Delayed explosion due to cluster shell

Scientists explore new technology to obtain improved snapshots of molecules

Violent forces are at work during research with free-electron lasers: every single light flash brings the analysed sample to a fast explosion. In the investigation, every femtosecond (a millionth of a billionth of a second) counts when decoding the molecular structure of the sample before its destruction. At DESY’s X-ray laser FLASH, a team of scientists from SLAC, TU Berlin and DESY has now studied a method to delay the explosion of the sample for potentially decisive fractions of a second. In their experiments which have now been presented in the scientific journal Physical Review Letters, the scientists were able to observe the exact process that takes place in little clusters of atoms intensely irradiated by X-ray light.

Impression of the explosion of a core-shell cluster in the X-ray light of FLASH.

X-ray lasers as DESY’s FLASH or the European XFEL currently under construction are high-speed cameras for the nanocosm. Their high-energy and ultra-short X-ray pulses allow insights into the smallest dimensions and ultra-fast processes, opening up new scientific investigation possibilities in various areas, ranging from the imaging of single molecules up to filming the movement of electrons in atoms. With these experiments, the scientists obtain important information about how molecular reactions take place, e.g. during photosynthesis. The problem is that during measurement, the extreme intensity of each single X-ray flash inevitably transforms the investigated sample into a dense plasma and eventually destroys it. When the X-ray flash hits the sample, a great many of the electrons bound in the atomic shell are abruptly knocked out, leaving a molecular structure of highly charged ions with the same charge repelling each other and flying off in all directions in a so-called Coulomb explosion.

 One strategy to prevent the loss of electrons in the sample too quickly is to surround it with a shell layer. During the intense interaction with the X-ray light, the layer is ionised as well; however, the electrons released from the shell will be transferred into the core of the sample, thus compensating the loss of core electrons for a short time.

For the investigation of this explosion delay, a team of researchers led by DESY scientist Tim Laarmann prepared small clusters with a core consisting of about 80 Xenon atoms and a shell of about 400 Argon atoms. They irradiated this cluster with the intensive FLASH X-ray beam and analysed the fluorescent light emitted by the cluster within the first femto- to picoseconds. “For the first time, we were able to observe the extremely short-lived states generated within the first few hundred femtoseconds by the interaction with the FLASH pulses and exactly see which states of charge are forming in the cluster at the beginning,” said Laarmann. The obtained “fingerprints” of these transient states could not be revealed in previous experiments.

Apart from these fundamental questions, this study will provide important impetus for future measurements with free-electron lasers. “Our experiments show that the cluster shell provides the core with electrons for a short time, thus delaying the Coulomb explosion,” said Laarmann. In imaging experiments at FELs, this is valuable time to obtain scattering images of molecules before destruction. In further experiments, the scientists will investigate in more detail the interdependence between delay and size of the cluster.

PRL Publication

By analysing the fluorescence light (right) the scientists can deduce exacly what happens when and where in the cluster.