for a transmission spectrum. Further more, the shape of the band provides qualitative information concerning the spatial orientation of the vibrational mode. For modes that are oriented perpen dicular to the surface, the band shapes are similar to those expected for a con ventional transmission spectrum. In contrast, those modes with an orienta tion parallel to the surface exhibit a shape that resembles the dispersion of ni. Both band shapes are evident in Figure 8. The absorbance-like band at 1435 cm -1 is the vs(COO~); the transi tion dipole of this mode has an average orientation along the surface normal, coupling strongly with the Ez compo nent of the incident electric field. As such, the band shape is governed pri marily by k2. In contrast, the deriva tive-like band centered at 1538 cm -1 represents v„(COO-). This mode has its transition dipole parallel to the sur face and, therefore, is governed strong ly by the dispersion of n-i. Several ef forts are currently underway to define further the interpretation of such spec tra (7-9).
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Prospectus
We have described a few of the ap proaches that may be employed to probe the molecular details of chemi cally modified surfaces with IR-ERS. Inherent to these approaches is the need to consider the differences be tween ERS and transmission measure ments before relating any band shape or intensity differences to structure or bonding changes of the interfacial structure. We have also discussed ef forts to extend such characterization from surfaces with high IR reflectivity to the more complex analysis of materi als with low IR reflectivity, such as glassy carbon. With continued im provements in the performance of FTIR spectrometers, the characterization of structures at submonolayer cover ages at such materials will become pos sible. These advances promise to open a new dimension in probing the compo sition, spatial arrangement, and dy namics of heterogeneous chemical pro cesses, enhancing our ability to design interfaces with new and improved per formance characteristics. The author expresses his appreciation to B. Theis and David Allara for many enlightening discus sions regarding various aspects of IR-ERS. The support of a Dow Corning Assistant Professorship and a Society of Analytical Chemists of Pittsburgh Starter Grant Award also are acknowledged. Ames Laboratory is operated for the U.S. Department of Energy by Iowa State University under Contract No. W-7405-Eng-82. Portions of this work were supported by the Office of Basic Energy Sciences, Chemical Sciences Division.
References (1) Eischens, R. P.; Pliskin, W. A. Adv. Catal. 1958,10,1-56. (2) Allara, D. In Industrial Applications of
Chemistry
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