Surface-enhanced non-linear Raman scattering at the single-molecule level

Surface-enhanced hyper-Raman scattering and surface-enhanced anti-Stokes Ra man scattering were studied as potential tools for non-linear single-molecu le Raman spectroscopy. Experiments were performed using near-infrared excit ation on crystal violet adsorbed on colloidal silver or gold clusters. Stro ng enhancement factors on the order of 10(20) were inferred from hyper-Rama n scattering experiments on colloidal silver. Such extremely high enhanceme nt factors overcome the inherently weak nature of the effect, and surface-e nhanced hyper-Raman scattering appears on comparable intensity levels as su rface-enhanced Raman scattering. Surface-enhanced anti-Stokes Raman scatter ing starts from vibrational levels, that are populated by the very strong s urface-enhanced Raman process. Thus, the anti-Stokes Raman scattering signa l depends quadratically on the excitation laser intensity. For the first ti me, surface-enhanced anti-Stokes and Stokes Raman scattering was detected f rom single molecules on colloidal gold clusters. (C) 1999 Published by Else vier Science B.V. All rights reserved.