  Mini-Neptunes and volatile-poor super-Earths coexist on adjacent orbits in
proximity to host stars such as Kepler-36 and Kepler-11. Several post-formation
processes have been proposed for explaining the origin of the compositional
diversity: the mass loss via stellar XUV irradiation, degassing of accreted
material, and in-situ accumulation of the disk gas. Close-in planets are also
likely to experience giant impacts during the advanced stage of planet
formation. This study examines the possibility of transforming volatile-rich
super-Earths / mini-Neptunes into volatile-depleted super-Earths through giant
impacts. We present the results of three-dimensional giant impact simulations
in the accretionary and disruptive regimes. Target planets are modeled with a
three-layered structure composed of an iron core, silicate mantle and
hydrogen/helium envelope. In the disruptive case, the giant impact can remove
most of the H/He atmosphere immediately and homogenize the refractory material
in the planetary interior. In the accretionary case, the planet can retain more
than half of the gaseous envelope, while a compositional gradient suppresses
efficient heat transfer as its interior undergoes double-diffusive convection.
After the giant impact, a hot and inflated planet cools and contracts slowly.
The extended atmosphere enhances the mass loss via both a Parker wind induced
by thermal pressure and hydrodynamic escape driven by the stellar XUV
irradiation. As a result, the entire gaseous envelope is expected to be lost
due to the combination of those processes in both cases. We propose that
Kepler-36b may have been significantly devolatilized by giant impacts, while a
substantial fraction of Kepler-36c's atmosphere may remain intact. Furthermore,
the stochastic nature of giant impacts may account for the large dispersion in
the mass--radius relationship of close-in super-Earths and mini-Neptunes.
