Wetting, surface melting, and freezing of thin films of methane adsorbed on MgO(100)

Neutron-diffraction experiments are reported on thin films of methane (10 layers) adsorbed on magnesium oxide (100) for temperatures ranging from below to above the melting point: 50≤T≤95 K. A continuous wetting transition is observed simultaneously with an increase of the thickness of the disordered layer when the melting point is approached from below. The adsorbed film is stratified (disordered layer on top of ordered layer), and the crystallinity profile is measured at the ordered-disordered phase interface. Above the triple point a few layers remain ordered. The results are discussed and compared with the previous works on surface melting performed by quasielastic neutron scattering