Broadband sum frequency generation (BB-SFG) spectroscopy has been used to investigating the potential-dependent adsorption behaviors of thiocyanate ions on polycrystalline Au electrode in alkaline solutions. During the positive scan, a change in the Stark slope (dωC-N/dE) for C—N stretching mode can be observed in the range of -1.1 V ~ 0.2 V (vs. SCE), which implies that the orientation of adsorbed thiocyanate ions at Au electrode surface changes from N-bound (dωC-N/dE=45 cm-1·V-1) form to S-bound (dωC-N/dE = 16.2 cm-1·V-1) form . At more positive potentials, the electrons in Au are ongoing interband transition, from d-band to sp-band, instead of intraband transition, from sp-band to sp-band, during the SFG process, and this difference will change the relative phase between the substrate and the adsorbed SCN- SFG signals. As a consequence, when the applied potential is more positive, the lineshapes of the BB-SFG spectrum for C—N stretching mode become Fano (dispersive) lineshapes. From the potential-dependent BB-SFG lineshapes, the information about relative positions for the electric energy levels of the electrode surface can be obtained.
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