INSTRUMENTATION both modify the magnitude and phase of an existing surface tensor element and create new ones. If the adsorbate reduces the average surface symmetry, additional nonzero elements of χ will appear in the rotational anisotropy. For example, iodine is known to adsorb onto the P t ( l l l ) surface i n a / / X -ft overlayer structure (23) shown in Fig ure 8b. The iodine atoms form a Cz„ monolayer with a symmetry axis that is rotated by 19.1° from the substrate crystal symmetry axis. This reduces the combined symmetry of the sub strate and monolayer from C3„ to C3, and leads to the possibility of nonzero Xxyz and Xyyy tensor elements for the surface. Figure 9a shows the IPyS(2w) SHG sig nal at 302 nm for the P t ( l l l ) - i o d i n e monolayer system that was obtained with p-polarized input light and s-polarized output light (as compared with the /sp(2a>) data shown in Figure 8, which used s-polarized input light and p-polarized output light). This SHG anisotropy can be interpreted in terms of the existence o f a nonzero xxyz ele ment. The / J X J 7 iodine monolayer reduces the surface symmetry and leads to the threefold pattern shown in Figure 9a. If only the χχχχ element con tributed to the rotational anisotropy, the SHG signal would be a sixfold sym metric pattern. Upon removal of the iodine by CO displacement and subse quent oxidation, the sixfold pattern ex pected from a Czv substrate is observed (Figure 9b). Figure 9c shows the IPiS(2w) anisotro py observed for the P t ( l l l ) surface in the potential region of hydrogen ad sorption; the pattern indicates that the adsorbed hydrogen also alters the six fold symmetry of the SHG response. Note that in this particular choice of input and output polarizations, no iso tropic contributions to the SHG signal occur; only terms indicating ordered adsorption contribute to the surface second harmonic response. A final caveat that must be men tioned is that in all of the anisotropy experiments the SHG signal measures the average surface symmetry (much like the average molecular orientation). In other words, a surface that has equal amounts of two possible C3 domains would have an average surface symme try of C3„, not C3. In the experiments on platinum single crystal electrodes an unequal domain distribution for the overlayers permits the determination of the average surface symmetry from the SHG measurements (15). SHG an isotropy measurements are sensitive not only to surface symmetry changes upon chemisorption, but also to sym metry changes resulting from surface
reconstruction. In addition to plati num, SHG rotational anisotropy mea surements currently are being made on silver and gold single crystal electrodes (9,10,16,17). Future extensions
As laser technology continues to im prove, the quality and number of SHG experiments at electrode surfaces will undoubtedly increase and branch out in several directions. For resonant mo lecular SHG studies, experiments at transparent electrodes instead of metal surfaces should lead to orientation mea surements from a wider variety of mol-
I î
Relative intensity
Figure 9. Changes in the SHG rotational symmetry plots for a Pt(111) electrode upon chemisorption. Polar plots of the /ρ5(2ω) (p-polarized input, s-polarized output) SHG signal for a flame-annealed Pt(111) electrode (a) with an adsorbed monolayer of atomic iodine in 0.1 M HCI04 at an electrode potential of +0.200 V, (b) at an electrode poten tial of +0.200 V after the removal of the iodine monolayer, and (c) at a potential of —0.175 V in the hydrogen adsorption region. A surface with C3„ symmetry will yield a symmetric sixfold pat tern; the presence of a monolayer of iodine or hy drogen reduces the combined surface symmetry and results in a threefold symmetry for the SHG signal. The polar plots are labeled with the rela tive intensities of the SHG signal from the three surfaces along the χ and y crystal axes.
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