Laser-Induced Thermal Decay of Pyridine and Chloride Surface-Enhanced Raman Scattering as a Probe of Silver Surface-Active Sites

The activation parameters for the temperature-dependent irreversible loss of surface-enhanced Raman scattered (SERS) intensity from pyridine and chloride adsorbed at silver surfaces in an electrochemical environment have been determined. Laser-induced heating is used to promote a first-order surface reaction at these roughened silver electrodes and is introduced as a probe of the chemical nature of SERS-active sites. Accurate laser-induced surface temperatures must be known for these experiments and are calculated from spectroscopic data. It is found that the kinetic data follow Arrhenius behavior for both pyridine and chloride. The activation energies associated with the destruction of SERS-active sites at a surface roughened by an illuminated oxidation-reduction cycle (ORC) are 12.8 ± 3.2 and 27.7 ± 3.1 kcal/mol for pyridine at two different types of sites on the Ag surface. Similarly, values for coadsorbed chloride are found to be 11.1 ± 2.4 and 24.5 ± 3.8 kcal/mol. Experiments designed to elucidate the decay mechanism suggest that the laser-induced surface reaction involves desorption of pyridine and chloride from active sites allowing their incorporation into the Ag lattice. The observation of two activation energies in the same temperature region is interpreted in terms of SERS-active sites, which exist at a surface defect and on a terrace plane, respectively. This interpretation is based on the relative activation energies and preexponential factors. The activation parameters for pyridine and chloride desorption are also evaluated at a silver surface roughened by a nonilluminated ORC. An activation energy of 27.4 ± 1.9 kcal/mol is obtained for pyridine on these surfaces. This result suggests that a greater concentration of terrace plane SERS-active sites is present at a surface roughened by an illuminated ORC.