Freeze-framing nanosecond movements of nanoparticles

A new method allows monitoring fast movements of hard X-ray lasers.

Scheme of the experiment: An autocorrelator developed at DESY splits the ultrashort X-ray laser pulses into two equal intensity pulses which arrive with a tunable delay at the sample. The speckle pattern of the sample is collected in a single exposure of the 2-D detector (picture: W. Roseker/DESY).
Scheme of the experiment: An autocorrelator developed at DESY splits the ultrashort X-ray laser pulses into two equal intensity pulses which arrive with a tunable delay at the sample. The speckle pattern of the sample is collected in a single exposure of the 2-D detector (picture: W. Roseker/DESY).

A collaboration of scientists from DESY, the Advanced Photon Source APS, and National Accelerator Laboratory SLAC, both in the USA have created and incorporated another technique for checking ultrafast developments of nanoscopic frameworks. With the light of the X-ray laser LCLS, they took pictures of the developments of nanoparticles taking just the billionth of a moment (0,000 000 001 s).

During experiments, they conquered the gradualness of current two-dimensional X-ray detectors by splitting individual laser flashes of LCLS, postponing one portion of it by a nanosecond and recording a solitary photo of the nanoparticle with these sets of X-ray pulses.

The tunable light splitter for hard X-rays which the researchers created for these tests empowers this new procedure to screen developments of nanometer estimate changes down to femtoseconds and at a nuclear determination. For examination: current synchrotron radiation light sources like PETRA III can typically quantify developments on millisecond timescales.

The intense light flashes of X-beam lasers are rational which implies that the waves of the monochromatic laser light spread in a stage to each other. Diffracting coherent light via sample typically brings about a speckle diffraction design demonstrating clearly arbitrarily requested light spots. In any case, this spot is additionally a guide of the example course of action, and developments of the example constituents result in a different speckle pattern.

To conduct experiments with some 1000 detected photons on the one-million-pixels size detector, scientists developed a special optical setup – a so-called optical autocorrelator – capable of splitting 100 femtosecond long XFEL pulses into two sub-pulses, deviate them into separated detours and recombining their paths with a tunable time delay between zero and a few nanoseconds.

These sets of XFEL beats hit the example with the tuned delay, detecting the example’s structure at the two presentation times. The aggregate of both spot pictures was recorded by a two-dimensional photon detector inside one presentation time.

If the constituents of the sample move during the two illuminations, the speckle pattern changes. Thus, it leads to an integrated picture of less contrast at the detector. The contrast is a measure on how strong the photon intensity varies on the detector. However, the intensity and especially the intensity difference measured at the detector are very weak.

Scientists also used a suspension of two nanometers estimate gold particles experiencing Brownian movement. The test was in culminate concurrence with the hypothetical forecasts subsequently demonstrating the execution of the autocorrelator setup as well as the legitimacy of the information investigation technique, exhibiting the principal effective analysis of this kind.

First author Wojciech Roseker from DESY said, “Such type of experiments has been done for much slower movements of nanoparticles at storage ring light sources. But now, the high coherence and intensity of the X-ray laser light at XFELs open up the opportunity to get pictures bright enough to provide reasonable information about quick movements in the nanosecond to femtosecond regime.”

Gerhard Grübel, head of the DESY FS-CXS group said, “This experiment paves the way to dynamics experiments of materials on atomic length and femtosecond-nanosecond timescales. Split-pulse X-ray Photon Correlation Spectroscopy (XPCS) can potentially track atomic-scale fluctuations in liquid metals, multi-scale dynamics in water, heterogeneous dynamics about the glass transition, and atomic scale surface fluctuations.”

The study is published in the journal Nature Communications.