the rheological properties of the solution as well as the shape of steady-state (and not measured transient) SANS profiles.Acknowledgment. We thank the Institute Laue-Langevin for providing the neutron beam facilities and W. Griessl for his assistance in computer work. This work has been supported by the Bundesministerium für Forschung und Technologie Grant No. 03-KA1BAY-0. Part of the support of S.H.C. came from the Alexander von Humboldt Stiftung and from the U.S. National Science Foundation.
Methylviologen adsorbed on a roughened silver electrode is reduced to its cation radical upon irradiation with laser light at liquid nitrogen temperature. Surface-enhanced Raman scattering (SERS) spectra were obtained with different excitation wavelengths between 406 and 752 nm and compared to those obtained at room temperature in an electrochemical cell under potential control. From two-color experiments, in which one laser frequency was used to generate the radical and a second to excite the SERS spectra, it was determined that radical formation occurs mainly with excitation in the blue spectral region. A comparison of the SERS spectra of the dication and cation radical forms of methylviologen with their solution spectra suggests that the former interacts more strongly with the surface than the latter. The cation radical appears to be stable for several hours in liquid nitrogen but has a short lifetime at room temperature. Two mechanisms for the photoreduction are discussed: plasmon-assisted electron transfer from the metal to the methylviologen dication and formation of a resonance charge transfer complex. The current experimental data are insufficient to determine the particular role of these mechanisms.
2163 Figure 7 shows the variation of PH, desorption peak area as a function of PH, exposure. Comparison of Figure 5 with Figure 7 shows that PH, desorption approaches saturation more slowly than the AES P/(Fe + 0) ratio. This suggests, not surprisingly, that PH, is sensitive to electron beam damage. While this was not investigated here, it is of interest for future work.Near saturation, PH, desorbs around 160 K from the oxidized iron surface (about 10 K lower than on the clean Fe surface).Assuming a preexponential factor" of IO', s-l, the desorption energy for the chemisorbed PH, is 11.5 kcal/mol in the limit of low coverage. For near-saturation coverage, the desorption energy drops to 9.1 kcal/mol (9.6 kcal/mol on the unoxidized surface).That PH3 adsorption on the oxidized Fe surface at 100 K is molecular is further supported by the fact that no detectable residue was left on the oxidized iron surface a t the end of the desorption as monitored by AES ( Figure 8). Thus there is a marked difference between the behavior of PH, on clean and oxidized iron, the latter inhibiting dissociative adsorption. Comparison of the amount of PH3 desorbed from clean and oxidized iron indicates that at low coverages 70 f 15% of the adsorbed PH3 decomposes on a clean iron surface.The adsorption of D2 is strongly inhibited on oxidized Fe, with or without the presence of PH3. A 2000-langmuir D2 adsorption and subsequent TPD showed no D2 desorption from either. SummaryThis work can be summarized as follows: 1. At 100 K, PH3 adsorption on clean Fe is both molecular and dissociative, while on oxidized Fe it is molecular. 2. Preadsorbed phosphorus and/or deuterium on clean iron inhibit PH3 decomposition but do not change the PH, desorption energetics. 3. Postdosed PH3 displaces some preadsorbed deuterium from the Fe surface and lowers the D2 desorption peak positions. 4. Preadsorbed PH3 inhibits D2 adsorption. 5. Coadsorption of D2 and PH3 leads to no incorporation of D into the molecular PH, desorption, showing that PH3 decomposition on clean Fe is irreversible under the transient conditions of these experiments. 6. The adsorption of D2 is inhibited on oxidized Fe in the presence or absence of coadsorbed PH3.The surface-enhanced Raman scattering (SERS) excitation profiles of chromate, molybdate, and tungstate ions on colloidal silver were obtained by using excitation wavelengths between 457.9 and 676.4 nm. The intensity of the strongest SERS band between 800 and 900 cm-I, assignable to v,(M-0), was in each case referenced to the 1020-cm-' band of the internal standard, methanol, in silver hydrosols to which methanol had been added. These relative SERS intensities were compared against similar intensity ratios for each oxo anion in solution and then appropriately scaled for the concentration of Mod2-in solution and on the surface of the silver particles in the sols. Pedk SERS enhancements occurred at 560 nm for Cr042-and at 600 nm for both MOO: -and WOZ-. The latter peak positions matched the secondary absorption band maxima that origi...
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