Time-of-flight measurements of the low-energy scattering of CH₄ from Ir(111)

We have measured high-resolution time-of-flight (TOF) spectra of methane scattered from an Ir(111) surface at an incident energy of 81 meV. The angular distributions of scattered CH4 reveal the presence of a sharp and intense specular peak in addition to sharp features corresponding to rotationally inelastic diffraction (RID) peaks along the two main symmetry directions of Ir(111). TOF spectra have been recorded at several RID positions for the two high-symmetry directions. The data show that the scattering dynamics of CH4 is more complex than the one reported for H-2/D-2, where energy losses in TOF correspond to the expected excitation/deexcitation RID energy transitions. For CH4, this is the case only for RID peaks showing up far from the specular peak, whereas those appearing close to the specular position present different behaviors, depending on the incident direction. The results are compared with Ne scattering TOF data, which allows to assess the relevance of multiphonon scattering in the energy-exchange process. Finally, we report experimental evidence of selective adsorption resonances detected with CH4 beams. This will allow characterizing the CH4-metal surface physisorption well by measuring angular distributions with CH4 beams.