• surface-enhanced Raman scattering;
  • combinational resonance bands;
  • Darling–Dennison resonance

We report surface enhanced Raman spectroscopy (SERS) of Darling–Dennison resonance of thiourea on Ag electrode excited at 514.5 nm laser excitation. Darling–Dennison resonance indicates that two degenerate Raman modes interact with each other and their degenerate first-order overtone modes obtain energy and appear in Raman spectra. Our study showed that the ratio of intensity of the Darling–Dennison resonance is up to 0.24 of its fundamental Raman intensity, when the applied electrode voltage is at –0.4 V versus the saturated calomel electrode. This phenomenon was also observed on the Ag island film surface at ambient condition. These observations demonstrated strong evidence for Darling–Dennison resonance band in SERS. The implications of these observations are also discussed. Copyright © 2013 John Wiley & Sons, Ltd.