Direct observation of heterogeneous photochemistry on aggregated Ag nanocrystals using Raman spectroscopy:: The case of photoinduced degradation of aromatic amino acids

We show that aromatic amino acids on aggregated and immobilized Ag nanocrystals degrade upon laser illumination (lambda = 514.5 nm). Surface-enhanced Raman scattering (SERS) data, using tyrosine as the precursor, point to a linear one-photon induced reaction. This interpretation is supported by ab initio calculations of tyrosine-Ag+ complexes. Spectral fluctuations and bleaching-effects suggest that the observed SERS signal is due to a variety of chromophores that continuously form from aromatic precursors and eventually break down at the Ag surface. The final photoproduct is similar to amorphous carbon (AC) but the spectral details are determined by the precursor. We found that precoating the Ag particles with thiophenol inhibits photoinduced degradation of tyrosine in H2O.