  Ultraviolet (UV) processing in the insterstellar medium (ISM) induces the
dehydrogenation of hydrocarbons. Aliphatics, including alkanes, are present in
different interstellar environments, being prevalently formed in evolved stars;
thus, the dehydrogenation by UV photoprocessing of alkanes plays an important
role in the chemistry of the ISM, leading to the formation of unsaturated
hydrocarbons and eventually to aromatics, the latter ubiquitously detected in
the ISM. Here, through combined experimental results and \textit{ab-initio}
calculations, we show that UV absorption (mainly at the Ly-$\alpha$ emission
line of hydrogen at 121.6 nm) promotes an alkane to an excited Rydberg state
from where it evolves towards fragmentation inducing the formation of olefinic
C=C bonds, which are necessary precursors of aromatic hydrocarbons. We show
that photochemistry of aliphatics in the ISM does not primarily produce direct
hydrogen elimination but preferential C-C photocleavage. Our results provide an
efficient synthetic route for the formation of unsaturated aliphatics,
including propene and dienes, and suggest that aromatics could be formed in
dark clouds by a bottom-up mechanism involving molecular fragments produced by
UV photoprocessing of aliphatics.
