Low-frequency vibrations of molecular submonolayers detected by time-domain Raman spectroscopy

Time-domain Raman spectroscopy using a single color pump-probe technique was performed to observe low-frequency vibrations of a submonolayer (4×1013 molecules cm−2) film of cresyl violet on a fused-silica surface. A16-fs pump pulse at 650 nm created Raman-active nuclear motions (vibrational coherence) in the film. The vibrational coherence was monitored as the time-dependent reflectance change of the sample. This time-domain method is promising for sensitive detection of low-frequency (terahertz) Raman bands of molecular submonolayer films, especially when the films contain highly luminescent compounds.