Junction Plasmon Driven Population Inversion of Molecular Vibrations: A Picosecond Surface-Enhanced Raman Spectroscopy Study

Molecular surface-enhanced Raman spectra recorded at single plasmonic nanojunctions using a 7 ps pulse train exhibit vibrational up-pumping and population inversion. The process is assigned to plasmon-driven, dark, impulsive electron-vibration (e-v) excitation. Both optical (Raman) pumping and hot-electron mediated excitation can be rejected by the characteristic spectra, which allow the simultaneous measurement of vibrational temperature of the molecules and electronic temperature of the metal. Vibrational populations are determined from anti-Stokes to Stokes intensity ratios, while the electron temperature is obtained from the anti-Stokes branch of the electronic Raman scattering continuum. Population inversion survives in high-frequency vibrations that effectively decouple from the metal.