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tokens: in the formation of van der Waals complexes. The heat of evaporation of a single He atom is about 5 cm −1 (Toennies Vilesov 2004 ). Assuming an extra 7% depletion when Ar gas is replaced with dihydrogen, we can estimate the lower limit of the energy released in the droplets to be about 4200 cm −1 . The real reaction energy is expected to be much higher because not all helium droplets contain both reactants. Additionally, an ejection of the hot reaction products from the helium droplets, as well as a dissipation of the reaction energy by photon emission, are expected (Krasnokutski Huisken 2010b ). This estimate is only consistent with the reaction leading to the formation of the HCH molecule becuase the formation of the CH 2 molecule would lead to the release of only a small amount of energy ∼600 cm −1 (Harding et al. 1993 ). The high rate of this reaction found at ultra-low temperature, assumes no energy barrier in the reaction pathway. Therefore, our experiment is in line with the results of quantum chemical computations that predict the barrierless reaction C + H 2 → HCH (Harding et al. 1993 ). In the case of doping of helium droplets by more than a single dihydrogen molecule, the formed HCH molecule can also further react with a H 2 molecule leading to the formation of methane. However, the quantum chemical computations predict the presence of a barrier for this reaction (Ge et al. 2010 ; Lu et al. 2010 ). Therefore, we expect that this reaction should not proceed inside the helium droplets and that the reactions of carbon atoms are terminated after the formation of HCH molecules. 3.2. Mass Spectrometry In the next step, we applied mass spectrometry for the characterization of products of chemical reactions. In this experiment, larger helium droplets (up to 10 5 He atoms) were used. The droplets were first doped with multiple dihydrogen molecules and, in the second pick-up chamber, one or a few carbon atoms were added. We used two different conditions for the doping with dihydrogen. The helium droplets picked up about 10 and 100 dihydrogen molecules on average at low and high doping conditions, correspondingly. Although, even at the high doping conditions, the C/H ratio is much higher than that present in the ISM, it is low enough to saturate the chemistry with hydrogen (i.e., more than five hydrogen atoms per carbon atom). The doped helium droplets were ionized by electron impact in the head of the TOF mass spectrometer. The collision of a doped helium droplet with an electron, having energy larger than 25 eV, results in the formation of He + . This positive hole migrates within the droplet until it becomes localized either at a dopant or at a helium dimer to form (Scheidemann et al. 1993 ). Therefore, the cations of dopants and helium clusters as well as their complexes are produced. Figure 2 shows the mass spectra of helium droplets doped with dihydrogen alone and together with carbon atoms. Both mass spectra were recorded one after