On the trail of the mystery of organic matter in space

For several years, there has been strong evidence that huge amounts of complex organic compounds have already formed in the interstellar clouds in the early days of the universe. This is suggested by about 400 diffuse absorption bands (DIBs) that astronomers were able to detect in the light from such clouds. However, the exact assignment of DIBs to concrete compounds is hardly possible so far. The fact that it could actually be the suspected polycyclic aromatic hydrocarbons (PAHs / PAHs) is now supported by new experiments carried out at the Max Born Institute (MBI) in Berlin together with international partners. The results were published in Nature Communications.

With the help of ultrafast UV lasers, the scientists were able to decode the dynamics of highly excited molecular states. Among the hydrocarbons, which are likely to be the carriers of the absorption bands, the polycyclic aromatic hydrocarbons were particularly promising. The presence of PAH / PAH molecules has previously been deduced in many astronomical objects, such as the interstellar matter clouds of our Milky Way galaxy, but even in ten-billion-year-old matter from the early days of the universe. However, astronomers also had doubts about the hypotheses because the lifetime of the unusual molecular states was unknown. The MBI researchers in collaboration with scientists from the University of Lyon, supported by theoretical calculations by scientists at the Universities of Leiden, Heidelberg and Hyderabad, have now shown that the lifetime of the electronic states of small to medium-sized PAHs coincides with the line widths be observed in the diffuse absorption bands.

In the experiments, a series of small to medium-sized PAH molecules (naphthalene, anthracene, pyrene, and tetracene, each containing several condensed aromatic rings) were ionized with an ultra-short ultra-violet laser pulse (XUV). The absorption of an XUV photon not only led to the removal of one of the electrons, but also to the electronic excitation of the resulting positively charged molecule ion. The lifetime of these excited cationic electronic states was measured by means of a time-delayed infrared laser pulse.

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.

Original publication

XUV excitation followed by ultrafast non-adiabatic relaxation in PAH molecules as a femto-astrochemistry experiment

A. Marciniak, V. Despré, T. Barillot, A. Rouzée, M. C. E. Galbraith, J. Klei, C-H. Yang, C. T. L. Smeenk, V. Loriot, Nagaprasad Reddy, S. , A. G. G. M. Tielens, S. Mahapatra, A. I. Kuleff, M. J. J. Vrakking, F. Lépine

Nature Communications 6 (2015) 7909-7915

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