Two-dimensional melting transitions of rod-like molecules analyzed by reflection-absorption infrared spectroscopy
- Univ. of Illinois, Urbana, IL (United States)
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. Specifically there are two RAIR-allowed methylene v{sub 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. 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 {yields} 1D transition temperatures are 187 K for n-hexane, 212 K for n-octane, and 225 K for n-decane. 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 defined largely by the degree of site homogeneity and thus are acutely sensitive to the conformational, rotational, and translational order of the adsorbate overlayer. 55 refs., 8 figs., 4 tabs.
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- FG02-91ER45439
- OSTI ID:
- 136098
- Journal Information:
- Journal of Physical Chemistry, Vol. 99, Issue 41; Other Information: PBD: 12 Oct 1995
- Country of Publication:
- United States
- Language:
- English
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