When a small particle, consisting of around one hundred atoms, is exposed to intense laser pulses, an extremely hot plasma ball is formed. This ball expands explosively, emitting fast electrons and high-energy ions, as well as X-rays. A research team from Rostock succeeded in using laser experiments on silver particles with precisely selected atom numbers to actively influence the plasma development. For the first time, clear indications of the initial shell structure of the clusters became apparent in the motional energies of the ions.
In the 1895 film Démolition d’un mur by Louis Lumière, workers are shown demolishing a wall. The Lumière brothers projected the film in reverse temporal order, so that the stone wall seemed to reassemble itself.[1]
What can easily be accomplished in films is rarely observed in reality. Nature prefers the path of disorder. Reverse motion, as this trick is known in the film industry, is therefore rather unlikely for physical processes – making a corresponding experimental approach unlikely to succeed.
What can be done?
If the initial object is unknown and one is only looking at a pile of stones, the scientific challenge might be to deduce the shape and structure of the initial object based on the motion of the fragments.
The process described in the film, when transferred to the level of molecules and nanoparticles, would be called the reconstruction of a Coulomb explosion.
First, a group of atoms is stripped of a large number of electrons within an extremely short time. The remaining ions experience electrostatic repulsion from the other charges and rapidly fly apart. Due to their charge states and their initial positions within the atomic assembly, they follow characteristic trajectories.
By determining the ion velocities, it is possible to calculate backwards and thereby reconstruct the initial molecular structure. This method has become known as Coulomb explosion imaging. In the early stages, electrons were stripped off using ultrathin metal foils [2].
Nowadays, intense, ultrashort laser pulses are used to trigger the Coulomb explosion. For molecules consisting of only a few atoms, this method works well [3].
But what happens when one considers significantly larger particles with more than a hundred atoms – in scientific terms, these are referred to as clusters?
Compared to studies on small molecules, larger particles experience a so-called phase transition when extensive electron stripping occurs, which leads to the creation of a tiny plasma at the microscopic level. The amount of positive charge forms an energy cage that prevents most of the electrons from escaping. Within this cage, the electrons can move almost freely. Because of their small size, such objects are called nanoplasmas [4].
The first nanoplasmas were already produced in the 1990s, and since then, their properties and behavior have been studied extensively [5].
Nanoplasmas are extremely fragile, and their expansion proceeds in a much more complex manner compared to molecules. Can structural information still be extracted?
This has now been achieved in a pioneering experiment by the Tiggesbäumker/Meiwes-Broer research group at the University of Rostock [6], using shell-structured silver clusters as test objects.
A particular challenge lies in the fact that charged particles in the plasma cannot be considered in isolation: electrons shield the ions or constantly alter their charge states as the ions accelerate. Due to the high degree of ionization, the particles move at approximately 0.1% the speed of light, which represents a major challenge for the diagnostics used and the subsequent analysis.
A clever approach was necessary to obtain the desired structural information:
The storage of size-selected clusters in a special ion trap, the analysis of narrowly defined intensity ranges, charge-resolved measurements, and simulations of particle motion made it possible to clearly assign the velocity signatures to the geometric structure of the clusters. The researchers also took advantage of the fact that every plasma has a characteristic frequency that can be excited resonantly.
This resonance allows energy to be pumped massively into the system via the collective motion of the plasma electrons. By irradiating the system with two temporally delayed laser pulses, the nanoplasma was first formed and then resonantly excited a short time later. Many blurring effects caused by electron motion – which would otherwise mask the Coulomb explosion of the ions – were minimized by the increased energy input. This enabled the researchers to demonstrate that in Ag55− and Ag147−, the ions left their binding sites within the individual shells and reached characteristic energies, allowing conclusions about their initial positions. The new method is so sensitive that even structural rearrangements in the initial geometry can be resolved when the cluster size is changed by just one atom.
Meanwhile, the scientists have detected the shell structure in a cluster with more than 300 atoms based on the energy signatures.
What began in 1895 with Démolition d’un mur as a cinematic reversal is now becoming reality at the molecular level: the work of these researchers marks another step toward the goal of analyzing the dynamics of structural changes in reverse over time – and reconstructing the initial geometry.
References
[1] Wikipedia. The free encyclopedia. https://en.wikipedia.org/wiki/Dmolition_d%27un_mur. last visit: 2025-06-13.
[2] Z. Vager, R. Naaman, and E. P. Kanter. Coulomb explosion imaging of small molecules. Science, 244:426–431, 1989.
[3] M. Pitzer, M. Kunitski, A. S. Johnson, T. Jahnke, H. Sann, F. Sturm, L. Ph. H. Schmidt, H. Schmidt-Böcking, R. Dörner, J. Stohner, J. Kiedrowski, M. Reggelin, S. Marquardt, A. Schießer, R. Berger, and M. S. Schöffler. Direct Determination of Absolute Molecular Stereochemistry in Gas Phase by Coulomb Explosion Imaging. Science, 341:1096–1100, 2013.
[4] Th. Fennel, K.-H. Meiwes-Broer, J. Tiggesbäumker, P.-G. Reinhard, P. M. Dinh, and E. Suraud. Laser-driven nonlinear cluster dynamics. Rev. Mod. Phys., 82:1793, 2010.
[5] A. McPherson, B.D. Thompson, A.B. Borisov, K. Boyer, and C.K. Rhodes. Multiphoton-induced Xray emission at 4-5 keV from Xe atoms with multiple core vacancies. Nature (London), 370:631–633, 1994.
[6] K. Raspe, N. Iwe, L. Kazak, B. Krebs, F. Martinez, K.-H. Meiwes-Broer, and J. Tiggesbäumker. Ion recoil energy signatures of geometric shells from the coulomb explosion of size-selected silver clusters. J. Phys. Chem. Lett., pages 5952–5959, 2025.
Contact for scientific information:
Josef Tiggesbäumker
Tel.: +49 381 498 6805
email: josef.tiggesbaeumker@uni-rostock.de
Institut für Physik
Universität Rostock