In Coulomb Explosion Imaging as many binding electrons as possible are stripped very rapidly from a molecule, before nuclear motion becomes appreciable. Few-cycle infrared laser pulses of very high intensity (>1015 W/cm2) or extremely short XUV or X-ray pulses are suited tools for this purpose. The highly-charged molecular ion will then “explode” on a repulsive, ideally purely Coulombic, potential. When the momenta of all fragments are mapped in full coincidence, the three-dimensional nuclear configuration at the time of the probe can be retrieved. In this way, molecular structure can be imaged one molecule at a time, while no a priori spectroscopic knowledge or assumptions about the molecule are required.
Figure 1: Dalitz plot of the fragments I+, Br+, and CH2+ from explosion of triply charged CH2IBr molecules. “I” marks the direct Coulomb explosion channel, while “II-V” mark different sequential breakup mechanisms of the molecule. For structural imaging, it is important to distinguish the different pathways.
Thus far, laser-driven Coulomb Explosion Imaging has mostly been applied to determine the static structure of diatomic and triatomic molecules. Accurate retrieval of evolving molecular structure requires very short, intense laser pulses at high repetition rates, to enable efficient coincidence momentum spectroscopy and the suppression of unwanted sequential channels. Here we combine very intense, ultrashort laser pulses at high repetition rate with coincidence momentum imaging to investigate evolving molecular structure and chemical dynamics with time-resolved Coulomb Explosion Imaging, employing unusual molecular targets. Such a probe gives direct insight into reaction mechanisms from a structural point of view.
Figure 2: Kinetic energy release of NO2+ fragments from strong-field ionization of N2O4 molecules, as a function of delay between a pump pulse, which excites a vibrational wavepacket, and an intense laser field. The molecular vibration can be accurately characterized from such a measurement.