Violation of the Condon Approximation in Semiconducting Carbon Nanotubes
A long-held but untested fundamental assumption in Raman spectroscopy of carbon nanotubes is that intensities arising from resonance with incident and scattered photons are equal. We present here the first complete Raman excitation profiles (REPs) for the nanotube G-band for ten semiconducting chiralities. In contrast to expectations, a strong asymmetry is observed in the REPs for all chiralities, with the scattered resonance always appearing weaker than the incident resonance. The behavior results from violation of the Condon approximation. Quantum chemical calculations indicate the behavior is intrinsic. The chirality dependence of the non-Condon effect is anti-correlated to expectations for exciton-phonon coupling in nanotubes. Observation of this behavior has significant implications for a range of nanotube photophysical behaviors. The result calls for a re-evaluation of how we describe Raman and other optical processes in carbon nanotubes.