  The reactions of carbon atoms with dihydrogen have been investigated in
liquid helium droplets at $T$ = 0.37 K. A calorimetric technique was applied to
monitor the energy released in the reaction. The barrierless reaction between a
single carbon atom and a single dihydrogen molecule was detected. Reactions
between dihydrogen clusters and carbon atoms have been studied by
high-resolution mass spectrometry. The formation of hydrocarbon cations of the
type C$_m$H$_n^+$, with $m$ = 1-4 and $n$ = 1-15 was observed. With enhanced
concentration of dihydrogen, the mass spectra demonstrated the main "magic"
peak assigned to the CH$_5^+$ cation. A simple formation pathway and the high
stability of this cation suggest its high abundance in the interstellar medium.
