Secondly, the two vertical motion frequencies decrease with an increasing coverage, while horizontal motion frequencies increases. According to the predictions with vibrational scaling relations developed by Lansford et al. , the vertical motion vibrational frequency of adsorbed CH X (X=0-3) on atop sites negatively scales with the adsorption energy (ω vertical 2 ∝ -∆E ads ). Contrarily, the horizontal motion vibrational frequency was reported to be relatively constant with adsorption energy. Our result is in agreement with the literature: as the adsorption energy increases (i.e. weaker binding) with the increasing coverage, the frequency of vertical motion decreases. However, the frequency of horizontal motion remains unchanged-regardless of the increased adsorption energy-until the lateral repulsion becomes significant.