The Vibrational Pumping Mechanism in Surface-Enhanced Raman Scattering: A Subpicosecond Time-Resolved Study

摘要

The origin of vibrational pumping in surface-enhanced Raman scattering is investigated applying subpicosecond Raman spectroscopy. We observe selective excitation of distinct fingerprint modes resulting in a pronounced nonequilibrium population distribution followed by a decay with a common time constant for all vibrations of about 1 ps. In particular, the Raman-active mode with the highest frequency in the fingerprint region is strongly excited. Vibrational excitation is delayed with respect to the excitation pulses. Our findings clearly rule out Stokes Raman scattering, but point to electronic excitation as the main source of vibrational pumping. After electronic excitation, energy is transferred to the vibrations by fluorescence and ultrafast internal conversion within about 0.8 ps.