Fig. Schematic representation of the experiment.
(a) Schematic representation of XUV-induced dynamics in PAH molecules. The excited states show up in the valence shell of the cation in one of two ways: the formation of a single-hole configuration or the formation of a two-hole single-particle configuration that occurs with increasing energies (left). IP stands for the ionization potential. The cation can be ionized by the IR test laser, provided that the non-adiabatic relaxation has not yet occurred (center). After relaxation it is no longer possible to ionize the cation with the IR test laser (right).
(b) Two-color XUV IR ion signals measured on anthracene as a function of the detected mass-to-charge ratio and the XUV-IR delay. The XUV only and the IR signals were subtracted. The XUV pump and IR test laser pulses overlap at zero delay (black dashed line). A red color corresponds to a rise in signal, while a blue color indicates fading. For positive XUV IR delays, very fast dynamics were observed for doubly charged anthracene ions (A²⁺, m / q = 89). As explained in the text, the measurement reflects a non-adiabatic relaxation in the anthracene cation (A⁺). The dynamics observed in the first fragment (A-C₂H₂⁺) are not discussed in this article.

Once an electron has been removed from the molecule, the electronic excitation is highest so that only one or a few infrared photons are needed to remove a second electron. A short time later, the ion "relaxes", it now requires more IR photons to knock out a second electron. In other words, the monitoring of the formation of doubly charged ions as a function of the delay time between the laser pulses XUV and IR allows the measurement of the lifetime of the various states. The measurements, which were supported by theoretical high-level calculations, showed that the lifetime of the organic PAH ions in the range of a few 10 femtoseconds is consistent with that, which is also measured in the diffuse absorption bands (DIBs) from space ,

The experiments have implications for the further development of attosecond physics. For in chemistry, precise knowledge of the charge migration is of great interest, i. ultrafast movements of an electron or a hole through a molecular structure. They occur in the unimaginably short time of attoseconds (one billionth of a billionth of a second) to a few femtoseconds (10-15 seconds). Controlled charge migration could give rise to completely new possibilities for controlling chemical reactions, an objective that is as old as the chemical research itself. Initial indications that charge migration can be controlled on a time scale ranging from attoseconds to a few femtoseconds have been identified by researchers University of Milan last year.

The PAH / PAH molecules studied in the experiments at MBI are the largest to date for which ultra-fast XUV-IR pump-probe spectroscopy has been applied. Further experiments are in preparation.