Molecular beam scattering of linear and branched butane on graphite (HOPG)

The adsorption of n-butane and iso-butane on HOPG (highly-oriented pyrolitic graphite) has been studied by molecular beam scattering. The initial adsorption probability, S0, decreases with impact energy, Ei, and is independent of surface temperature, Ts, i.e., molecular adsorption is present. The adsorption probability, S, increases with coverage, Θ, which is most distinct at large Ei and low Ts. Thus, precursor mediated adsorption is concluded. Whereas S0 of the linear alkane is larger than the one of the branched alkane, consistent with their molecular structure, the shapes of S(Θ) curves are approximately identical. The rotational excitation of the molecules appears to affect S0 for n-butane but not for iso-butane. Monte Carlo simulations (MCS) have been conducted to extract dynamics parameters from the S(Θ) curves.