Surface enhanced Raman scattering of ammonia

A surface enhanced Raman scattering (SERS) spectrum of 0.5 M NH₃ in 4.0 M KCl has been observed on a silver electrode. An approximate enhancement factor of 3 × 10⁵ is calculated, and additional evidence for the enhanced nature of the spectrum is provided by the observation that totally symmetric vibrations are depolarized and by the strong potential dependence of the intensity of surface lines. Assignments have been given to the SERS lines with the low-frequency lines assigned to a AgCl and AgN stretch. The positive shift of the maximum of the intensity versus voltage curve with a lower laser excitation frequency is taken as evidence for the occurrence of a charge transfer process from ammonia to the silver electrode. The fact that the SERS spectrum of NH₃ on Ag can only be observed at large electrolyte concentrations is attributed to the breaking of hydrogen bonding at the electrode-solution interface.