X-ray Induced Coulomb Explosion Imaging of Complex Molecules

Recording images of individual molecules with ultrashort “exposure times” has been a long-standing dream in molecular physics, chemistry, and biology, because this would allow one to follow the motion of atoms on their inherent timescale. While X-ray and electron diffraction have been successfully used for larger molecules, both are very challenging to apply to small gas-phase molecules. Moreover, these techniques are relatively insensitive to hydrogen atoms, which are key to many chemical reactions.

We could recently demonstrate that snapshot images of the complete structure of a molecule with eleven atoms, including all hydrogens, can be recorded by Coulomb explosion imaging (CEI) when using very intense, femtosecond soft X-ray pulses from the European XFEL [1]. The intriguingly clear momentum images allow us to identify each atom’s position in the molecule unambiguously. While it was possible to record up to six-fold ion coincidences in the experiment, even three-fold ion coincidences can be sufficient to image the full structure of a molecule. The X-ray intensity is high enough to produce extreme charge states (e.g. up to 42+ in xenon atoms), and to Coulomb-explode molecules into individual atoms very quickly, such that the initial molecular structure is well preserved in the recorded momenta of all ions.

The sensitivity of CEI to the molecular structure at the instant of ionization allows studying processes such as intramolecular charge rearrangement [1], the influence of transient molecular resonances [2], and fragmentation dynamics [3]. The femtosecond pulse duration opens the door to monitoring the temporal evolution of the molecular structure [4] - one of the key future goals for this technique, towards which we have already taken the first steps.