The understanding of complex many-body dynamics in laser-driven polyatomic molecules is crucial for any attempt to steer chemical reactions by means of intense light fields. Ultrashort and intense X-ray pulses from accelerator-based free electron lasers (FELs) now open the door to directly watch the strong reshaping of molecules by laser fields.
A prototype molecule, the famous football-shaped "Buckminsterfullerene" C60 was studied both experimentally and theoretically by physicists from two Max Planck Institutes, the one for Nuclear Physics (MPIK) in Heidelberg and the one for the Physics of Complex Systems (MPI-PKS) in Dresden in collaboration with groups from the Max Born Institute (MBI) in Berlin and other institutions from Switzerland, USA and Japan. For the first time, the experiment carried out at the Linac Coherent Light Source (LCLS) of the SLAC National Accelerator Laboratory could image strong-laser-driven molecular dynamics in C60 directly.
Analysing the X-ray diffraction pattern of the time-dependent response of the molecule to a strong infrared (IR) laser pulse, two parameters can be extracted: The (average) radius R of the molecule and the so-called Guinier amplitude A. The latter is a measure for the strength of the X-ray scattering signal. It is proportional to N2, the squared (effective) number of atoms in the molecule, which act as scattering centres. Whereas R is directly related to expansion or deformation of the molecule and its fragments, A contains information about the fragmentation mode, in particular the size distribution of fragments.
Figure 2 shows the results from a "low" (1×1014 W/cm2) through an "intermediate" (2×1014 W/cm2) to a "high" (8×1014 W/cm2) laser-intensity regime. The parameters R and A are given relative to the values for negative delays where the X-ray pulse arrives before the IR pulse imaging an intact C60. The time evolution of the molecule (expansion, deformation, fragmentation) from model calculations performed at the MPI-PKS can be seen in the movies for the different intensities. Here, electrons freed from the molecule and driven by the laser field are depicted as small blue balls. Exemplary movie stills are shown in the upper part of Figure 2.
At low intensities, the molecule expands before some fragmentation sets in, indicated by the delayed and modest decrease of the Guinier amplitude. At intermediate intensities the expansion is followed by a decrease in the X-ray imaged radius. This signature of scattering off small fragments is in accordance with the slightly delayed drop in the Guinier amplitude indicating that a large fraction of molecules has already broken up.
At the highest intensity, fast expansion and simultaneously decreasing Guinier amplitude already set in at the leading edge of the strong laser pulse, removing almost all outer valence (binding) electrons. This consequence of the violent "kick" by the laser is also reproduced by the model calculations.
However, at low and intermediate intensities there is only some qualitative agreement with the experiment. In particular, the model predicts an oscillatory behaviour both in the radius and the amplitude caused by a periodic "breathing" of the molecule (see movies), which is completely absent in the observed data. Implementing an additional ultrafast heating mechanism acting on the atomic positions in the molecule led a better agreement with the experiment, showing that more work, experimentally as well as theoretically, is necessary to better understand, and finally steer, intense-laser interactions with matter.
Multi-electron dynamics driven by intense laser fields still poses a challenge for the theoretical description as a full quantum mechanical treatment is currently out of reach. Thus, X-ray movies of structural dynamics as this one in C60 are an ideal testbed for the understanding of fundamental quantum processes in molecular systems of increasing size and complexity, illuminating our path towards the control of chemical reactions with laser fields.
