The aim of the present project is to study fundamental chemical reactions of astrophysical interest in helium droplets. Although the temperature is lower (T = 0.37 K) than the ones encountered in the Interstellar Medium and in dense molecular clouds (T = 10 – 50 K), the expected data will provide extremely useful information. We will learn whether the reaction occurs at all at the low temperature and, if it does, the rate constant for 10 – 50 K may be estimated by interpolation of the new ultralow-temperature data obtained for the first time and the available higher-temperature data. On the other hand, the helium droplets provide an extraordinary opportunity to study binary reactions under very well defined conditions where translational, rotational, and vibrational degrees of freedom are in equilibrium. Most notable is the fact that the reaction occurs between two individual molecules, atoms, or radicals and that the time when the two reactants start to interact is very well defined. Up to now, helium droplet experiments were mostly applied to the spectroscopic characterization of either the helium cluster itself or the incorporated molecule. Chemical reactions in helium droplets are only very scarcely studied and they are just beginning to attract increasing interest. As a result, the techniques to prepare the reactant species (molecules, atoms, or radicals), to incorporate them into the helium droplets, to detect the products and to specify them, and to obtain detailed information on branching ratios, reaction rates, and the enthalpy of formation (ΔHf) are still under development.

This is also true for the present project. On the production side, it has to be explored how the radicals of interest (H, OH, CH, etc.) are most conveniently produced (thermal dissociation, dc or rf discharge, or laser photolysis) and embedded into the helium droplets without destroying them. Refractory materials must be sublimated at rather high temperature when astrophysically interesting atoms (C, Si, Fe, etc.) are chosen as reaction partners. Also on the detection side, some new development seems necessary. Thus, for example, mass spectrometric detection of reaction products with proper discrimination against ion-molecule reactions possibly occurring in the ionizer must be optimized under the given conditions. Lasers of various wavelengths (uv, vis, and ir) should be employed to resonantly excite the products and to observe their fluorescence or the energy transfer to the helium droplet (depletion spectroscopy). Finally, dynamic and kinetic experiments will be carried out to determine reaction rates and enthalpies of formation. Both methods have to be explored and eventually established.

Various reactions of astrophysical interest will be chosen. It is foreseen that, for each of these reactions, somewhat different methods on the production and/or detection side must be developed. Therefore, the project will present a mix of new development and investigation. The expected data will be used as input parameters for astrochemical reaction network calculations. Aside from this more practical application, the proposed experiments will also address a more fundamental issue and provide important information on the role and effectiveness of quantum tunneling at 0.37 K.

Another issue which will be addressed is the formation of interstellar grain analogues. For this purpose, we will investigate the formation of carbon and silicon clusters by the sequential pick-up of individual atoms. Such experiments may provide a basis for interesting subsequent studies, as for example the coverage of carbon clusters by water, methanol, and ammonia molecules as ice analogues.


Schematic view of the setup

Fig. 1. Schematic view of the setup used for the study of reactions between R1 and R2 in helium droplets employing a mass spectrometer for product detection.


Several reactions involving atoms (Mg, Si, Al, Fe, and C) and molecules (O2, benzene C6H6, SiO, H2, H2O, CO, CO2, C2H2, fullerene C60, and polycyclic aromatic hydrocarbons) have been studied and reported [1-12].


[1] S. A. Krasnokutski and F. Huisken: Ultra-low-temperature reactions of Mg atoms with O2 molecules in helium droplets, J. Phys. Chem. A 114, 7292-7300 (2010). [DOI]
[2] S. A. Krasnokutski and F. Huisken: Oxidative reactions of silicon atoms and clusters at ultralow temperature in helium droplets, J. Phys. Chem. A 114, 13045-13049 (2010). [DOI]
[3] S. A. Krasnokutski and F. Huisken: Low-temperature chemistry in helium droplets: Reactions of aluminum atoms with O2 and H2O, J. Phys. Chem. A 115, 7120–7126 (2011). [DOI]
[4] S. A. Krasnokutski, G. Rouillé, C. Jäger, F. Huisken, S. Zhukovska, and Th. Henning: Formation of silicon oxide grains at low temperature, Astrophys. J. 782, 15/1-15/10 (2014). [DOI][]
[5] S. A. Krasnokutski and F. Huisken: Reactivity of iron atoms at low temperature, J. Phys. Chem. A 118, 2612-2617 (2014). [DOI]
[6] S. A. Krasnokutski and F. Huisken: Ultra-low-temperature reactions of C(3P0) atoms with benzene molecules in helium droplets, J. Chem. Phys. 141, 214306/1-214306/5 (2014). [DOI]
[7] S. A. Krasnokutski, M. Kuhn, M. Renzler, C. Jäger, Th. Henning, and P. Scheier: Ultra-low-temperature reactions of carbon atoms with hydrogen molecules, Astrophys. J. Lett. 818, L31/1-L31/5 (2016). [DOI][]
[8] S. A. Krasnokutski, M. Kuhn, A. Kaiser, A. Mauracher, M. Renzler, D. K. Bohme, and P. Scheier: Building carbon bridges on and between fullerenes in helium nanodroplets, J. Phys. Chem. Lett. 7, 1440-1445 (2016). [DOI]
[9] S. A. Krasnokutski, F. Huisken, C. Jäger, and Th. Henning: Growth and destruction of PAH molecules in reactions with carbon atoms, Astrophys. J. 836, 32/1-32/7 (2017). [DOI]
[10] S. A. Krasnokutski, M. Goulart, E. B. Gordon, A. Ritsch, C. Jäger, M. Rastogi, W. Salvenmoser, Th. Henning, and P. Scheier: Low-temperature condensation of carbon, Astrophys. J. 847, 89/1-89/7 (2017). [DOI]
[11] T. K. Henning and S. A. Krasnokutski: Experimental characterization of the energetics of low-temperature surface reactions, Nat. Astron. 3, 568-573 (2019). [DOI]
[12] S. A. Krasnokutski, O. Tkachenko, C. Jäger, and T. Henning: Formation of a long-lived cyclic isomer of ethylenedione, Phys. Chem. Chem. Phys. 21, 12986-12990 (2019). [DOI]


For more information contact Dr. Sergiy Krasnokutskiy.

Related funding(s): DFG HU 474/22-1, DFG HE 1935/26-1, DFG HU 474/22-3, DFG JA 2107/4-1, DFG KR 3995/3-1.

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