Application of attenuated total reflection surface-plasmon-polariton Raman spectroscopy to gold and copper

An attenuated total reflection (ATR) method to excite the surface plasmon polariton (SPP) was applied to gold and copper to improve the sensitivity level of classical Raman spectroscopy. Reflectivity and Raman band intensity from copper phthalocyanine (CuPc) adsorbates on these metals were measured in an Otto configuration as a function of the angle of incidence and gap size at various excitation wavelengths. The SPP's on gold and copper were most efficiently excited at a wavelength of 632.8 nm according to the sufficiently large real part and the small imaginary part in the dielectric constants. It yields an enhancement factor of approximately 50 and 23 for Raman band intensity from CuPc 6 nm thick on gold and copper, respectively. Furthermore the significantly larger enhancement factor of ~100 was obtained for a self-assembled monolayer of p-nitrothiophenol on gold at 632.8-nm excitation, probably owing to its smaller imaginary part value. It enabled us to observe the electrochemical reduction to p -aminothiophenol in situ and gave clear evidence of charge transfer resonance as observed for the silver system. Conclusively, the ATR-SPP method is essential to enhancing a quite weak Raman signal from adsorbates on gold and copper surfaces as well as silver.