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 $\nu =1\mathrm{}$ to $\nu =2\mathrm{}$ Raman transitions in the SERS Stokes signals.