2-DIMENSIONAL MELTING TRANSITIONS OF ROD-LIKE MOLECULES ANALYZED BY REFLECTION-ABSOSPTION 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 temperatu res (<160 K) that is characterized by both quasi-long-range translatio nal 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. T he 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, t here are two RP;IR-allowed methylene upsilon(C-H) stretching modes: on e which largely involves motion of the methylene C-H bond that project s 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 hav e assigned these modes to the bands at similar to 2909 and similar to 2760 cm(-1), respectively. The assignments for the ''methyl'' modes se en at similar to 2947, 2929, and 2810 cm(-1) follow in this same spiri t: one C-H bond of the methyl groups interacts with the surface, and t he 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 (poss ibly ''hexatic'') phase in which translational order has been lost alo ng the long axis of the molecules but orientational ordering is mainta ined; the approximate 2D --> 1D transition temperatures are 187 K for n-hexane, 212 K for n-octane, and 225 K for n-decane. Upon transformin g to this hexatic phase, the C-H stretching bands (especially the soft modes) broaden substantially and shift in frequency; these spectrosco pic changes reflect the inhomogeneity generated upon translation of th e chains relative to their positions in the ordered 2D phase. At highe r temperatures (210 K for hexane, 240 K for octane, and 270 K for deca ne), the RAIR Spectra suggest that the order-disorder transition noted by LEED yields a population of alkane molecules in which trans segmen t conformations predominate, but a small population of gauche kink def ects is present as well. As expected for true phase transitions, all c hanges noted by RAIRS are reversed when the samples are cooled. The co nformational dynamics seen on a Pt(lll) surface are compared with thos e that occur in the premelting and melting transitions of bulk n-alkan e crystals. The present data also answer a long standing question abou t the mechanism of mode softening: the line widths of soft modes are d etermined largely by the degree of site homogeneity and thus are acute ly sensitive to the conformational, rotational, and translational orde r of the adsorbate overlayer.