Two-dimensional melting transitions of rod-like molecules analyzed by reflection-absorption infrared spectroscopy

The melting of ordered monolayers of n-hexane, n-octane, and n-decane adsorbed on Pt(111) has been studied by reflection-absorption infrared spectroscopy (RAIRS). Each alkane forms an overlayer at low temperatures (<160 K) that is characterized by both quasi-long-range translational and orientational order. Previous low-energy electron diffraction (LEED) studies suggested that the alkanes are arranged on the surface in the all-trans conformation, and this has been confirmed by RAIRS. The low-temperature ordered state is distinguished by the presence of a sharp, resolved set of soft modes near 2760 cm^-1. The perturbation of the frequencies of some of the C-H oscillators due to the presence of M⋯H-C interactions between the adsorbed n-alkane molecules and the surface has a dramatic effect on the RAIR spectra. Specifically, there are two RAIR-allowed methylene v_C-H stretching modes: one which largely involves motion of the methylene C-H bond that projects away from the surface (the distal C-H bond) and another that largely involves motion of the near-surface (i.e., proximal) C-H bond. We have assigned these modes to the bands at ∼2909 and ∼2760 cm^-1, respectively. The assignments for the "methyl" modes seen at ∼2947, 2929, and 2810 cm^-1 follow in this same spirit: one C-H bond of the methyl groups interacts with the surface, and the resulting shift in the frequency of this oscillator to 2810 cm^-1 serves to decouple it from the vibrations of the other two. At higher temperatures, a transition occurs to a one-dimensionally ordered (possibly "hexatic") phase in which translational order has been lost along the long axis of the molecules but orientational ordering is maintained; the approximate 2D → 1D transition temperatures are 187 K for n-hexane, 212 K for n-octane, and 225 K for n-decane. Upon transforming to this hexatic phase, the C-H stretching bands (especially the soft modes) broaden substantially and shift in frequency; these spectroscopic changes reflect the inhomogeneity generated upon translation of the chains relative to their positions in the ordered 2D phase. At higher temperatures (210 K for hexane, 240 K for octane, and 270 K for decane), the RAIR spectra suggest that the order-disorder transition noted by LEED yields a population of alkane molecules in which trans segment conformations predominate, but a small population of gauche kink defects is present as well. As expected for true phase transitions, all changes noted by RAIRS are reversed when the samples are cooled. The conformational dynamics seen on a Pt(111) surface are compared with those that occur in the premelting and melting transitions of bulk n-alkane crystals. The present data also answer a long standing question about the mechanism of mode softening: the line widths of soft modes are determined largely by the degree of site homogeneity and thus are acutely sensitive to the conformational, rotational, and translational order of the adsorbate overlayer.