Population pumping of excited vibrational states by spontaneous surface-enhanced Raman scattering

We observed significant transfer of ground state population to the first excited vibrational state by spontaneous surface-enhanced Raman scattering (SERS) which can be explained by unexpectedly large SERS cross sections. Evidence for this pumping includes (i) anti-Stokes to Stokes ratios which exceed those expected from a Boltzmann distribution, (ii) a quadratic dependence of the anti-Stokes signal on the excitation intensity, whereas the Stokes signal remains linearly dependent, and (iii) a component of nu = 1 to nu = 2 Raman transitions in the SERS Stokes signals.