  At the low temperatures found in the interior of dense clouds and
circumstellar regions, along with H$_2$O and smaller amounts of species such as
CO, CO$_2$, or CH$_3$OH, the infrared features of CH$_4$ have been observed on
icy dust grains. Ultraviolet (UV) photons induce different processes in ice
mantles, affecting the molecular abundances detected in the gas-phase. This
work aims to understand the processes that occur in a pure CH$_4$ ice mantle
submitted to UV irradiation. We studied photon-induced processes for the
different photoproducts arising in the ice upon UV irradiation. Experiments
were carried out in ISAC, an ultra-high vacuum chamber equipped with a cryostat
and an F-type UV-lamp reproducing the secondary UV-field induced by cosmic rays
in dense clouds. Infrared spectroscopy and quadrupole mass spectrometry were
used to monitor the solid and gas-phase, respectively, during the formation,
irradiation, and warm-up of the ice. Direct photodesorption of pure CH$_4$ was
not observed. UV photons form CH$_x\cdot$ and H$\cdot$ radicals, leading to
photoproducts such as H$_2$, C$_2$H$_2$, C$_2$H$_6$, and C$_3$H$_8$. Evidence
for the photodesorption of C$_2$H$_2$ and photochemidesorption of C$_2$H$_6$
and C$_3$H$_8$ was found, the latter species is so far the largest molecule
found to photochemidesorb. $^{13}$CH$_4$ experiments were also carried out to
confirm the reliability of these results.
