dynamic range covers 100 photons pulse-1 and is only technically limited by the utilized APD.
Highlights
We have implemented a setup for TR X-ray absorption (XAS) at the dynamics beamline P01 of PETRA III with two key components, which improve the data collection efficiency by a total of up to six orders of magnitude
Compared with a traditional kHz titanium : sapphire (Ti): Sa laser system this allows for a thousand-fold faster acquisition of laser-excited XAS data in total X-ray fluorescence yield (TFY) mode
The second element is a MHz data acquisition (DAQ) system developed at European XFEL, which acts similar to a gated integrator, but operates, in principle, up to the full RF of PETRA III (500 MHz)
Summary
Pump–probe experiments using pulsed hard X-rays from synchrotron radiation (SR) sources are still a growing field, since the first proof-of-principle experiments were performed about 15 years ago with 1 kHz amplified laser systems (Lindenberg et al, 2000; Wulff et al, 1997; Chen et al, 2001; Saes, Gawelda et al, 2003; Bressler & Chergui, 2004; Schotte et al, 2003). For comparison reasons the conditions for 1 kHz experiments at SLS are listed at the bottom of Table 1, emphasizing the dramatic boost in signal quality when performing pump–probe experiments at MHz repetition rates Success of this avenue requires a sufficient amount of sample molecules to be excited in the X-ray probed volume, or laser pulse energies (at the selected UV–Vis wavelength) of the order of a few mJ, when laser and X-ray spot sizes of < 50 mm can be realised at the beamline. Our data acquisition (DAQ) scheme was applied at beamline P11 of PETRA III at 0.13 MHz probe repetition rates for experiments using a 65 kHz laser system (Goeries et al, 2016), but could operate reliably at even higher repetition rates towards the inverse pulse width of typical electrical photodiode signals ($ 30 MHz)
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