Population Pumping of Excited Vibrational States by Spontaneous Surface-Enhanced Raman Scattering

Abstract

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 $\ensuremath{\nu}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1$ to $\ensuremath{\nu}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}2$ Raman transitions in the SERS Stokes signals.