Substrate Morphology's Influence on the Overlayer Structure and Oscillator Strength

I used infrared spectroscopy (IRS) and electron energy loss spectroscopy (EELS) to probe the influence of substrate morphology on the interaction strength among coadsorbates and hence on the overlayer structure and on the oscillator strength. We compared the coadsorption behavior of H and CO on both steps and terraces of the Pt(335) surface, and compared our results with previous studies on similar surfaces. We also compared the cross section and Stark tuning rate of edge and terrace CO. Our infrared spectroscopy study of coadsorption of H and edge CO on Pt(335) show that along the step edges of the Pt(335) surface, coadsorbed H and edge CO actually mixed together. In contrast, on Pt(111) and Pt(112) surfaces, coadsorbed CO and H segregate into islands. We proposed an overlayer structure model to explain our data, in which adsorbed H continuously shifts CO from atop to bridge binding. The different results on Pt(112) and Pt(335) mean that the interaction strength among the coadsorbates changes with the terrace width. With EELS, we directly verified the proposed CO site shift. We also surprisingly found that coadsorbed H produced no observable effect on the HREEL spectra of terrace CO. With IRS and EELS, we found that edge atop CO has twice the cross section of terrace atop CO, and that edge atop CO's Stark tuning rate is also twice that of terrace atop CO. We explained these and several previous results with an electrostatic model. This model also partly accounts for the much smaller difference found on surfaces with much wider terraces. Our data show that the screening of IR and static fields is different, whether by changing coadsorbate coverage or by changing the substrate sites. This is not explained by the standard dipole-dipole coupling model. We also found that the Stark tuning rate measured in electrochemical cells is 3 times larger than our data if standard models of electrochemical double layers are used. Coadsorption of H also produces different effects in the two environments. These results require much better understanding of how the adsorbate responds to the applied fields.